3.3 Reversible Reactions & Equilibria

Separate: Chemistry Only

Nitrogen dioxide, NO₂, is a brown gas.

The two gases can exist together in dynamic equilibrium according to the equation:

N₂O₄(g)         2NO₂(g)

Explain what is meant by the term dynamic equilibrium.

Separate: Chemistry Only

Some N₂O₄ and some NO₂ are put into a sealed gas syringe and allowed to form an equilibrium mixture. 

This equilibrium mixture is brown.

Carbon monoxide, CO, and oxides of nitrogen are produced in a car engine when petrol is burned.

These oxides can be partly removed by using a catalytic converter fitted to the car’s exhaust system.

Separate: Chemistry Only

This question is about reactions involving gases.

Potassium carbonate reacts with dilute hydrochloric acid to produce carbon dioxide gas.

The equation for the reaction is

Calculate the volume, in cm³, of carbon dioxide gas produced when 6.9 g of potassium carbonate reacts with excess dilute hydrochloric acid.

[M_r of K₂CO₃ = 138]

[molar volume of CO₂ at rtp = 24 dm³]

volume = .............................................................. cm³

Separate: Chemistry Only

This reaction involving gases is in dynamic equilibrium at a temperature of 225°C.

H₂ (g) + CO₂ (g) → CO (g) + H₂O (g) ΔH = + 41 kJ/mol

(2)

(2)

Separate: Chemistry Only

The industrial production of sulfuric acid involves several steps.

One of these steps is the reaction of sulfur dioxide, SO₂, with oxygen to form sulfur trioxide, SO₃.

What volume of sulfur trioxide, in dm³, is produced by the complete reaction of 750 dm³ of sulfur dioxide?
(all volumes of gases are measured under the same conditions of temperature and pressure)

Separate: Chemistry Only

Calculate the volume of oxygen needed to react completely with 750 dm³ of sulfur dioxide.
(all volumes of gases are measured under the same conditions of temperature and pressure)

Separate: Chemistry Only

Calculate the mass, in kilograms, of 750 dm³ of sulfur dioxide, measured at room temperature and pressure.
(relative formula mass: SO₂ = 64;
1 mol of any gas at room temperature and pressure occupies 24 dm³)

Separate: Chemistry Only

The reaction to produce sulfur trioxide reaches an equilibrium.

 2SO₂ (g) + O₂ (g) 2SO₃ (g) 

The forward reaction is exothermic.
The rate of attainment of equilibrium and the equilibrium yield of sulfur trioxide are affected by pressure and temperature.

A manufacturer considered two sets of conditions, A and B, for this reaction.
In each case sulfur dioxide is mixed with excess oxygen.
The manufacturer changed the temperature and the pressure and only used a catalyst in B.
The sets of conditions A and B are shown in Figure 7.

Figure 7

The manufacturer chooses set of conditions B rather than set of conditions A.

Explain, by considering the effect of changing the conditions on the rate of attainment of equilibrium and on the equilibrium yield of sulfur trioxide, why the manufacturer chooses the set of conditions B rather than the set of conditions A.

This question is about equilibrium. 

An ionic equation for the reaction between iron (III), Fe³⁺ and thiocyanate ions, SCN²⁺ is shown below:

At room temperature, the mixture of the equilibrium solution is orange. 

Which solvent would be used to dissolve the ions in this reaction?

Separate: Chemistry Only

Separate: Chemistry Only

A few drops of a colourless solution containing a high concentration of Fe³⁺ ions are added to the orange equilibrium mixture.

Explain the colour change observed.

Separate: Chemistry Only 

A water bath is set up at a temperature below room temperature.

When a test tube containing the orange equilibrium mixture is placed in the water bath, the mixture becomes more red.

Explain what this shows about the energy change for the forward reaction.

Separate: Chemistry Only

The decomposition of ammonium chloride is endothermic:

NH₄Cl (s) NH₃ (g) + HCl (g)

Which changes to the conditions would both decrease the amount of product in this reaction?

Hydrogen gas can be produced by reacting a mixture of methane and steam in the presence of a nickel catalyst.

The reaction conditions are a temperature of 700°C and a pressure of 5 atmospheres.

The equation for the reaction is

CH₄ (g) + H₂O (g) CO (g) + 3H₂ (g)                 ΔH = +206 kJ/mol

What does the symbol  represent?

Separate: Chemistry Only

(2)

(2)

Separate: Chemistry Only

Calculate the volume, in dm³, of hydrogen gas at rtp that is produced when 10 tonnes of methane gas completely react with steam.

[molar volume of hydrogen at rtp is 24 dm³]

Give your answer in standard form.

Separate: Chemistry Only

The gasification of carbon is a reversible endothermic reaction:

C (s) + H₂O (g)  CO (g) + H₂ (g)

Which of the following conditions could be changed to increase the relative amount of H₂ gas produced?

This question is about chemical equilibrium and reversible reactions.

If carried out in a closed system, reversible reactions can reach a state of equilibrium. 

Explain what a closed system is.

When a reversible reaction reaches equilibrium, it appears to have stopped. 

Explain why.

In the Haber process, the reaction of nitrogen with hydrogen to produce ammonia is reversible.

                                  N₂ (g)     +     3H₂ (g)     ⇌      2NH₃ (g)

Explain what happens to the amount of ammonia produced at equilibrium if the pressure is increased.

Describe the effect, if any, of using a catalyst on:

• the rate of reaction
• the position of equilibrium