equilibrium and reaction rates

8/4/2019 Equilibrium and Reaction Rates

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RATE AND EXTENT OF REACTIONS

1. Explain what is meant by reaction rate and how it can be measured.

2. List the factors which affect the rate of chemical reactions.

3. Consider the reaction between calcium carbonate (CaCO3) and

hydrochloric acid:

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (ℓ)

Explain in terms of the collision theory how the increase in surface

area of CaCO3 will affect the rate of a chemical reaction.

4. Explain what the difference is between an open and closed system.Which of the following reactions should occur in a closedsystem?

(i) HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (ℓ)(ii) NH4Cl (s) ⇌ NH3 (g) + HCl (g)(iii) 2Cu (s) + O2 (g) → 2CuO (s)

5. Study the following reversible reaction equation:N2 (g) + 3H2 (g) ⇌ 2 NH3 (g)

What does it mean if a reaction is reversible?

6. What is dynamic equilibrium?

7. a) State Le Chatelier’s principle in words.

b) List the factors which will influence the position of an equilibrium ina closed system.

8. The following reaction reaches equilibrium at 30°C in a closed 1 dm3

container:

H2 (g) + I2 (g) ⇌ 2HI (g) ΔH = - 52 kJ

a) Write the equilibrium expression for the reaction.



b) Calculate the value of the equilibrium constant if 1 mole of H2 (g)

and 1 mole of I2 (g) are placed in the container and analysis shows

that 0,8 moles of HI (g) are present when equilibrium is reached.

c) Explain the significance of a high value of the equilibrium constant.

9. When ethane and oxygen are mixed in a closed container, the

following equilibrium is established at a constant temperature of

183°C:

2C2H6 (g) + 7O2 (g) ⇌ 4CO2 (g) + 6H2O (g) (ΔH < 0)

a) For the open spaces in the table below, select from increases/

remains constant/ decreases to indicate the effect upon the

specific quantity if the conditions are changed as indicated.

Change Amount of CO2

Value of Kc

Pressure is decreased by increasing thevolume at constant temperature

Temperature is increased at constantvolume

A catalyst is added

More C2H6 is added to the container

b) Give a reason for your answer for the value of Kc with temperature

change.

c) Use Le Chatelier’s principle and explain your answer for the amountof CO2 with more C2H6 added to the container.

10. Ammonia is a very important chemical in the production of fertilizers

and explosives. For a chemical such as ammonia, to be produced

economically on an industrial scale one must be able to produce

sufficient quantities of the chemical in a reasonably short time. The

Haber process is used to prepare ammonia industrially.



The equilibrium reaction involved is:

N2 (g) + 3H2 (g) ⇌ 2NH3 (g) ΔH = - 92 kJ.mol-1

a) Is the forward reaction exothermic or endothermic? Give a reason

for your answer. b) Calculate the value of ΔH per mole of ammonia formed.

c) State how each of the following changes affect the equilibrium

concentration of ammonia (only answer: Increases, Decreases,

Remains the same.)

(i) The temperature of the system is increased.

(ii) More H2 (g) is added to the system.

(iii) A catalyst is added.

(iv) The pressure of the system is decreased.



SOLUTIONS

1. The rate of a reaction is the speed of the reaction and it can bemeasured as follows:the amount of reactant used up per unit time or

the amount of product formed per unit time.

2. a) Surface area (solid)b) Concentration (solution)c) Pressure (gas)d) Temperaturee) Catalyst

3. Increasing the surface area of the solid CaCO3 will increase the

frequency of collisions between reactant molecules (in solution) andthe solid substance. Since the rate of a reaction is directlyproportional to the frequency of collisions the reactant moleculesundergo, the greater the surface area of a given solid, the greater thefrequency of collisions and hence the greater the reaction rate.

4. a) A closed system is one in which no substances in the reactioncan escape, where in an open system substances especiallygases can escape.

b) Reaction (ii), as NH3 is produced as a gas.

5. Reversible reactions can go in both directions. This means that theproducts can break down and return to the starting reactant(s). Areversible reaction is symbolized using double arrows ⇌

6. Dynamic equilibrium means that the forward and reverse reactions

continue to take place while the overall concentrations of reactantsand products no longer change.

7. a) If a reversible reaction that is in a state of dynamic equilibrium isdisturbed by changing the conditions, the reaction will what it canto oppose that change.

b) Changing the concentration, the temperature or pressure.

8. a) Kc = [HI] 2 [H2].[I2]



b) H2 + I2 ⇌ 2HI

start 1 1 -

reacted 0,4 0,4 -

at equilibrium 0,6 0,6 0,8

[equilibrium] 0,6 0,6 0,8

Kc = [HI] 2 [H2].[I2]

= (0,8)2

(0,6).(0,6)= 1,78

c) A high equilibrium constant indicates that there is more productsand less reactants

9. 2C2H6 (g) + 7O2 (g) ⇌ 4CO2 (g) + 6H2O (g) (ΔH < 0)

a)

Change Amount of CO2

Value of Kc

Pressure is decreased by increasingthe volume at constant temperature

increases remainsconstant

Temperature is increased atconstant volume

decreases decreases

A catalyst is added remainsconstant

More C2H6 is added to the container increases

b) Kc is not constant with a change in temperature. An increase in

temperature will favour the reverse reaction and therefore lessproducts will be produced. Kc will thus decrease.

c) If more C2H6 is added to the container, then according to LeChatelier’s principle, the forward reaction will be favoured to haveless of C2H6 and O2 and more of CO2 and H2O.

10. a) Exothermic because ΔH is smaller the zero (a negative value).

b) ΔH =- 92

= -46 kJ per mole NH3

2



c) (i) Decrease

(ii) Increases

(iii) Remains the same

(iv) Decreases

equilibrium and reaction rates

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