CHEMICAL CHANGE - Doc Scientia...CHEMICAL CHANGE 274 Doc Scientia - Chemistry Textbook and Workbook Book 1 - Grade 12 A small balloon must be inflated with hydrogen gas. The balloon

CHEMICAL CHANGE

274 Doc Scientia - Chemistry Textbook and Workbook Book 1 - Grade 12

A small balloon must be inflated with hydrogen gas. The balloon is attached to the top of one of the flasks into which zinc was added. To inflate the balloon as quickly as possible, which ONE of the combinations will be the most suitable?

(Gr. 12: September 2018, Gauteng) A. P and R B. P and S C. Q and P D. Q and S4. The graph represents the change in potential energy against time for the following reaction: A2(g) + 2B2(g) 2AB2(g) Which ONE of the following statements is correct? (Gr. 12: September 2018, KwaZulu-Natal) A. The forward reaction is exothermic. B. The heat of reaction for the forward reaction is Q–P. C. The activation energy for the forward reaction is R–Q. D. The activation energy for the reverse reaction is R–P.5. Consider the chemical reaction represented by the balanced equation below:

CaCO3(s) + 2HCℓ(aq) CaCℓ2(s) + CO2(g) + H2O(ℓ) Which ONE of the following changes will increase the rate of CO2(g) production?

(Gr. 12: September 2018, Limpopo) A. Increase in pressure B. Increase in the mass of CaCO3 C. Increase in the volume of HCℓ(aq) D. Increase in the concentration of HCℓ(aq)6. The Maxwell-Boltzmann distribution curves below show the number of particles as a function of their kinetic energy for a reaction, at four different temperatures. The minimum kinetic energy needed for effective collisions to take place, is represented by E.

Which ONE of the curves represents the reaction that takes place at the highest temperature?(Gr. 12: September 2018, Mpumalanga)

A. I B. II C. III D. IV

course of reaction

EP

P

Q

R

num

ber o

f pa

rtic

les

kinetic energy E

I

IIIII

IV

RATE OF REACTIONS Exam Questions 1

275Doc Scientia - Chemistry Textbook and Workbook Book 1 - Grade 12

7. The following graphs A and B represent the Maxwell-Boltzmann distribution curve for a reactant at two different temperatures: (Gr. 12: September 2018, Northern Cape) Consider the following statements which refer to the graph: I: Graph A represent a lower temperature and it has a larger percentage of molecules with enough kinetic energy to react. II: Graph A represent a lower temperature but graph B has a larger percentage of molecules with enough kinetic energy to react. III: The reaction rate for molecules represented by graph A will be lower than that is represented by graph B. Which of the above statements are TRUE? A. I and III B. I and II C. II and III D. I, II and III8. The following graph represents the change in potential energy for the reaction:

CO(g) + NO2(g) CO2(g) + NO(g) (Gr. 12: September 2018, Northern Cape) The activation energy, and the heat of reaction for the reverse reaction is:

Activation energy (kJ⋅mol-1)

Heat of reaction (kJ⋅mol-1)

A. 35 50

B. 50 -50

C. 85 50

D. -50 -50

9. The sketch graph represents the change in potential energy for an hypothetical chemical reaction: K → L (Gr. 12: September 2018, North West) ΔH for the forward reaction is given by: A. B–C. B. C–D. C. D–B. D. D–C.10. A small piece of Mg is added to 5 cm3 of a dilute hydrochloric acid solution in a test tube. Which one of the following changes will affect the rate at which hydrogen gas is produced in this reaction?

(Gr. 12: September 2018, North West) A. Using 10 cm3 of the same dilute hydrochloric solution B. Increasing the concentration of the hydrochloric solution C. Decreasing the concentration of the hydrochloric solution D. Decreasing the temperature of the hydrochloric solution

frac

tion

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mol

ecul

e w

ith

a sp

ecif

ic k

inet

ic e

nerg

y

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EA

A

B

reaction coordinate

E P (kJ⋅

mol

-1)

4030

0

-50

B

C

D

K

L

EP

course of reaction

CHEMICAL CHANGE


11. The potential energy graph for a hypothetical chemical reaction is shown alongside. (Gr. 12: September 2018, Cape Winelands PLC) What type of reaction is taking place and what are the correct methods to calculate ΔH and EA?

Type of reaction ΔH EA

A. Exothermic b–a c–b

B. Endothermic b–a c–a

C. Endothermic a–b a–c

D. Exothermic a–b b–c

12. Which one of the reaction rate versus time graphs below best represents the reaction between magnesium and excess dilute hydrochloric acid? (Gr. 12: Feb./Mar. 2018, DBE)

A. B.

C. D.

13. Study the hypothetical reaction below: 2P(g) + 3Q(g) → 4R(g) + 2Z(g)

(Gr. 12: June 2018, DBE) The rate of the reaction in terms of the number of moles of substance P used up, is 1 × 10-3 mol⋅dm-3⋅s-1. What is the rate (in mol⋅dm-3⋅s-1) at which product R is formed? A. 1 × 10-3 B. 4(1 × 10-3) 1 × 10-3

C.

2 D. 2(1 × 10-3)

14. One mol of zinc reacts with EXCESS hydrochloric acid of concentration 1 mol⋅dm-3 at 25°C according to the balanced equation:

Zn(s) + 2HCℓ(aq) → ZnCℓ2(aq) + H2(g)(Gr. 12: June 2018, Eastern Cape)

Which one of the following changes will increase the average kinetic energy of reacting particles? A. Add more zinc. B. Increase temperature. C. Add a suitable catalyst. D. Increase concentration of HCℓ.

course of reactionpote

ntia

l ene

rgy

(kJ⋅

mol

-1)

products

reactants

c

b

a

reac

tion

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time

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rate

time

reac

tion

rate

time

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time



15. The rate of a chemical reaction is increased when the surface area of the reactant is increased. This change in the rate is due to the: (Gr. 12: September 2017, Gauteng) A. increase in the density of the reactant particles. B. increase in the concentration of the reactant. C. increase in exposure of more reactant particles to a possible collision. D. alteration of the electrical conductivity of the reactant particles.16. Consider the following results of experiments:

Experiment 1 Experiment 2Reactants Powdered Cu and HCℓ Powdered Cu and HNO3

Temperature 20°C 10°C

Concentration of acid 0,6 mol⋅dm-3 of HCℓ 0,4 mol⋅dm-3 of HNO3

Rate Low High

(Gr. 12: September 2017, Gauteng) Which one of the following factors would account for the lower rate in Experiment 1? A. Temperature B. Concentration of acid C. Surface area of Cu D. Nature of reactants17. 50 cm3 of 0,1 mol⋅dm-3 solution of hydrochloric acid is poured on to 5 g of magnesium powder inside a small test tube at room temperature. Which one of the following factors will increase the rate of this reaction? (Gr. 12: September 2017, KwaZulu-Natal) A. Using 100 cm3 of 0,1 mol⋅dm-3 solution of hydrochloric acid at room temperature. B. Using 5 g of magnesium ribbon. C. Decreasing the temperature of the acid solution to 10°C. D. Increasing the temperature of the acid solution to 80°C.18. Graph B below represents the Maxwell-Boltzmann energy distribution curve for a reaction mixture at a temperature of 300°C. Area X represents the number of molecules in the mixture that have enough kinetic energy for the reaction to take place.

(Gr. 12: September 2017, KwaZulu-Natal)

The temperature of the mixture is now increased to 500°C. Which one of graphs A to D represents the distribution curve of the mixture at this higher temperature? A. A B. B C. C D. D

Y

A

B

C

D

kinetic energy

num

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ules

CHEMICAL CHANGE


19. Given the reaction: N2(g) + O2(g) → 2NO(g) The activation energy for the forward reaction is 181,5 kJ and for the reverse reaction is 192,8 J. What is dthe heat of reaction for the forward reaction? (Gr. 12: June 2017, Mpumalanga) A. -374,3 kJ B. +374,3 kJ C. -11,3 kJ D. +11,3 kJ20. Hydrogen gas is produced in the reaction between hydrochloric acid and magnesium. Which ONE of the following graphs of volume versus time best represents the formation of hydrogen gas until the reactants are used up? (Gr. 12: September 2017, Free State) A. B.

C. D.

Contextual questions

1. Two molecules that can react are sealed in the same container. Name 2 conditions that are required for the molecules to react during collisions with each other.

2. Explain the role of reactivity that reactants play during the determination of the rate of the reaction.

3. Use the collision theory and explain why icing sugar heated over a hot flame will caramelise faster than normal sugar heated over the same flame.

4. Use the collision theory and explain why the increase in concentration of a reactant will increase the rate of the reaction.

5. Use the collision theory and explain why the increase in temperature of the reaction mixture will increase the reaction rate. Refer in your answer to the Maxwell-Boltzmann distribution of the molecules.

6. Explain, by drawing a graph, why the addition of a catalyst to the reaction mixture will accelerate an exothermic reaction.

time (s)

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CHEMICAL CHANGE


7 Study the accompanying diagram:7.1 How much energy does the reactants have?7.2 How much energy does the products have?7.3 What is the activation energy for the forward reaction?7.4 What is the activation energy for the reverse reaction?7.5 What is the value of the change of enthalpy of the reaction?7.6 Is the reaction endothermic or exothermic?

8 Magnesium reacts with an excess of hydrochloric acid to form magnesium chloride and hydrogen gas. The accompanying setup is used to measure the rate at which hydrogen gas is collected:

Study the accompanying graph and answer the following questions:

8.1 During which interval of 5 s is the reaction rate fastest?8.2 At which point in time did the reaction stop?8.3 What volume of gas is produced in total?8.4 Calculate the instantaneous reaction rate for the reaction at 20 seconds.8.5 Sketch on the same set of axes the different graph lines that would represent the rate of reaction if the experiment is repeated with the following changes:8.5.1 The temperature is increased (mark this line as HT).8.5.2 The concentration of the acid is reduced (mark this graph as LC).8.6 Calculate the average rate of the reaction for the first 45 s in cm3⋅s-1.8.7 Give two reasons why this reaction cannot reach equilibrium.8.8 If 250 mℓ hydrochloric acid with an initial concentration 2 mol⋅dm-3 is used, calculate the mol of hydrochloric acid in excess at STP conditions.8.9 What mass of magnesium was initially sealed in the container?

E P (kJ⋅

mol

-1)

reactants

products

20

30

40

B

50

reaction progress

gas syringe

stop watch

magnesium ribbon

hydrochloric acid

side-arm flask

time (seconds)

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as (c

m3 )

0 10 20 30 40 50 60 70 80 90 100

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CHEMICAL CHANGE - Doc Scientia...CHEMICAL CHANGE 274 Doc Scientia - Chemistry Textbook and Workbook Book 1 - Grade 12 A small balloon must be inflated with hydrogen gas. The balloon