chem quiz
Hydrogen peroxide decomposes to water and oxygen. 2H2O2(aq) + 2H2O(1) + O2(g) The concentration of a solution of hydrogen peroxide was measured over time and the data plotted to give the following (a) Deduce the average rate of the reaction for the first ten seconds. (b) Calculate the actual rate of the reaction at 20 seconds.
(a) At time t=0 s, [H2O2(aq)] = 1.0 mol dm-3 At time t = 10 s, (H2O2(aq)] = 0.75 mol dm-3 Average rate = (1.0 - 0.75) = 10 = 0.025 mol dm-3 s-1 (b) Take the tangent at time t = 20 s Rate = 0.30 = 18.0 = 0.017 mol dm-3 s-1 (Note that the rate is given as a positive value as it is for the decrease in concentration of hydrogen peroxide)
Factors that affect the rate of a chemical reaction include particle size, concentration of reactants and the temperature of the reaction. (i) Define the term rate of a chemical reaction. (ii) List the three characteristic properties of reactant particles which affect the rate of reaction as described by the collision theory.
(i) Increase in concentration of product(s) per unit of time. on decrease in concentration of reactants per unit of time. (ii) 1) Frequency of collisions 2) Kinetic energy of particles 3) Collision geometry (orientation)
The graph below shows how the volume of carbon dioxide formed varies with time when a hydrochloric acid solution is added to excess calcium carbonate in a flask. (i) Explain the shape of the curve. (ii) The experiment is repeated using a sample of hydrochloric acid with double the volume, but half the concentration of the original acid. Draw a second line on the graph to represent this change. Explain why the shape of the curve is different.
(i) Initially the concentration of HCl is the highest and the rate of production of CO2 at a maximum. As HCl becomes more dilute the production of CO2 slows and the curve flattens. (ii) The lower concentration of HCl will result in a slower rate of reaction, but will produce the same volume of CO2 as the total number of moles of HCl is unchanged.
The Haber process enables the large-scale production of ammonia needed to make fertilizers. The equation for the Haber process is given below. N2(g) + 3H2(g) = 2NH3(g) The percentage of ammonia in the equilibrium mixture varies with temperature. (i) Use the graph to deduce whether the forward reaction is exothermic or endothermic and explain your choice. (ii) State and explain the effect of increasing the pressure on the yield of ammonia. (iii) Explain the effect of increasing the temperature on the rate of reaction.
(i) N2(g) + 3H2(g) = 2NH3(g) + heat Exothermic; as the temperature increases the rate of production of NH3, decreases. (ii) Increasing pressure favors the reaction with fewer moles of gaseous products, so the reaction shifts to the right. (iii) Increasing temperature both increases the number of collisions and the number of colliding molecules with sufficient activation energy (Ea). * per unit of time
A solution of hydrogen peroxide, H2O2, is added to a solution of sodium iodide, NaI, acidified with hydrochloric acid, HCl. The yellow colour of the iodine, I2, can be used to determine the rate of reaction. H2O2(aq) + 2Nal(aq) + 2HCl(aq) → 2NaCl(aq) + I2(aq) + 2H2O(l) The experiment is repeated with some changes to the reaction conditions. For each of the changes that follow, predict, stating a reason, its effect on the rate of reaction. (i) The concentration of H2O2 is increased at constant temperature. (ii) The solution of NaI is prepared from a fine powder instead of large crystals. (b) Explain why the rate of a reaction increases when the temperature of the system increases.
(i) Rate increases due to a greater number of H2O2 molecules, resulting in more collisions per unit of time. (ii) No effect; solid NaI is not reacting, only an aqueous solution of NaI. (b) Increasing temperature both increases the number of collisions and the number of colliding molecules with sufficient activation energy (Ea). * per unit of time
12. Which changes increase the rate of the reaction below? C4H10(g) + Cl2(g) - CAHCI(I) + HCl(g) I. Increase of pressure II. Increase of temperature III. Removal of HCl(g) A. I and II only B. I and III only C. II and III only D. I, II and III
A
8. The Haber process uses an iron catalyst to convert hydrogen gas, H2(g), and nitrogen gas, N2(g), to ammonia gas, NH3(g) 3H2(g) + N2(g) >< 2NH3(g) Which statements are correct for this equilibrium system? I. The iron catalyst increases the rates of the forward and reverse reactions equally. II. The iron catalyst does not affect the value of the equilibrium constant, Kc. III. The iron catalyst increases the yield of ammonia gas, NH3(g). A. I and II only B. I and III only C. II and III only D. I, II and III
A
Consider the reaction between gaseous iodine and gaseous hydrogen. 12(g) + H2(g) = 2HI(g) AH = -9 kJ Why do some collisions between iodine and hydrogen not result in the formation of the product? A. The l2 and H2 molecules do not have sufficient energy. B. The system is in equilibrium. C. The temperature of the system is too high. D. The activation energy for this reaction is very low
A
Which quantities in the enthalpy level diagram are altered by the use of a catalyst? A. I and II only B. I and III only C. II and III only D. I, II and I
A
Which statement is true about using sulfuric acid as a catalyst in the following reaction? CH3-CO-CH3(aq) + I2(aq) --> CH3-CO-CH2-I(aq) + HI(aq) I. The catalyst increases the rate of reaction. II. The catalyst lowers the activation energy for the reaction. III. The catalyst has been consumed at the end of the chemical reaction. A. I and II only B. I and III only C. II and III only D. I, II and III
A
On the axes below sketch two Maxwell-Boltzmann energy distribution curves for the same sample of gas, one at a temperature T and another at a higher temperature T'. Label both axes. Explain why raising the temperature increases the rate of a chemical reaction.
At higher temperatures, more particles will have sufficient activation energy for a reaction to occur.
Curve X on the graph below shows the volume of oxygen formed during the catalytic decomposition of a 1.0 mol dm' solution of hydrogen peroxide. 2H2O2(aq) + O2(g) + 2H2O(1) Which change would produce the curve Y? A. Adding water B. Adding some 0.1 mol dm-3 hydrogen peroxide solution C. Using a different catalyst D. Lowering the temperature
B
Hydrochloric acid is reacted with large pieces of calcium carbonate, the reaction is then repeated using calcium carbonate powder. How does this change affect the activation energy and the collision frequency? A. Activation energy increases, Collision frequency increases В. Activation energy stays constant, Collision frequency increases C. Activation energy increases, Collision frequency stays constant D. Activation energy stays constant, Collision frequency stays constant
B
In the reaction between marble (calcium carbonate) and hydrochloric acid, which set of conditions would give the highest rate of reaction? Caco (s) + 2HCl - Cacizlaq) + CO2(g) + H20(0) A) marble chips and 1.0 mol dmCI B) marble powder and 1.0 mol dm-3 HC C) marble chips and 0.1 mol dm-3 HCI D) marble powder and 0.1 mo dm 3 HCI
B
Which of the following statements is correct? A) A catalyst increases the rate of the forward reaction only. B) A catalyst increases the rate of the forward and backward reactions. C) A catalyst increases the yield of product formed, D) A catalyst increases the activation energy of a reaction.
B
Which statements are correct for the effects of catalyst and temperature on the rate of reaction? Adding a catalyst Increasing the temperature A) Adding a catalyst collision frequency increases, Increasing the temperature collision frequency increases B) Adding a catalyst activation energy decreases, Increasing the temperature collision frequency increases C) Adding a catalyst collision frequency increases, Increasing the temperature activation energy increases D) Adding a catalyst activation energy increases, Increasing the temperature activation energy decreases
B
Which statements describe the action of a catalyst? I. It does not alter the △H for a reaction. II. It increases the E, for the reaction. III. It alters the mechanism (pathway) of a reaction. A. I and II only C. II and III only B) I and III only D. I, II and III
B
7. Powdered manganese(IV) oxide, MnO2(s), increases the rate of the decomposition reaction of hydrogen per- oxide, H2O2(aq). Which statements about MnO2 are correct? I. The rate is independent of the particle size of MnO2. II. MnO2 provides an alternative reaction pathway for the decomposition with a lower activation energy III. All the MnO2 is present after the decomposition of the hydrogen peroxide is complete. A I and II only B I and III only C. II and III only D. I, II and III
C
Under which conditions will the reaction between 1.0 g calcium carbonate and excess hydrochloric acid be the fastest? Assume that all reactions are carried out at the same temperature. A. One large piece of calcium carbonate and 2 mol dm^-3 hydrochloric acid B. One large piece of calcium carbonate and 1 mol dm^-3 hydrochloric acid C. Powdered calcium carbonate and 2 mol dm^-3 hydrochloric acid D. Powdered calcium carbonate and 1 mol dm^-3 hydrochloric acid
C
What is the function of iron in the Haber process? A. It shifts the position of equilibrium towards the products. B. It decreases the rate of the reaction. C. It provides an alternative reaction pathway with a lower activation energy. D. It reduces the enthalpy change of the reaction.
C
Which unit could be used for the rate of a chemical reaction? A. mol B. mol dm-3 C. mol dm-3s-1 D. dm
C
At 25 °C, 200 cm^3 of 1.0 mol dm^-3 nitric acid is added to 5.0 g of magnesium powder. If the experiment is repeated using the same mass of magnesium powder, which conditions will result in the same initial reaction rate?
D
Sodium carbonate and hydrochloric acid react according to the equation below. Na2CO3(s) + 2HCl(aq) - CO2(g) + 2NaCl(aq) + H2O(1) Which conditions will produce the fastest initial rate with 2.0 g of powdered sodium carbonate? A. 100 cm^-3 of 1.0 mol dm^-3 hydrochloric acid at 323 K B. 50 cm^-3 of 2.0 mol dm^-3 hydrochloric acid at 323 K C. 100 cm^-3 of 1.0 mol dm^-3 hydrochloric acid at 348 K D 50 cm^-3 of 2.0 mol dm^-3 hydrochloric acid at 348 K
D
The formation of nitric acid, HNO3(aq), from nitrogen dioxide, NO2(g), is exothermic and is a reversible reaction. 4NO2(g) + O2(g) + 2H2O(l) >< 4HNO3(aq) What is the effect of a catalyst on this reaction? A. It increases the yield of nitric acid. B. It increases the rate of the forward reaction only. C. It increases the equilibrium constant. D. It has no effect on the equilibrium position.
D
What is the best definition of rate of reaction? A. The time it takes to use up all the reactants B. The rate at which all the reactants are used up C. The time it takes for one of the reactants to be used up D. The increase in concentration of a product per unit time
D
Which factors can affect reaction rate? I. The state of the reactants II. The frequency of the collisions between particles III. The average kinetic energy of the particles A. I and II only B. I and III only C. II and III only D I, II and III
D
Which of the following can increase the rate of a chemical reaction? I. Increasing the temperature II. Adding a catalyst III. Increasing the concentration of reactants A. I and II only B. I and III only C. II and III only D I, II and III
D
A sugar cube cannot be ignited with a match but a sugar cube coated in ashes will ignite Suggest a reason for this observation.
The ashes must contain a catalyst that speeds up the reaction between sugar and oxygen. (Deduced from the fact that all other factors that affect reaction rate can be ruled out.)
The rate of a chemical reaction usually decreases over time. Suggest two possible reasons why the rate of a chemical reaction might increase after an initial slower rate even though nothing else is added and the conditions do not change.
The reaction is exothermic so the temperature increases as the reaction proceeds. One of the products acts as a catalyst for the reaction. (This is known as an autocatalytic reaction)
