3.1.6 KINETICS

Rate of reaction can be increased by:

- Increasing concentration (aq) - Increasing pressure (g) - Increasing surface area (s) - Increasing temperature - Adding a catalyst

Q: The curve below shows how the volume of oxygen evolved varies with time when 50 cm3 of a 2.0 mol dm-3 solution of hydrogen peroxide, H2O2, decomposes at 298 K. Sketch curves, on the above axes, to illustrate how the volume of oxygen evolved would change with time if the experiment was repeated at 298 K using the following. (a) 100 cm3 of a 1.0 mol dm-3 solution of H2O2. Label this curve X. (b) 25 cm3 of a 2.0 mol dm-3 solution of H2O2 in the presence of a catalyst. Label this curve Y. (4 MARKS)

(a) - Curve X is lower and starts at origin - And levels out at same volume as original curve (b) - Curve Y is steeper than original and starts at origin - Then levels out at half the volume of the original

Q: Explain qualitatively why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration. (3 MARKS)

- Reaction occurs when molecules have E>E, - Doubling T by 10 °C causes many more molecules to have this ENERGY - Whereas doubling concentration only doubles the number with this Energy

Q: Emp is the most probable value for the energy of the molecules (a) On the appropriate axis of this diagram, mark the value of Emp for this distribution. On this diagram, sketch a new distribution for the same sample of gas at a lower temperature. (3 MARKS) (b) With reference to the Maxwell-Boltzmann distribution, explain why a decrease in temperature decreases the rate of decomposition of this gas. (2 MARKS)

(a) - On the energy axis Emp at the maximum of the original peak - The peak of their new curve is displaced to the left and higher than the original. - All of the following are required: • The new curve starts at the origin and should begin to separate from the original almost immediately • and the new curve crosses the original curve once • and an attempt has been made to draw the new curve correctly towards the energy axis below the original curve but not to touch the original curve or the axis (b) 1. A decrease in the number / proportion of molecules with E ≥ Ea OR fewer molecules have E ≥ Ea OR fewer molecules have sufficient / enough energy to react / decompose 2. Fewer effective / productive / successful collisions in a given time / given period OR fewer frequent effective / productive / successful collisions OR lower rate of effective / productive / successful collisions

TIP!

If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions.

Q: Nitric acid is manufactured from ammonia in a process that involves several stages. In the first stage, ammonia is converted into nitrogen monoxide and the following equilibrium is established. 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g) ∆H = -905 kJ mol-1 The catalyst for this equilibrium reaction is a platinum-rhodium alloy in the form of a gauze. This catalyst gauze is heated initially but then remains hot during the reaction. (a) Suggest a reason why the catalyst must be hot. (1 MARK) (b) Suggest a reason why the catalyst remains hot during the reaction. (1 MARK)

(a) - to provide/overcome activation energy - to provide the minimum energy to make the reaction go/start (b) The reaction is exothermic OR releases heat (energy)

Q: A student carried out an experiment to determine the rate of decomposition of hydrogen peroxide into water and oxygen gas. The student used 100 cm3 of a 1.0 mol dm-3 solution of hydrogen peroxide at 298 K and measured the volume of oxygen collected. Curve R, shows how the total volume of oxygen collected changed with time under these conditions. (a) Draw a curve on Figure 1 to show how the total volume of oxygen collected will change with time if the experiment is repeated at 298 K using 100 cm3 of a 2.0 mol dm-3 solution of hydrogen peroxide. (2 MARKS) (b) Draw a curve on Figure 2 to show how the total volume of oxygen collected will change with time if the experiment is repeated at 298 K using 100 cm 3 of a 0.4 mol dm-3 solution of hydrogen peroxide. (2 MARKS) (c) Draw a curve on Figure 3 to show how the total volume of oxygen collected will change with time if the original experiment is repeated at a temperature higher than 298 K. You should assume that the gas is collected at a temperature of 298 K. (2 MARKS) (d) Explain why the slope (gradient) of curve R decreases as time increases.

(a) M1 curve is steeper than original and starts at the origin M2 curve levels at the top line on the graph (b) M1 curve is shallower than original and starts at the origin M2 curve levels at the first line on the graph (c) M1 curve would be steeper than original M2 curve levels at the same original volume of O2 (d) - The (concentration / amount of) H2O2 or reactant falls / decreases / used up OR The number of H2O2 or reactant molecules/ particles falls / decreases - The rate of reaction / rate of decomposition / rate of formation of oxygen / frequency of collisions / (effective) collisions in a given time decreases / is slower

SME: A chemist performed a reaction at three different temperatures, 100K, 300K and 700K as shown by the Maxwell-Boltzmann distribution graph in Figure 3. (a) Label each curve in Figure 3 with the correct temperature values, 100K, 300K and 700K. (b) Consider the following statement, 'All reacting molecules have higher kinetic energy at 700K than they do at 300K'. State whether you agree this statement is correct and justify your reasons. (4 MARKS)

(a) Temperatures labelled correctly going from low to high values (left to right); (b) - The given statement is incorrect; - The distribution curves for 700K and 300K show that there are molecules with both high and low energy values; - (But) The most probable energy value / Emp will be greater at 700K than 300K;

Draw and explain the Maxwell−Boltzmann distribution curve for an increase in temperature.

- Increase in temperature → increase in rate of reaction - Curve shifts down and to the right - Total area under curve remains constant as total no. of particles is constant - Higher K.E. of reactant particles - Higher frequency of collisions - Higher no. Of particles with sufficient energy to react - (NB, increased the proportion that have sufficient energy)

EQ: Methanol, for use as a fuel, can be produced by the reaction of carbon monoxide with hydrogen. CO(g) + 2H2(g) ⇌ CH3OH(g) ΔH = -90 kJ mol-1 The reaction is typically carried out at 300 °C and 3 × 107 Pa, in the presence of a catalyst. The graph shows the Maxwell-Boltzmann distribution for a mixture of carbon monoxide and hydrogen at 300 °C. (i) Sketch a second curve on the graph to show the distribution of molecular energies in this mixture at a higher temperature. (1 MARK) (ii) Explain with reference to both curves on the graph how a small change in temperature leads to a large change in the rate of reaction. (2 MARKS)

(i) Look at image (ii) - (Relative areas under curves indicate) many more molecules with E greater than or equal to Ea (at higher T) - (Large) increase in (number of) successful (owtte) collisions per unit time

State why the Maxwell-Boltzmann curve starts at the origin.

No molecules have no energy

Q: A student investigated how the initial rate of reaction between sulfuric acid and magnesium at 20 °C is affected by the concentration of the acid. The equation for the reaction is H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g) The student made measurements every 20 seconds for 5 minutes. The student then repeated the experiment using double the concentration of sulfuric acid. State a measurement that the student should make every 20 seconds. Identify the apparatus that the student could use to make this measurement. (2 MARKS)

- (Measure the) volume of gas / mass of the container + contents - Suitable named piece of equipment: - Gas syringe (or inverted burette or measuring cylinder, as long as student has referred to the cylinder being filled with water) / balance.

Increasing rate of reaction: increasing pressure (g)

- Adding more particles OR decreasing the volume for same no. of particles - More no. of reactant molecules per unit volume - Packed closer together - Higher frequency of collisions - Higher frequency of successful / effective collisions

Draw and explain the Maxwell−Boltzmann distribution curve when a catalyst is added.

- Addition of catalyst → increases rate of reaction - Catalyst lowers activation energy - Curve does not change - Activation energy line shifts to the left - Larger no. of particles now have sufficient energy to react

EQ: Figure 1 shows the volume of hydrogen gas collected when a sample of magnesium reacted with an excess of dilute hydrochloric acid. The rate of this reaction can be studied by measuring the time it takes for a given volume of hydrogen to be collected. State and explain what has happened to the rate of this reaction at point W in Figure 1. (2 MARKS)

- At W The rate / it is zero - The magnesium has all reacted / has been used up - OR Reaction is complete / it has stopped

Q: Explain what must happen for a reaction to occur between molecules of two different gases. (2 MARKS)

- Collision of molecules - With sufficient energy (activation energy)

Q: The diagram below shows the Maxwell-Boltzmann distribution of molecular energies in a sample of a gas. Explain the process that causes some molecules in this sample to have very low energies. (2 MARKS)

- Collisions - Cause some molecules to slow down or lose energy

Q: Calcium carbonate is an insoluble solid that can be used in a reaction to lower the acidity of the water in a lake. Explain why the rate of this reaction decreases when the temperature of the water in the lake falls. (3 MARKS)

- Decrease in T decreases the energy of the particles/ions/H+ /molecules - Decrease in the number of/less particles/ions/ H+ /molecules with E ≥ EAct or E ≥ minimum energy to react - Few(er)/Less effective/productive/successful collisions

SME: Aqueous hydrogen peroxide decomposes according to the following equation. 2H2O2(aq) → 2H2O(1) + O2(g) The decomposition is catalysed by adding 0.25 g manganese(IV) oxide. Manganese(IV) oxide acts as a heterogeneous catalyst. This can be investigated by measuring the volume of oxygen produced at various times as the reaction proceeds. Catalysts are not used up during a reaction. Describe and outline a method to show that the manganese(IV) oxide is not used up in the decomposition of hydrogen peroxide and that it still functions as a catalyst. (4 MARKS)

- Filter the solid (catalyst) from the solution after the experiment; - (Rinse with solvent / water) dry the solid; - Re-weigh the solid (should weigh approximately 0.25 g); - Repeat the experiment to see if repeatable results are obtained / catalyst still works;

EQ: Another type of hand-warmer uses sodium thiosulfate. Sodium thiosulfate is very soluble in water at 80 °C but is much less soluble at room temperature.When a hot, concentrated solution of sodium thiosulfate is cooled it does not immediately crystallise. The sodium thiosulfate stays dissolved as a stable 'super-saturated' solution until crystallisation is triggered. Heat energy is then released when the sodium thiosulfate crystallises. This handwarmer is reusable. Describe the two steps that you would take to make the sodium thiosulfate hand-warmer ready for re-use. (2 MARKS)

- Heat to / or above 80 °C (to allow thiosulfate to redissolve) - Allow to cool before using again

EQ: Methanol, for use as a fuel, can be produced by the reaction of carbon monoxide with hydrogen. CO(g) + 2H2(g) ⇌ CH3OH(g) ΔH = -90 kJ mol-1 The reaction is typically carried out at 300 °C and 3 × 107 Pa, in the presence of a catalyst. By considering both rate and yield, state why the reaction is carried out at a temperature of 300 °C rather than at a higher temperature. (2 MARKS)

- Higher T would increase rate but decrease yield/make less methanol OR Lower T decreases rate but increases yield; - Chosen T is a compromise/balance (between rate and yield) owtte

State and describe the two types of catalysts.

- Homogenous: catalyst in the same state / phase as the reactants - Heterogeneous: catalyst in a different state to the reactants

What are the benefits of using a catalyst in a reaction?

- Increased rate at lower temperatures + pressure - Less energy needed - Lower economic + environmental cost (CO2 emissions)

What is the y axis of the Maxwell-Boltzmann curve labelled with?

Number of molecules (NOT moles or atoms)

EQ: When an aqueous solution of ethanoic acid reacts with magnesium, the progress of reaction can be followed using the equipment shown in Figure 5 to measure the volume of hydrogen produced. Figure 6 shows how the volume of hydrogen produced varies with time when 396 mg of magnesium are added to 30.0 cm3 of 0.600 mol dm−3 ethanoic acid. The reaction was repeated using 20 cm3 of 0.800 mol dm−3 of ethanoic acid solution with all other conditions the same. The magnesium was still in excess. Sketch a line on Figure 6 to show how the volume of hydrogen produced varies with time in this second experiment. (2 MARKS)

- Initial reaction → 0.009 moles H2 - New reaction → 0.008 moles H2

EQ: A commercial hand-warmer uses powdered iron sealed in a plastic container.A valve allows air to enter the container, and oxygen in the air reacts slowly with the iron to form solid iron(lll) oxide. The heat released warms the container. One version of an iron-oxygen hand-warmer advertises that it is designed to stay warm for up to four hours. Other than by increasing the amount of iron in the container, state one change to the iron in the hand-warmer that would increase this time. Explain why this change to the iron might not be an advantage. (3 MARKS)

- Iron powder particle size could be increased / - Not all the iron reacts / less reaction / not all energy - Correct consequence of M2; An appropriate consequence, for example: • too slow to warm the pouch effectively • lower temperature reached • waste of materials

Q: A student investigated how the initial rate of reaction between sulfuric acid and magnesium at 20 °C is affected by the concentration of the acid. The equation for the reaction is H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g) State one condition, other than temperature and pressure, that would need to be kept constant in this investigation. (1 MARK)

- Mass of magnesium - Surface area of magnesium

Q: An equation for the decomposition of hydrogen peroxide is shown below. 2H2O2 → 2H2O + O2 State the measurements you would take in order to investigate the rate of this reaction. (2 MARKS)

- Measure volume of gas / mass loss - At (regular) time intervals

Q: Gas G decomposes as shown in the equation below. G(g) → X(g) + Y(g) At any time, most of the molecules of G have energy less than the activation energy. Suggest why, at a constant temperature, most of G eventually decomposes. (2 MARKS)

- Molecules gain energy (or always some molecules have E > Ea) - Due to collisions

Increasing rate of reaction: increasing concentration (aq)

- More no. of reactant molecules per unit volume - Packed closer together - Higher frequency of collisions - Successful / effective collisions are more frequent

Increasing rate of reaction: increasing surface area (s)

- More reactant molecules exposed → more can react - Higher frequency of collisions - Higher frequency of successful collisions

Draw and explain the Maxwell−Boltzmann distribution curve for a decrease in temperature.

- Peak increases in height - Curve moves to the left - Rate of reaction decreases - A decrease / fewer number / proportion of molecules with sufficient energy to react; energy greater than or equal to the activation energy - Fewer / Less frequent successful collisions per unit of time

SME: Ammonia, NH3, is manufactured by the chemical industry from nitrogen and hydrogen gases. N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) AH = - 92 kJ mol-1 An iron catalyst is used which provides several benefits for sustainability. The chemical industry uses operational conditions that are different from the conditions predicted to give a maximum equilibrium yield. Justify the above statements using your knowledge of kinetics and equilibrium. (6 MARKS)

- Pressure: Right-hand side has fewer number of (gaseous) moles; - Temperature: Forward reaction is exothermic; - Lower temperature and higher pressure required for maximum yield of ammonia AND Both cause the equilibrium to shift to the right; - Too low a temperature causes a slow rate of reaction; - Too high a pressure is expensive / safety risk; - Use of a catalyst: Reactions take place at lower temperatures with lower energy demand OR Reduce CO2 emissions / burning fossil fuel;

Increasing rate of reaction: adding a catalyst

- Provides a different route / mechanism / pathway - With lower activation energy - More particles have sufficient energy to react - Higher proportion of collisions are successful - Does not increase frequency of collisions

How can rate of reaction be measured?

- Quantity of reactant used / time taken - Quantity of product formed / time taken

EQ: Explain the effect that lowering the temperature would have on the rate of reaction. (2 MARKS)

- Rate of reaction decreases (no mark) - Fewer particles will have energy greater than or equal to the activation energy - Fewer successful collisions in a given time

SME: In a reaction, lumps of a reactant were used. Use collision theory to explain what would happen to the rate of the reaction if powdered reactants were used instead of lumps.

- Rate of reaction would increase - There is a bigger/greater/larger surface area for collisions to take place - The frequency of successful collisions would increase

Increasing rate of reaction: increasing temperature

- Reactant molecules gain K.E. - More have sufficient energy to react (required activation energy) - Greater proportion of collisions are successful - Higher frequency of collisions - Higher frequency of successful collisions per unit time

Q: For a particular reaction, the best yield is obtained by keeping the pressure as low as possible. Explain the effect that lowering the pressure will have on the rate of the reaction. (3 MARKS)

- Reduce the rate of reaction - Fewer gas molecules in a given volume/the concentration will be reduced/the molecules will be further apart - Frequency of collisions between molecules decreases

Q: Hydrogen peroxide decomposes more rapidly in the presence of aqueous hydrogen bromide. The decomposition proceeds as shown by the following equations. H2O2 + HBr → HBrO + H2O HBrO + H2O2 → H2O + O2 + HBr Give two reasons, other than an increase in the reaction rate, why these equations suggest that hydrogen bromide is behaving as a catalyst. (2 MARKS)

- Remains unchanged (or not used up or not in the overall reaction equation) - Offers alternative reaction route (or acts as an intermediate)

Q: The enthalpy change for a specific reaction is quite small. By reference to the number and type of bonds broken and made, explain how this might have been predicted. (2 MARKS)

- Similar (types) of bond broken and made - Same number of the bonds broken and made

EQ: The diagram below shows the Maxwell-Boltzmann distribution of molecular energies, at a constant temperature, in a gas at the start of a reaction. On this diagram the most probable molecular energy at this temperature is shown by the symbol Emp. The activation energy is shown by the symbol Ea. To answer the questions, you should use the words increases, decreases or stays the same. You may use each of these answers once, more than once or not at all. 1. State how, if at all, the value of the most probable energy (Emp) changes as the total number of molecules is increased at constant temperature. 2. State how, if at all, the number of molecules with the most probable energy (Emp) changes as the temperature is decreased without changing the total number of molecules. 3. State how, if at all, the number of molecules with energy greater than the activation energy (Ea) changes as the temperature is increased without changing the total number of molecules. 4. State how, if at all, the area under the molecular energy distribution curve changes as a catalyst is introduced without changing the temperature or the total number of molecules.

- Stays the same - Increases - Increases - Stays the same

EQ: A Maxwell-Boltzmann distribution curve was drawn for a second sample of the reaction mixture in the same reaction vessel. Figure 2 shows the results. Deduce the change that was made to the reaction conditions. Explain the effect that this change has on the rate of reaction. (3 MARKS)

- The amount of gas present (or number of molecules) has been reduced / or the pressure has been reduced - Rate of reaction decreases (no mark) - Particles are spread further apart - Fewer collisions between gas particles so fewer successful collisions

Q: A method of synthesising ammonia directly from nitrogen and hydrogen was developed by Fritz Haber. On an industrial scale, this synthesis requires a high temperature, a high pressure and a catalyst and is very expensive to operate. The enthalpy of formation of ammonia is -46.5 kJ.mol-1. A manager in charge of ammonia production wished to increase the daily production of ammonia and reduce the production costs. How would a chemist explain the factors that would influence the commercial efficiency of this production process? (8 MARKS)

- The higher the temperature the faster the reaction QWC - but, since the reaction is exothermic - the equilibrium yield is lower QWC - The higher the pressure the greater the equilibrium yield QWC - because there is a reduction in the number of moles of gas in the reaction - but higher pressure is expensive to produce or plant is more expensive to build QWC - A better catalyst would lessen the time to reach equilibrium - and allow more ammonia to be produced in a given time QWC

Q: State the meaning of the term activation energy. (2 MARKS)

- The minimum energy - For a reaction to start / for a successful/ effective collision

Q: Explain why, even in a fast reaction, a very small percentage of collisions leads to a reaction. (1 MARK)

Only a small percentage/very few collisions have E >Ea

EQ: Figure 1 shows the volume of hydrogen gas collected when a sample of magnesium reacted with an excess of dilute hydrochloric acid. The rate of this reaction can be studied by measuring the time it takes for a given volume of hydrogen to be collected. In terms of collision theory explain why, at a fixed temperature, the rate of this reaction doubles when the concentration of the hydrochloric acid doubles. (2 MARKS)

- Twice / double as many particles / hydrogen ions (in a given volume) - Twice / double as many frequency of effective / successful collisions (in a given time)

Draw and explain the Maxwell−Boltzmann distribution curve for an increase in concentration / pressure.

- Y AXIS SHOULD SAY NUMBER / PROPORTION / FRACTION OF MOLECULES NOT PARTICLES - Higher concentration(aq) / higher pressure of gas = higher rate - Curve shifts UP, shape of curve does not change - Area increases due to higher no. of particles - Higher frequency of collisions - No. of particles with sufficient energy increases - Higher no. of successful collisions (NB, same proportion relatively; eg here is an analogy, if you sit 50 students in an exam and 10% get A's, that is 5 students, if you make more students sit, eg 100, you will still get 10% A's, but that is 10 students who got A's) (The modal / most probable energy that the particles possess, the mean energy that particles possess and the activation energy all DONT change)

Define rate of reaction.

- change in concentration of a substance in unit time - amount of substance formed / used up in a unit of time

EQ: Hydrogen peroxide solution decomposes slowly to form water and oxygen. The reaction is much faster in the presence of a manganese(IV) oxide catalyst. 2H2O2(aq) ⟶ 2H2O(l) + O2(g) Three experiments, shown in Table 1, were carried out to investigate how the volume of oxygen produced varied over time under different conditions. The same mass of catalyst was used in each experiment. The graph shows how the volume of oxygen collected varied with time in Experiment 1. Draw two lines on Figure 1 to show how the volume of oxygen collected varied with time in Experiments 2 and 3. Label each line with the experiment number. (2 MARKS)

- line 2 = steeper than original and finishes at same height - line 3 = less steep than original and finishes at half the height

Q: The gas-phase reaction between hydrogen and chlorine is very slow at room temperature. H2(g) + Cl2(g) → 2HCl(g) Explain why a small increase in temperature can lead to a large increase in the rate of reaction between hydrogen and chlorine. (2 MARKS)

- many more molecules have energy greater than the activation energy - so proportion of collisons which are successful will increase

Q: Give one reason why most collisions between gas-phase reactants do not lead to a reaction. State and explain two ways of speeding up a gas-phase reaction other than by changing the temperature. (5 MARKS)

- molecules (or particles or collisions) do not have enough energy; - increase the pressure; - (or increase the concentration or reduce the volume) increases the collision frequency; - add a catalyst; - lowers activation energy (or Ea) (Q of L mark);

EQ: Hydrogen peroxide solution decomposes slowly to form water and oxygen. The reaction is much faster in the presence of a manganese(IV) oxide catalyst. 2H2O2(aq) ⟶ 2H2O(l) + O2(g) Three experiments, shown in Table 1, were carried out to investigate how the volume of oxygen produced varied over time under different conditions. The same mass of catalyst was used in each experiment. The graph shows how the volume of oxygen collected varied with time in Experiment 1.

- more particles/molecules/reactants/H,O2/moles in given/same volume/space - OR particles/molecules/reactants/H,O/moles closer together - successful collisions are more frequent (Many students referred to the number of successful collisions rather than the frequency of successful collisions when explaining why the concentration of hydrogen peroxide affects the rate of reaction.)

EQ: Group 2 metals and their compounds are used commercially in a variety of processes. Strontium is extracted from strontium oxide (SrO) by heating a mixture of powdered strontium oxide and powdered aluminium. 3SrO(s) + 2Al(s) -----> 3Sr(s) + Al2O3(s) ∆H = +101 kJ mol-1 1. The use of powdered strontium oxide and powdered aluminium increases the surface area of the reactants. Suggest one reason why this increases the reaction rate. (1 MARK) 2. Suggest one major reason why this method of extracting strontium is expensive. (1 MARK)

1. - To increase collision frequency / collisions in a given time / rate of collisions - To increase the surface contact / contact between the solids / contact between (exposed) particles 2. - Aluminium is extracted by electrolysis OR aluminium extraction uses (large amounts of) electricity - Reaction / process / It / the mixture requires heat - It is endothermic

EQ: The following figure shows the Maxwell.Boltzmann distribution of molecular energies in a sample of gas at temperature T. 1. One of the axes is labelled.Label the other axis. (1 MARK) 2. State why the curve starts at the origin. (1 MARK)

1. Amount / number / proportion / percentage / fraction / moles of molecules / particles Not 'atoms'. This is penalised. 2. - There are no molecules / particles with zero energy - OR All of the molecules / particles are moving / have some energy Not 'atoms'. This is penalised. Do NOT use atoms in this context.

Q: In an investigation of the rate of reaction between hydrochloric acid and pure magnesium, a student obtained the following curve. The reaction of magnesium with dilute hydrochloric acid is exothermic. Use your understanding of collision theory to explain why the student did not obtain a straight line. (3 MARKS)

1. As concentration increases the amount of heat given out increases / temperature increases 2. More successful collisions or reactions in a given time OR more particles have the activation energy 3. (An increase in temperature or more heat given out) increases the rate of a reaction

Q: An equation for the equilibrium reaction between hydrogen, iodine and hydrogen iodide is shown below. H2(g) + I2(g) ⇌ 2HI(g) State and explain the effect of an increase in total pressure on the rate of attainment of this equilibrium. (3 MARKS)

1. Increase/speed up/faster (rate of attainment of equilibrium) 2. More particles/molecules in a given volume/space OR the particles/molecules are closer together OR an increase in concentration. 3. More/higher chance of successful/effective/productive collisions (between particles) OR more collisions/higher chance of collisions (of particles) with E>EAct

Q: The diagram below shows a Maxwell-Boltzmann distribution for a sample of gas at a fixed temperature. Ea is the activation energy for the decomposition of this gas. 1. On this diagram, sketch the distribution for the same sample of gas at a higher temperature. (2 MARKS) 2. With reference to the Maxwell-Boltzmann distribution, explain why an increase in temperature increases the rate of a chemical reaction. (2 MARKS)

1. M1 The peak of the new curve is displaced to the right. M2 All of the following are required: - The new curve starts at the origin - The peak of the new curve is lower than the original - and the new curve only crosses the original curve once - and an attempt has been made to draw the new curve correctly towards the energy axis but not to touch the original curve - the new curve must not start to diverge from the original curve 2. - Increase in the number/proportion of molecules with E > Ea OR more molecules have E > Ea OR more molecules have sufficient energy to react - More effective/productive/successful collisions

Q: Pure magnesium reacts completely with an excess of dilute sulfuric acid. The reaction of pure calcium with an excess of dilute sulfuric acid is very rapid initially. This reaction slows down and stops before all of the calcium has reacted. Use your knowledge of the solubilities of Group 2 sulfates to explain why these reactions of magnesium and calcium with dilute sulfuric acid are so different. (3 MARKS)

1. Magnesium sulfate is soluble and calcium sulfate is insoluble / slightly soluble / magnesium sulfate is more soluble / calcium sulfate is less soluble / correct trend in solubility 2. Calcium sulfate coats the surface of the calcium 3. Coating prevents further contact with / reaction by the acid

Q: Sodium thiosulfate solution (Na2S2O3) reacts slowly with dilute hydrochloric acid to form a precipitate. The rate of this reaction can be studied by measuring the time (t) that it takes for a small fixed amount of precipitate to form under different conditions. The fixed amount of precipitate is taken as the amount needed to obscure a cross on paper. The equation for this reaction is shown below. Na2S2O3 + 2HCl →2NaCl + S + SO2 + H2O In terms of particles, explain why, at a fixed temperature, you might expect the rate of this reaction to double when the concentration of sodium thiosulfate is doubled and the concentration of hydrochloric acid remains the same. (2 MARKS)

1. Twice as many / double number of particles 2. More / twice / double (effective) collisions (in a given time)

EQ: The apparatus in the figure below was set up to measure the time taken for 20.0 cm³ of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a 100 cm3 conical flask at 20 °C. The timer was started when the sodium thiosulfate solution was added to the acid in the flask. The timer was stopped when it was no longer possible to see the cross on the paper. What is likely to decrease the accuracy of the experiment? A: Rinsing the flask with acid before each new experiment. B: Stirring the solution throughout each experiment. C: Using the same piece of paper for each experiment. D: Using different measuring cylinders to measure the volumes of acid and sodium thiosulfate. (Total 1 mark)

A

EQ: The question below is about the Maxwell-Boltzmann distribution shown for a sample of a gas, X, at two different temperatures. Which statement is correct for the higher temperature? A: The area under the curve to the left of E, decreases. B: The total area under the curve increases. C: The activation energy decreases. D: More molecules have the mean energy. (Total 1 mark)

A

Q: Line X in the diagram represents the volume (V) of gas formed with time (t) in a reaction between an excess of magnesium and aqueous sulfuric acid. Which line represents the volume of hydrogen formed, at the same temperature and pressure, when the concentration of sulfuric acid has been halved?

A

Define catalyst.

A catalyst increases reaction rate without being changed in chemical composition or amount by providing an alternative route for the reaction that has a lower activation energy.

What does a lower activation energy due to a catalyst mean in terms of rate of reaction?

A greater proportion of particles will have sufficient energy to react. Therefore there are more successful collisions.

Q: Explain why a small increase in temperature has a large effect on the initial rate of a reaction. (2 MARKS)

A small increase in temperature results in a much higher proportion of molecules/collisions with sufficient energy / minimum energy to react (compared with a small increase in concentration)

Q: The apparatus in the figure below was set up to measure the time taken for 20.0 cm3 of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a 100 cm3 conical flask at 20 °C. The timer was started when the sodium thiosulfate solution was added to the acid in the flask. The timer was stopped when it was no longer possible to see the cross on the paper. What is likely to decrease the accuracy of the experiment? A: Rinsing the flask with acid before each new experiment. B: Stirring the solution throughout each experiment. C: Using the same piece of paper for each experiment. D: Using different measuring cylinders to measure the volumes of acid and sodium thiosulfate.

A; - An unwanted reaction may occur (react with leftover sodium thiosulfate in the flask) OR It will change the total volume of acid used;

Q: The gas-phase reaction between hydrogen and chlorine is very slow at room temperature. H2(g) + Cl2(g) → 2HCl(g) Give one reason why the reaction between hydrogen and chlorine is very slow at room temperature. (1 MARK)

Activation energy is high / few molecules/particles have sufficient energy to react/few molecules/particles have the required activation energy

SME: Explain why the use of the catalyst in the Contact process, reduces energy demand and benefits the environment. (2 MARKS)

Any two from the following: - Lower temperatures / less heat / less thermal energy is required; - Less fossil fuels / oil / coal / gas / non-renewable fuels are required; - CO2 emissions are reduced / decrease;

SME: Four possible reactions (A, B, C and D) of the following equation are measured at the same temperature. X (g) + Y (g) → Z (g) Which reaction pathway diagram shows the reaction occurring rapidly with an overall negative enthalpy value? (1 MARK)

B; A is incorrect as; it has higher activation energy than B

EQ: The apparatus in the figure below was set up to measure the time taken for 20.0 cm³ of sodium thiosulfate solution to react with 5.0 cm3 of hydrochloric acid in a 100 cm3 conical flask at 20 °C. The timer was started when the sodium thiosulfate solution was added to the acid in the flask. The timer was stopped when it was no longer possible to see the cross on the paper. The experiment was repeated at 20 °C using a 250 cm³ conical flask. Which statement is correct about the time taken for the cross to disappear when using the larger conical flask? A: The time taken will not be affected by using the larger conical flask. B: The time taken will be decreased by using the larger conical flask. C: The time taken will be increased by using the larger conical flask. D: It is impossible to predict how the time taken will be affected by using the larger conical flask. (Total 1 mark)

C

What does collision theory state?

Chemical reactions can only occur when reactants physically collide with each other with sufficient energy to react (ie have the required activation energy)

EQ: Another type of hand-warmer uses sodium thiosulfate. Sodium thiosulfate is very soluble in water at 80 °C but is much less soluble at room temperature.When a hot, concentrated solution of sodium thiosulfate is cooled it does not immediately crystallise. The sodium thiosulfate stays dissolved as a stable 'super-saturated' solution until crystallisation is triggered. Heat energy is then released when the sodium thiosulfate crystallises. This type of hand-warmer is re-usable. Suggest one environmental advantage that a sodium thiosulfate hand-warmer has over the other two types. (1 MARK)

Conserves resources / fewer disposal problems / less use of landfill / fewer waste products

Q: Suggest one possible risk to a person who uses a hand-warmer containing sodium hydroxide and hydrochloric acid.

Container splitting and releasing irritant / corrosive chemicals

What is the x axis of the Maxwell-Boltzmann curve labelled with?

Energy

Q: Hydrogen is produced by the reaction between steam and methane when the following dynamic equilibrium is established. CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) ΔH = +206 kJ mol-1 State how, and explain why, the use of a catalyst might or might not change the equilibrium yield of hydrogen, and also the amount of hydrogen produced, in a given time. (4 MARKS)

Equilibrium yield: - Unaffected or equilibrium unchanged - Rate or speed increased - Forward and backwards reactions equally or by the same amount Amount of hydrogen produced: More hydrogen produced

Draw, label and describe the Maxwell-Boltzmann distribution curve.

Look at image. - Modal can also be called Emp: most probable energy - The mean energy of particles is not at the peak of the curve - No particles have zero energy - The energy distribution should never meet the x axis because there is no maximum energy for molecules

EQ: Figure 1 shows the Maxwell−Boltzmann distribution of molecular energies in a sample of gas at a fixed temperature. On Figure 1, sketch a distribution of molecular energies for this sample of gas at a higher temperature. (2 MARKS)

Look at image. - maximum peak height is lower and displaced to the right of the original - all of the following: - starts at the origin but does not follow the original line - shows separation as soon as possible from the original line - crosses the original curve once only - similar area to original curve - an attempt has been made to draw the new curve correctly towards the energy axis above the original curve but not to touch the original curve (or axis)

Q: A gas decomposes on heating. Explain why an increase in temperature increases the rate at which this gas decomposes.

M1 an increase in the number / amount / proportion / fraction of molecules with with activation energy / sufficient energy (to react) M2 More / Higher rate / frequency of successful / effective / productive collisions in a given time / period

EQ: The following figure shows the Maxwell.Boltzmann distribution of molecular energies in a sample of gas at temperature T. The pressure of the original sample of gas is doubled at temperature T. State the effect, if any, of this change on the value of Y. Y = most probable energy

No effect / stays the same

Q: In an experiment, two moles of gas W reacted completely with solid Y to form one mole of gas Z as shown in the equation below. 2W(g) + Y(s) → Z(g) The graph below shows how the concentration of Z varied with time at constant temperature. (a) On the axes above, sketch a curve to show how the concentration of W would change with time in the same experiment. Label this curve W. (b) On the axes above, sketch a curve to show how the concentration of Z would change with time if the reaction were to be repeated under the same conditions but in the presence of a catalyst. Label this curve Z. (c) In terms of the behaviour of particles, explain why the rate of this reaction decreases with time. (6 MARKS)

See image for answers (a) and (b) (c) - fewer collisions - W used up

Q: Sodium thiosulfate solution (Na2S2O3) reacts slowly with dilute hydrochloric acid to form a precipitate. The rate of this reaction can be studied by measuring the time (t) that it takes for a small fixed amount of precipitate to form under different conditions. The fixed amount of precipitate is taken as the amount needed to obscure a cross on paper. The equation for this reaction is shown below. Na2S2O3 + 2HCl →2NaCl + S + SO2 + H2O Consider the description of the way in which this experiment is carried out. Use your understanding of the term rate of reaction to explain why it is possible to use a simplified formula 1/t as a measure of the rate of this reaction. (1 MARK)

The measured change / amount (of precipitate) / cloudiness is fixed or constant or unchanged

When a graph of concentration of reactant is plotted vs time, the _________ of the curve is the rate of reaction.

gradient

For a reaction to occur successfully, collisions must have energy _______ than or __________ to the activation energy of the reaction and the particle _____________ must be correct.

greater, equal, orientation

Q: The gas-phase reaction between hydrogen and chlorine is very slow at room temperature. H2(g) + Cl2(g) → 2HCl(g) Suggest one reason why a solid catalyst for a gas-phase reaction is often in the form of a powder. (1 MARK)

increases the surface area

Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a ___________ to the curve (at different times) and calculating the gradient of the _________.

tangent, tangent