Reversible Reactions

A reversible reaction is one which can proceed in both directions. It is represented by the sign Reversible Reactions • Most reactions are not reversible. For examples: • 1. 2Mg + O2 2MgO • NaOH + HCl  NaCl + H2O • CaCO3 + 2HCl  CaCl2 + H2O + CO2 Can you think of an important reversible reaction which occurs in your body?

Heating of hydrated copper(II) sulfate

Equation for the reaction is : Example: The Decomposition of Ammonium Chloride What would happen to the moist litmus papers ? The moist red litmus paper turns blue first, then the moist blue litmus papers turn red again. (Ammonia which has a smaller relative molecular mass than hydrogen chloride diffuses faster.)

represents represents forward reaction which is NH4Cl  NH3 + HCl backward reaction which is NH3 + HCl  NH4Cl

The above reaction is also known as thermal dissociation. In a reversible reaction, the reactants are not completely converted to products. Instead an intermediate position or equilibrium is reached whereby both reactants and products are present. Reversible reactions are never complete, a mixture of reactants and products is obtained.

In a closed system, reversible reactions reach a Dynamic Equilibrium Forward reaction fastest, no backward reaction Forward reaction slows down as reactants are used up; backward reaction speeds up as concentration of products increases Rate of forward reaction = rate of backward reaction Start of reaction Half way At equilibrium

2. Dynamic Equilibrium • After a period of time, the rate of forward reaction becomes • equal to the rate of backward reaction. This situation is called • dynamic equilibrium. • At equilibrium in a reversible reaction: • - the rate of forward reaction = rate of backward reaction. • - the concentrations of the reactants and products are • constant. • Graph, see white board

How do you know when an equilibrium has been reached? • rate of forward reaction = rate of backward reaction • Concentration of reactants and products remain constant.

3. Factors Affecting Equilibrium Equilibrium is subject to changes in concentration, temperature and pressure. Best way to predict is by Le Chatelier’s Principle Le Chatelier’s Principle If a change is made to a system in equilibrium, the system reacts in such a way as to oppose the change, and a new equilibrium is formed.

a) Effect of Concentration yellow orange What colour would you observe when the concentration of acid is increased ? The solution would be orange in colour. As the concentration of acid increases, concentration of hydrogen ions increases. Thus forward reaction is favoured and equilibrium shifts to the right so as to decrease the concentration of hydrogen ions/ to offset the increase in concentration of hydrogen ions, producing more dichromate (VI) ions, causing the solution to become orange.

Note: • The term position of equilibrium is used to indicate whether the system contains a larger proportion of reactants or products. For example: • ‘equilibrium shifts to the left’ means the system contains a greater proportion of • reactants. • ‘equilibrium shifts to the right’ means the system contains a greater a proportion of • products.

yellow orange What happens when alkali is added to the reaction mixture ? The solution would be yellow in colour. As alkali is added, it neutralises the hydrogen ions, thus decreasing the concentration of hydrogen ions. Thus equilibrium shifts to the left to produce more hydrogen ions in order to offset the decrease in concentration of hydrogen ions , at the same time producing more chromate (VI) ions, causing the solution to turn yellow.

b) Effect of Temperature The above reaction shows the formation of ammonia from hydrogen and nitrogen. This reaction is known as the Haber process. It is used in the industry to make ammonia. Notice from the equation that  H = -184 kJ. This means forward reaction is exothermic while backward reaction is endothermic.

What happens when the temperature of the reaction is increased? In the above reaction, forward reaction is exothermic while backward reaction is endothermic. Increasing the temperature will favour the backwardreaction so as to remove the additional heat. Thus equilibrium shifts to the left, forming more nitrogen and hydrogen but less ammonia.

Always remember: Increase in temperature always favours endothermic reactions. Decrease in temperature always favours exothermic reaction.

c) Effect of Pressure Reversible reactions involving gases may also be affected by pressure. Pressure of a gas is due to the molecules bouncing off the walls of the container. The greater the number of gaseous particles, the greater the pressure.

c) Effect of Pressure What happens when the pressure of the equilibrium is increased? In the above reaction, when forward reaction takes place, 4 moles of gases are changed into 2 moles of gas. Hence there is a decrease in the no. of moles of gas, leading to a decrease in the pressure. Thus, when the pressure is increased, forward reaction is favoured and equilibrium shifts to the right so as to offset the increase in pressure, producing more ammonia.

The industrial conditions for the Haber process are as follows: • Temperature of • 450°C • Pressure of • 250 atmospheres. • iron catalyst

Example 2 What is the effect of increasing pressure on the following equilibrium? The number of moles of gases remain the same before and after the reaction. Hence increasing thepressure has no effect on the position of this equilibrium.

d) Effect of catalyst • Catalysts only • increase the rate of reaction so that equilibrium is reached more quickly. • A catalyst has no effect on the equilibriumposition because it increases the rates of forward and backward reactions to the same extent. • Catalyst does not affect the yield of product.

Critical thinking 2 – Contact Process, Pg 5 • 1. Which stage of the Contact Process is reversible? • Stage 2 • Explain the effect of increasing temp on the yield • of sulfur trioxide in stage 2. • In the above reaction, forward reaction is • exothermic while backward reaction is • endothermic. • Increasing the temperature will favour the • backwardreaction so as to remove additional heat to counteract the effect of increasing temperature. Thus equilibrium shifts to the • left, decreasing the yield of sulfur trioxide.

Always remember: Increasing temperature always favour endothermic reactions. Decreasing temperature always favour exothermic reaction.

Stage 2 is carried out at 450°C. Explain why stage 2 is not carried out at low temp. to increase the yield of sulfur trioxide. • At low temperature, although yield of sulfur trioxide is high, rate of reaction would be too slow, causing the production of sulphur trioxide to be uneconomical. • Thus 450C chosen is a • compromise so as to obtain an optimum yield at a resonable rate and cost.

Explain the effect of increasing pressure on the yield of sulphur trioxide in stage 2. • When forward reaction occurs, 3 moles of gases are converted to 2 moles of gases, leading to a decrease in the number of moles of gases and hence a decrease in pressure. • An increase in pressure will favour the • forward reaction and equilibrium shifts to the right so as to offset the increase in pressure. Yield of sulfur trioxide will therefore • increase.

5. Suggest why stage 2 is carried out at one atmospheric pressure and not at a pressure of 1000 atm. The reaction is 98% complete at equilibrium at one atmospheric pressure, so high pressure is not necessary. (Read question, the line just before 2nd equation). If high pressure (1000 atm) is used, yield increases but it would be too costly to maintain the high pressure as expensive engineering equipment such as special pumps and stronger pipes are required.

What is the effect of vanadium (V) oxide on • a) rate of formation of sulphur trioxide • As vanadium (V) oxide is a catalyst, it would increase the rate of formationof sulfur trioxide. (Read 2nd paragraph of question) • b) Vanadium (V) oxide has no effect on the yield sulfur trioxide. • Recall catalyst increases the rate of reaction but has no effect on the yield.

Summary of the effects on Equilibrium, Pg 7 Concepts needed: Increase in temp, pressure and concentration and presence of catalyst lead to increase in rate of reaction. Rate of reaction NOT predicted by Le Chatelier’s Principle. Le Chatelier’s Principle is applied to predict the yield/composition of eqm mixture.

Summary of the effects on Equilibrium Change: Addition of catalyst Rate of reaction: increased Composition of equilibrium mixture: no effect

Summary of the effects on Equilibrium Change: Addition of more A (g) Rate of reaction: increased Composition of equilibrium mixture: Equilibrium shifts to right. More C (g), less B (g)

Summary of the effects on Equilibrium Change: increase in pressure Rate of reaction: increased Composition of equilibrium mixture: (Equilibrium shifts to the right.) more C (g), less A (g) and B (g)

Summary of the effects on Equilibrium Change: Decrease in pressure Rate of reaction: decrease Composition of equilibrium mixture: (Equilibrium shifts to the left) Less C (g), more A (g) and B (g)

Summary of the effects on Equilibrium Change: Increase in temperature Rate of reaction: increase Composition of equilibrium mixture: (Equilibrium shifts to the left) Less C (g), more A (g) and B (g)

Summary of the effects on Equilibrium Change: Decrease in temperature Rate of reaction: decrease Composition of equilibrium mixture: (Equilibrium shifts to the right) More C (g), Less A (g) and B (g)

Quick check

Reversible Reactions