ocr 3.2 chemistry- enthalpy changes
Calculate the bond enthalpy for the H-I bond in equilibrium 3.1, given the following information.
- (436 +151-(-9) =) (+)596 - 596/2 = =) (+)298 kJ mol-1
An important reaction in the manufacture of nitric acid is the catalytic oxidation of ammonia. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g) ΔH = - 909 kJ mol-1 (a) Low pressures and low temperatures would give the maximum equilibrium yield of NO. Explain why.
-Low pressure because more (gas) molecules on right hand side of equation OR low pressure because ∆V = positive -Low temperature because the (forward) reaction is exothermic
The enthalpy change of combustion for alcohol J can also be determined indirectly from standard enthalpy changes of formation. (i) Write an equation, including state symbols, for the chemical change that represents the standard enthalpy change of formation of the liquid alcohol J, C5H12O.
5C(s) + 6H2(g) + 1⁄2O2(g) ⎯→ C 5H12O(l)
The standard enthalpy change of formation of hexane can be determined indirectly. Calculate the standard enthalpy change of formation of hexane using the standard enthalpy changes of combustion below. substance Hc / kJ mol-1 C -394 H2 -286 C6H14 -4163
6 × -394 + 7 × -286 shown OR calculated as -4366 -4366 and -4163 added OR subtracted correct answer -4366 - (-4163) = -203
What is the value for the enthalpy change of formation of nitrogen monoxide?
66/2= +33
An incomplete equation is shown below for the chemical change that takes place to produce the standard enthalpy change of formation of hexane. Add state symbols to the equation to show each species in its standard state. 6C(.......) + 7H2(.......) C6H14(.......)
6C(s) + 7H2(g) -> C6H14(l)
What is the enthalpy change of formation, ΔHf , of N2O(g)?
820/10 = (+)82
What is the value of the enthalpy change of formation of ammonia?
92/2= 46 -46 (kJ mol-1)
State the catalyst required for Method 1.
Acid
Solid ammonium thiocyanate, NH4SCN, reacts with solid barium hydroxide, Ba(OH)2, as shown in the equation below. 2NH4SCN(s) + Ba(OH)2(s) Ba(SCN)2(s) + 2H2O(l) + 2NH3(g) A research chemist carries out an experiment to determine the enthalpy change of this reaction. In the experiment, 15.22 g of NH4SCN is reacted with a slight excess of Ba(OH)2. The reaction absorbs energy, cooling the 50.0 g of water from 21.9 °C to 10.9 °C. (i) Calculate the energy absorbed, in kJ, during this reaction.
Q = 50 × 4.2 × 11.0 2.3
A research chemist investigates the reaction between nitrogen and oxygen. She mixes nitrogen and oxygen gases in a sealed container. She then heats the container at a constant temperature for one day until the gases reach a dynamic equilibrium. (i) Explain, in terms of the rate of the forward reaction and the rate of the backward reaction, how the mixture of N2(g) and O2(g) reaches a dynamic equilibrium containing N2(g), O2(g) and NO(g).
Rate of forward reaction slows down and rate of backward reaction speeds up (Until) rate of forward reaction is the same as the rate of the backward reaction
A student mixes hydrogen and iodine at room temperature and pressure and allows the mixture to reach dynamic equilibrium. State one other feature of this dynamic equilibrium.
Rate of the forward reaction is equal to the rate of the reverse reaction
Hess' law can be used to calculate enthalpy changes of reaction. The equation for the reaction that gives the enthalpy change of formation, ΔHf of N2O(g) is as follows. N2(g) + 1⁄2O2(g) N2O(g) (i) It is not possible to measure the enthalpy change of formation of N2O(g) directly. Suggest why it is not possible.
Reaction gives NO OR reaction gives NO2 OR reaction gives a mixture of oxides OR activation energy too high OR rate of reaction is too slow
Nitrogen monoxide is an atmospheric pollutant, formed inside car engines by the reaction between 2NO(g) ΔH = +66 kJ mol-1 nitrogen and oxygen. N2(g) + O2(g) This reaction is endothermic. (a) (i) Explain the meaning of the term endothermic.
Reaction in which energy enters the system (from the surroundings)
Glucose, C6H12O6 , can be completely combusted to give carbon dioxide and water. C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l) (a) In the body, the conversion of glucose into carbon dioxide and water takes place in a number of stages catalysed by enzymes. What name is given to this oxidation process in the body?
Respiration
What is meant by the term standard enthalpy change of formation? Give the standard conditions.
The energy change that takes place when one mole of a compound is formed from its constituent elements in their standard state under standard condition. Conditions: 298 K / 25 °C AND 1 atm / 100 kPa
Calculate the activation energy, Ea, for the reverse reaction.
(+)182
Calculate the enthalpy change of combustion of pentan-1-ol. Give your answer to three significant figures.
(-)2821.5 = (-)2820 (3 SF) correct minus sign
Nitrogen forms several oxides including N2O4, N2O and NO. (a) A rocket uses the reaction between N2O4 and methylhydrazine, CH3NHNH2, equation 5.1, to release a large amount of energy. 4CH3NHNH2(l) + 5N2O4(l) 4CO2(g) + 12H2O(g) + 9N2(g) Using the enthalpy changes of formation, ΔHf , calculate the enthalpy change of reaction in equation 5.1.
(4x54) + (5x-20) = 116 (-116) (4x394) + (12x -242) = -4480 -116-4480 = -4596 answer = -4596,
Calculate the enthalpy change of reaction, ΔHr for preparing 1 mol of butan-2-ol by Method 1.
(Energy for bonds broken) = 5538 (kJ) (Energy for bonds made) = 5580 (kJ) ∆Hr = -42 (kJ mol-1)
The student wants to determine the enthalpy change of formation of calcium carbonate, CaCO3(s). Ca(s) + C(s) + 11⁄2O2(g) CaCO3(s) (i) What is meant by the term standard enthalpy change of formation? You should state the standard conditions in your answer.
(Enthalpy change) when one mole of a compound is formed from its elements at 25 oC/298 K AND 1 atmosphere/101 kPa
Standard enthalpy changes of reaction can also be determined using average bond enthalpies. (i) What is meant by the term average bond enthalpy?
(Enthalpy change) when one mole of bonds of (gaseous covalent) bonds is broken
Explain the bonding in a C=C double bond. Use the orbital overlap model.
(Sideways) overlap of p orbitals Forming a π/pi bond
Enthalpy changes of reaction can be determined by experiment or by using bond enthalpies. (a) What is meant by the term enthalpy change of reaction?
(enthalpy change when) the number of moles of reactants as specified in the (balanced) equation react together
Calculate the enthalpy change of formation of NO(g) using the data above.
- (3832 - 885) = 2947 - (2947-2677)= 270 270/3 = = (+)90
Use the ΔHc values in the table to calculate the enthalpy change of formation of C6H12O6. C(s) = -394 H2(g) = -286 C6H12O6(s) = -2801
- (394 × 6) + (-) (286 × 6)= -4080 -4080 + 1279 = 2801
A student carries out an experiment to find the enthalpy change of reaction, ΔHr for the reaction below. Na2CO3(s) + 2HCl(aq) 2NaCl(aq) + CO2(g) + H2O(l) In the experiment, 3.18 g of Na2CO3 are added to 50.0 g of 2.00 mol dm−3 HCl, an excess. The temperature of the reaction mixture increases by 5.5 °C. Calculate ΔHr in kJ mol−1.
- q calculated correctly = 1149.5(J) OR 1.1495 (kJ) - Moles: 3.18/(46+12+48)= 0.03 - answer = -38.3 (kJ mol-1)
The uses of catalysts have great economic and environmental importance. For example, catalysts are used in ammonia production and in catalytic converters. (a) Nitrogen and hydrogen react together in the production of ammonia, NH3. N2(g) + 3H2(g) 2NH3(g) Complete the enthalpy profile diagram for this reaction between nitrogen and hydrogen.
-2NH3 added as product -ΔH labelled with product below reactant AND arrow downwards -Ea labelled correctly AND above reactants
Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.
-Bond breaking absorbs energy AND bond forming releases energy -More energy released than absorbed
Outline the stages that allow nitrogen monoxide and carbon monoxide to react in a catalytic converter.
-CO and NO are adsorbed (onto surface) OR reactants are adsorbed (onto surface) -weakening of bonds OR chemical reaction OR new bonds are made OR carbon dioxide and nitrogen are made -CO2 and N2 desorbs (from the surface) OR products desorbs (from the surface)
Hydrogen iodide, HI, is a colourless gas that can be made from the reaction of hydrogen, H2 and equilibrium 3.1 iodine, I2. This reversible reaction is shown in equilibrium 3.1 below. H2(g) + I2(g) 2HI(g) ΔH = −9 kJ mol−1 Complete the enthalpy profile diagram below for the forward reaction in
-H2 and I2 on LHS 2HI on RHS -Ea is under curve to the reactant line (arrow up) - ΔH is from reactant line to product line (arrow down)
The actual conditions used in the catalytic oxidation of ammonia include 900 °C and an increase in pressure. Suggest why these conditions are a compromise.
-Increased pressure speeds up reaction -900 C increases the rate OR increased temperature speeds up reaction -Idea that high enough temperature without compromising yield OR idea that high enough pressure without compromising yield
The student heats the equilibrium mixture keeping the volume constant. Predict how the composition of the equilibrium mixture changes on heating.
-More H2 and I2 (less HI) -(equilibrium position shifts) to the left AND Forward reaction is exothermic
Aeroplane engines produce nitrogen monoxide. Describe, with the aid of equations, how nitrogen monoxide catalyses ozone depletion in the stratosphere.
-NO is not consumed OR overall reaction is O3 + O -> 2O2 -NO + O3 -> NO2 + O2 -NO2 + O -> NO + O2
Outline the use of infrared spectroscopy in identifying air pollutants such as NOx.
ANY TWO FROM: -To identify the functional groups (in pollutants) OR to identify the bonds (in pollutants) -Match spectrum to known pollutants OR each pollutant will have a different spectrum -Idea that you can measure the concentration or abundance of pollutant
Methane is a greenhouse gas. Scientists are concerned that the concentration of methane in the atmosphere is slowly increasing. (i) Explain how atmospheric methane molecules can contribute to global warming.
Absorbs IR radiation Bonds vibrate
Explain the meaning of the term activation energy.
Activation energy is the minimum amount of energy needed for the reactants to react
A student uses Method 2 to prepare 3.552 g of butan-2-ol from 2-bromobutane. The percentage yield of butan-2-ol is 80.0%. Calculate the mass of 2-bromobutane that the student uses. Give your answer to three significant figures.
Actual n(C4H9OH) produced = 3.552/74 = 0.048 (mol) theoretical n(C4H9OH) =n(C4H9Br) = 0.048 × 100/80 = 0.06 (mol) Mass of C4H9Br = 0.06 × 136.9 = 8.21 (g)
A branched-chain alcohol J is a liquid and has the molecular formula C5H12O. (a) A student does an experiment to measure the enthalpy change of combustion, ΔHc, of alcohol J. The student found that combustion of 1.54 g of alcohol J changes the temperature of 180 g of water from 22.8 °C to 75.3 °C. Calculate the amount, in mol, of alcohol J that burns. Calculate the enthalpy change of combustion, ΔHc, of alcohol J, in kJ mol−1. (3 sig figs)
Amount, n, C5H12O calculated correctly = 0.0175 (mol) q calculated correctly = 39501 (J) OR 39.5(01) (kJ) 39.501/.0175= -2257.2 ∆Hc = -2260 (kJ mol -1)
Chemical companies are using catalysts to develop processes that are more sustainable. These processes reduce costs and are less harmful to the environment. Suggest two ways in which the use of catalysts helps chemical companies to make their processes more sustainable.
Any F OUR from: -catalyst not used up in reaction -reactions take place at lower temperatures -different reactions can be used with better atom economy -catalysts or enzymes can generate specific products
Ethane reacts with bromine in the presence of ultraviolet radiation to form many organic products. (i) Two of these products are bromoethane and hydrogen bromide. Describe the mechanism of the reaction between ethane and bromine that forms bromoethane and hydrogen bromide. Include in your answer • the type of bond fission that occurs • equations for each step of the reaction • the name of each step of the reaction.
Homolytic Br2 -> 2Br Br + C2H6 -> HBr + C2H5 C2H5 + Br2 -> C2H5Br + Br Br + C2H5 -> C2H5Br initiation propagation termination
Calculate the enthalpy change of reaction. Give your answer to three significant figures.
IF answer = -134 (kJ) award 3 marks 2.68 ÷ 0.02 Correctly calculates the value to 3 sig figs
The enthalpy change of formation, ΔHf , of glucose, C6H12O6, cannot be determined directly. The equation for this enthalpy change is shown below. 6C(s) + 6H2(g) + 3O2(g) C6H12O6(s) Suggest why the enthalpy change of formation of C6H12O6 cannot be determined directly.
Would make carbon dioxide and water instead OR activation energy (too) high OR rate is (too) slow OR do not react together
Calculate the amount, in moles, of magnesium used by the student.
amount = 0.02(00) (mol)
Propane can be cracked to make ethene. Using the average bond enthalpies in Table 3.1, calculate the enthalpy change of this reaction.
bonds broken = (+)4010 AND bonds formed = (-)3931 Overall enthalpy change = +79
Nitrogen monoxide is a radical. What does this tell you about a molecule of nitrogen monoxide?
has an unpaired electron
Suggest how this energy can be used to reduce the cost of making NO.
Used to heat the incoming gases
Suggest equations for two termination steps.
H + H ->H2 Cl + Cl -> Cl2 H + Cl -> HCl
The research chemist repeats the experiment at the same temperature using the same initial amounts of N2(g) and O2(g). This time she carries out the experiment at a much higher pressure. Suggest why • much less time is needed to reach dynamic equilibrium • the composition of the equilibrium mixture is the same as in the first experiment.
-Reaction is faster -Increasing pressure mean more particles per unit volume OR increasing pressure gives more crowded particles OR increasing pressure gives more concentrated (particles) -So more collisions per second OR higher collision frequency OR collisions more often -(Changes of pressure) do not change the (position of) equilibrium -Both sides of equation have same number of moles (of gas)
The branched-chain alcohol J, C5H12O, was heated under reflux with excess H2SO4 / K2Cr2O7 to form an organic compound K with the infrared spectrum below. -Determine the structures for the branched-chain alcohol J and compound K. • Write an equation for the reaction of J when heated under reflux with excess H2SO4 / K2Cr2O7 to form K. Use [O] to represent the oxidising agent.
-There is evidence of infrared absorption at 1720 (cm-1), indicating the presence of a carbonyl group. -There's no peak between 2500-3300 cm-1, so carboxylic acid OH absorption is indicated. -so J is a secondary alcohol - structure of J: OH H I I H3C-------------------C-------------C---------------CH3 I I H CH3 - K is structure of a carbonyl compound that could be obtained from alcohol J - CH3CHOHCH(CH3)2 + [O] ⎯→ CH3COCH(CH3)2 + H2O
The reaction is repeated at a higher pressure. Describe and explain what happens to the rate of the reaction between H2(g) and Cl 2(g).
-more concentrated (particles) OR more particles per (unit) volume -more collisions per second OR more frequent collisions
The reaction is repeated again. This time the temperature is decreased. Describe and explain, by drawing appropriately labelled Boltzmann distributions, what happens to the rate of reaction between H2(g) and Cl 2(g).
-y-axis label is '(number of) molecules' AND x-axis label is 'energy' AND one correct curve -Correct curve for lower temperature (labelled) -Activation energy does not change OR clearly labelled on diagram, e.g. Ea OR E -Fewer molecules have energy above activation energy OR fewer molecules have enough energy to react -So fewer successful collisions
A student carries out an experiment to determine the enthalpy change of combustion of glucose. In the experiment, 0.831 g of glucose is burned. The energy released is used to heat 100 cm3 of water from 23.7 °C to 41.0 °C. (i) Calculate the energy released, in kJ, during combustion of 0.831 g glucose.
100 × 4.18 × 17.3 7.23 (kJ)
The data below can be used to calculate the enthalpy change of formation, ΔHf of N2O(g). C(s) + N2O(g) -> CO(g) + N2(g) = -193 C(s) + 1⁄2O2(g) -> CO(g) = -111 Calculate ΔHf for N2O(g).
193-111= +82 (kJ mol-1)
Write the equation, including state symbols, for the reaction that represents the standard enthalpy change of formation of NH4ClO4(s).
1⁄2N2(g) + 2H2(g) + 1⁄2Cl2(g) + 2O2(g) -> NH4ClO4(s)
The reaction between nitrogen and hydrogen can be catalysed. Suggest a possible value for the activation energy of the catalysed forward reaction.
200 kJmol-1
What is the value of the activation energy for the uncatalysed reverse reaction (the decomposition of ammonia into nitrogen and hydrogen)?
250+92= +342 (kJ mol-1)
In a catalytic converter, nitrogen monoxide reacts with carbon monoxide. (i) Write the equation for this reaction.
2CO + 2NO -> 2CO2 + N2
Oxides of nitrogen, NOx, are atmospheric pollutants. (i) Nitrogen monoxide reacts with oxygen to form NO2. Write an equation for the formation of NO2 from nitrogen monoxide and oxygen.
2NO + O2 -> 2NO2
Complete the enthalpy profile diagram for the reaction between nitrogen and oxygen. On your diagram • add the product • label the activation energy as Ea • label the enthalpy change as ΔH.
2NO added for product ΔH labelled with product above reactant AND arrow upwards Ea labelled correctly AND above products
When the reverse reaction takes place hydrogen iodide, HI, decomposes to form iodine and hydrogen. Calculate the enthalpy change when 336 dm3 of hydrogen iodide, measured at room temperature and pressure, decomposes. Include the sign for enthalpy change in your answer.
336/24= 14 moles of HI 14/2 = 7 7x9= 63 Enthalpy Change = +63 kJ
Calculate the activation energy, Ea, for the reverse reaction.
447-164= (+)283
The equation for the complete combustion of alcohol J is shown below. C5H12O(l) + 71⁄2O2(g) 5CO2(g) + 6H2O(l) Calculate the enthalpy change of combustion, ΔHc, of alcohol J from the information given above.
5 × -394 + 6 × -286 - -366 = -3320 (kJ mol-1)
A factory makes 2.50 × 10^5 mol of NO a day. (i) How much energy is released every day?
5.68 × 10^7 / 5.7 × 10^7
A student carried out an experiment to determine the enthalpy change of combustion of pentan-1-ol, CH3(CH2) 4OH. In the experiment, 1.76 g of pentan-1-ol was burnt. The energy was used to heat 250 cm3 of water from 24.0 °C to 78.0 °C. (i) Calculate the energy released, in kJ, during combustion of 1.76 g pentan-1-ol.
56.430 (kJ) 78-24= 54 54x4.18x250= 56430 56430/1000= 56.43
Explain why the actual conditions used in the chemical industry might be different from those in (a) above.
Too expensive to use a high pressure Too slow to use a low temperature
Explain why it is important to establish international cooperation to reduce pollution levels.
Any one from: Idea that pollution travels (across country) borders OR idea that all countries contribute towards pollution OR Cooperation means that scientists can share ideas OR scientists can warn governments of risk OR world-wide legislation can be introduced OR allows monitoring of pollution in different countries OR richer countries can help poorer countries introduce pollution controls OR One country cannot control pollution unless all countries do
One way that scientists hope to minimise global warming is by developing Carbon Capture and Storage, CCS, techniques. Describe two of these CCS techniques.
Any two from: (liquid) injected deep into the oceans Stored in (old) geological formations OR stored underground in rocks OR stored in (old) mines OR stored in (old) oil wells Stored by reaction with metal oxides OR reaction to form (solid) carbonates OR stored as a carbonate OR equation to show formation of metal carbonate
Give two reasons why there are many organic products of the reaction between bromine and ethane.
Any two from: -More than one C-H bond can be substituted OR multi- substitution can occur OR more than one substitution can happen -Lots of termination steps -termination steps can give products that will also react with (bromine) radicals
Scientists monitor pollutant gases in the atmosphere. (i) State two modern analytical techniques that scientists can use to monitor environmental pollution.
Any two from: IR (spectroscopy) Mass spectrometry UV (spectroscopy) NMR GC
The actual value for the enthalpy change of this reaction is +81 kJ mol-1. Suggest a reason why the actual value for the enthalpy change of this reaction is different from the calculated value.
Bond enthalpies may not be the same as the average bond enthalpy OR The idea that bonds have different strengths in different environments
Many organisms use the aerobic respiration of glucose, C6H12O6 to release useful energy. (a) The overall equation for aerobic respiration is the same as for the complete combustion of C6H12O6. (i) Write the equation for the aerobic respiration of C6H12O6.
C6H12O6 + 6O2 -> 6CO2 + 6H2O
Catalysts are increasingly being used in chemical processes. A catalyst speeds up a reaction without being consumed by the overall reaction. A catalyst provides an alternative reaction route with a lower activation energy. (i) Chlorine radicals, Cl • , catalyse some reactions. Choose a reaction that you have studied that is catalysed by chlorine radicals. Write down an equation for the overall reaction and show how chlorine radicals are not consumed by the overall reaction.
Cl + O3 -> ClO + O2 ClO + O -> Cl + O2 overall: O3 + O -> 2O2
The mechanism of the reaction between H2(g) and Cl 2(g) involves initiation, propagation and termination. (i) The initiation step is the homolytic fission of the covalent bond in a chlorine molecule. Write an equation to show this homolytic fission.
Cl2 -> 2Cl
Methane and ethane are important fuels. (a) Methane could be manufactured by the reaction between carbon dioxide and hydrogen. CO2(g) + 4H2(g) CH4(g) + 2H2O(g) Using the table of bond enthalpies, calculate the enthalpy change of reaction for this manufacture of methane.
Energy associated with bond breaking = 3354 OR (2 × 805) + (4 × 436) Energy associated with bond making = 3516 OR (4 × 415) + (4 × 464) Enthalpy change = -162 (kJmol-1)
Using the following data and enthalpy cycle, • fill in the boxes on the enthalpy cycle with the correct enthalpy change values • calculate the enthalpy change of formation, ΔHf , of CaCO3(s).
From left to right -16, -28, -39, -5 ∆Hf -792 (kJ mol-1)
The alcohol J is soluble in water. Explain why alcohol J is soluble in water. Use a labelled diagram to support your answer. Include relevant dipoles and lone pairs.
H H I I H3C------------C------------C---------O I I \ CH3 CH3 H -O bonds to H of a water molecule. O is slightlet neg, H is slightly pos -O has one lone pair
Complete the following equations which show the propagation steps. Cl + H2 ................... + ................... H + Cl2 ................... + ...................
HCl + H HCl + Cl
Enthalpy changes can be determined directly or indirectly. (a) A student investigates the reaction between magnesium and dilute hydrochloric acid. Mg(s) + 2HCl(aq) MgCl 2(aq) + H2(g) The student determines the enthalpy change for this reaction. In her experiment, she reacts 0.486 g of magnesium with 50.0 cm3 of 2.00 mol dm-3 HCl(aq). The HCl(aq) is in excess. The temperature of the solution changes from 19.2 °C to 32.0 °C. (i) Calculate the energy released, in kJ, during this reaction.
IF answer = 2.68 (kJ) award 2 marks q = mc∆T OR = 50.0 × 4.18 × 12.8 = 2.68 (kJ)
Hydrogen and chlorine are reacted together to form hydrogen chloride. H2(g) + Cl 2(g) 2HCl(g) ΔH = -184 kJ mol-1 (a) Calculate the bond enthalpy for the H-Cl bond using the information in the table below. bond bond enthalpy/ kJ mol-1 H-H +436 Cl-Cl +243
IF answer = 431.5 (kJ mol-1) award 2 marks Energy required to break bonds = (+)679 kJ so bond enthalpy = (+)431.5
Butan-2-ol can be prepared using two different methods. Comment on the atom economy of each method, giving your reasons.
In method one, there is only one product and no waste, the atom economy is 100%. In method two, there are two products and NaBr is a waste product, so the atom economy is less than 100%.
Under certain conditions nitrogen reacts with oxygen to make N2O. 2N2(g) + O2(g) 2N2O(g) equation 5.2 Calculate the enthalpy change when 240 dm3 of N2O(g), measured at room temperature and pressure, is formed from N2 and O2.
Moles of N2O: 240/24= 10 moles 164x5= 820 1640-820= 820 answer = +820
Calculate the amount, in moles, of glucose that is burned.
Mr = 180 amount = 4.62 × 10^-3 (mol)
Calculate the amount, in moles, of pentan-1-ol that was burnt.
Mr [CH3(CH2)4OH] = 88.0 n = 0.02 mol
In the stratosphere, NO catalyses the breakdown of ozone. Write two equations to show how NO catalyses this breakdown.
NO + O3 -> NO2 + O2 NO2 + O -> NO + O2
Predict and explain what effect, if any, an increase in the pressure would have on the position of the equilibrium.
No effect b/c same number of (gaseous) moles on both sides
The reaction between nitrogen and oxygen in a car engine does not reach a dynamic equilibrium. Suggest why not.
Not a closed system
Describe and explain, using equations, how the concentration of ozone in the stratosphere is maintained.
O3 ->O2 + O AND O + O2 -> O3 rate of ozone decomposition (almost) equals rate of ozone formation
In the stratosphere, nitrogen monoxide, NO, is linked with ozone depletion. Complete the equations below that describe how NO contributes to ozone depletion. step 1 NO + O3 ........... + ........... step 2 NO2 + ........... NO + ........... overall ........... + ........... 2O2
Step 1 NO + O3 -> NO2 + O2 Step 2 NO2 + O -> NO + O2 overall O3 + O -> 2O2
Average bond enthalpies can be used to calculate enthalpy changes. (i) What is meant by the term average bond enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule
Enthalpy changes of combustion, Hc , are amongst the easiest enthalpy changes to determine directly. (a) Define the term enthalpy change of combustion.
The enthalpy change for the complete combustion of 1 mol (of a substance)
In the chemical industry methanol, CH3OH, is synthesised by reacting together carbon monoxide and hydrogen in the presence of copper, zinc oxide and alumina which act as a catalyst. This is a reversible reaction. CO(g) + 2H2(g) CH3OH(g) H = -91 kJ mol-1 (a) High pressures and low temperatures would give the maximum equilibrium yield of methanol. Explain why.
high pressure as fewer moles (of gas) on right-hand side OR high pressure as volume of products less than that of reactants low temperature as (forward) reaction is exothermic
The calculated value of ΔHc from this experiment is different from the value obtained from data books. Apart from heat loss, suggest two reasons for the difference.
incomplete combustion non-standard conditions evaporation of alcohol/water specific heat capacity of beaker/apparatus
It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why.
many different hydrocarbons would form OR activation energy too high OR reaction too slow OR they don't react together
Calculate the amount, in moles, of NH4SCN used by the research chemist.
moles = 0.200
The standard enthalpy change of formation of hexane can be defined as: The enthalpy change when 1 mol of hexane is formed from its constituent elements in their standard states under standard conditions. Hexane melts at -95 °C and boils at 69 °C. (i) What are standard conditions?
pressure: 100 kPa OR 101 kPa AND temperature: 298 K OR 25 °C
What is meant by the term enthalpy change of combustion?
when one mole of a substance is completely combusted OR burns in excess oxygen
Using the axes below, sketch an enthalpy profile diagram for an exothermic reaction to show how a catalyst provides an alternative reaction route with a lower activation energy. Include on your diagram labels for: • enthalpy change, H; • activation energy for the catalysed route, Ec; • activation energy for the uncatalysed route, Ea.
ΔH shown and products below reactants Ea shown Ec shown < Ea
Suggest why the average bond enthalpy of a C=C bond is not twice the bond enthalpy of a C-C bond.
π bond is weaker (than the σ bond) OR σ bond is stronger (than the π bond)
Calculate the enthalpy change of reaction. Include the sign in your answer. Giv ye our answer to two significant figures.
∆Hr = 2 × (2.3 ÷ 0.200) 23 + sign
