Equilibrium

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Question 1

Which statement is true about chemical reactions at equilibrium?

The forward reaction is exothermic.

The concentrations of the reactants adnd products are equal.

The forward and reverse reactions proceed at equal rates.

The forward and reverse reactions have stopped.

Wrong Answer!The equation is dynamic with the rates of forward and reverse reactions equal.

Question 2

Which statement(s) is/are true for a mixture of ice and water at equilibrium?

I. The rates of melting and freezing are equal.

II. The amounts of ice and water are equal.

III.The same position of equilibrium can be reached by cooling water or heating ice.

I only II only

II and III onlyI and III only

Wrong Answer!I is correct as the equation is dynamic, III is correct as the equilibrium can be reached from either direction.

II and III only

Question 3Which statements are correct for a reaction at equilibrium?

I. The forward and reverse reactions both continue.

II. The rates of forward and reverse reactions are equal.

III.The concentrations of reactants and products are equal.

I and II only I and III only

I, II and III

Wrong Answer!I and II are correct for the equilibrium involved is a dynamic process; III is not necessary true, the balance, that is the position of equilibrium depends on the conditions.

Question 4The following equilibrium can be set up in a closed

container.

N2O4(g) 2NO2(g) ΔH = +57 kJmol-1

colourless brown

Which of the following changes would produce a darkening of the colour of the gaseous mixture in the container?

An increase in temperature Adding a catalyst

A decrease in temperatureAn increase in pressure

Wrong Answer!The forward reaction is endothermic and favoured by an increase in temperature, so more NO2 is present in the equilibrium mixture (so the mixture is browner)

Question 5What changes occur when the temperature is increased in the

following reaction at equilibrium?

Br2(g) + Cl2(g) 2BrCl(g) ΔH = +14 kJmol-1

Position of equiibrium Value of equilibrium constant

Shifts towards the products Decreases

Shifts towards the reactants Decreases

Shifts towards the products Increases

Shifts towards the reactants Increases

Wrong Answer!The forward reaction is endothermic, favoured by an increased temperature. So the equilibrium shifts to the right; Kc is a measure of how far to the right an equation lies.

Question 6The volume of the reaction vessel containing the following

equilibrium mixture

SO2Cl2 (g) SO2(g) + Cl2(g) is increased.

When the equilibrium is re-established, which of the following will have occured?

The amount of SO2Cl2

will have increased.The amount of SO2Cl2

will have decreased.

The amount of Cl2 will have decreased.

The amount of Cl2 will have remained unchanged.

Wrong Answer!The pressure is decreased favouring a shift to the products side of the equilibrium as this involves fewer moles of gas, so the amount of SO2Cl2 is decreased.

Question 7Which changes will shift the position of the equilibrium to

the right in the following reaction?

2CO2(g) 2CO(g) + O2(g)

I. Adding a catalyst

II. Dereasing the oxygen concentration

III.Increasing the volume of the container

I, II and III I and II only

I and III onlyII and III only

Wrong Answer!I is correct as removing oxygen shifts the reaction to the right. III is correct as reduced pressure favours the side of the equation involving fewer moles of gas, in thies case the products side.

Question 8 I2(g) + 3Cl2(g) 2ICl3(g)

What is the equilbrium constant expression for the reaction above?

32 2

23

[ ][ ]

[ ]c

I ClK

ICl 3

2 2

2[ ]

[ ] 3[ ]c

IClK

I Cl

3

2 2

2[ ]

3[ ][ ]c

IClK

I Cl

23

32 2

[ ]

[ ][ ]c

IClK

I Cl

Wrong Answer!This is correctly constructed expression which fits the equilibrium law.

Question 9

The smaller an equilibrium constant, Kc,

the slower the reaction rate.

The lower the concentration of products at equilibrium.

The faster the reactants are converted to products.

the more endothermic the reaction.

Wrong Answer!The higher the value of Kc, the greater the value of the numerator (the top figures) in the equilibrium expression, so the greater the concentration(s) of the products. There is no connection between Kc and the rate at which equilibrium is achieved.

Question 10For the gaseous reaction, the equilibrium constant

expression is:

Which equation corresponds to this equilibrium expression?

4NH3 + 5O2 4NO + 6H2O 4NO + 6H2O 4NH3 + 5O2

2NO + 3H2O 2NH3 + O2 8NH3 + 10O2 8NO +12H2O

5 42 3

4 62

[ ] [ ]

[ ] [ ]c

O NHK

NO H O

5

2

Wrong Answer!This is correctly constructed expression which fits the equilibrium law.

Question 11The key reaction in the manufacture of sulfuric acid can be

represented by the equation below.

SO2(g) + O2(g) 2SO3(g) ΔH = -197 kJmol-1

What happens when a catalyst is added to an equilibrium mixture from this reaction?

The rate of the forward reaction increases and the that of the reverse reaction decreases.

The rates of both the forward and reverse reactions increase.

The yield of SO3 increases.

The value of ΔHƟ increases.

Wrong Answer!A catalyst speeds up both the forward and reverse reactions equally. The composition of the equilibrium mixture remains unchanged.

Question 12In which of the following reactions does the

position of equilibrium remain unaffected by change in pressure?

2O3(g) 3O2(g) 2NO2(g) N2O4(g)

N2(g)+O2(g) 2NO(g)2NO(g)+Cl2(g) 2NOCl(g)

Wrong Answer!There are the same number of moles of as on both sides of the equation.

Congratulations!