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4.6 The rate and extent of chemical change 4.6.2 Reversible reactions and dynamic equilibrium 4.6.2.4 The effect of changing conditions on equilibrium The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle. Specification Describe Le Chatelier’s principle. Explain the effects on equilibrium of changing conditions using suitable examples. Research the work of Le Chatelier or the life of Fritz Haber. Highlight the moral ambiguity of Haber’s work. Scientific Communication Skills 4.6 The rate and extent of chemical change 4.6.2 Reversible reactions and dynamic equilibrium Use data to predict the effect of concentration on equilibrium. Justify answers. 4.6.2.5 The effect of changing concentration If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again. If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. If the concentration of a product is decreased, more reactants will react until equilibrium is reached again. Specification Scientific Communication Skills 4.6 The rate and extent of chemical change 4.6.2 Reversible reactions and dynamic equilibrium Use data to predict the effect of temperature on equilibrium. Justify answers. 4.6.2.6 The effect of temperature changes on equilibrium If the temperature of a system at equilibrium is increased: •the relative amount of products at equilibrium increases for an endothermic reaction, •the relative amount of products at equilibrium decreases for an exothermic reaction. If the temperature of a system at equilibrium is decreased: •the relative amount of products at equilibrium decreases for an endothermic reaction, •the relative amount of products at equilibrium increases for an exothermic reaction. Specification Scientific Communication Skills 4.6 The rate and extent of chemical change 4.6.2 Reversible reactions and dynamic equilibrium Use data to predict the effect of pressure on equilibrium. Justify answers. 4.6.2.7 The effect of pressure changes on equilibrium For gaseous reactions at equilibrium: •an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction, •a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction. Specification Scientific Communication Skills The effect of changing conditions on equilibrium Friday, 10 June 2022 Do Now An aqueous solution of blue copper(II) ions can react with aqueous chloride ions to form a yellow copper compound. The ionic equation for this reaction is: Cu2+ + 4Cl- ⇋ [CuCl4]21. 2. 3. 4. 5. The forward reaction is endothermic. What two things can you say about the enthalpy change for the reverse reaction? A solution containing copper(II) ions is mixed with a solution containing chloride ions in a flask. The solution quickly turns green. When observed for a few minutes no further change in colour can be seen. Explain these observations. Suggest two changes that could be made to the mixture that would change its colour. Why would a change in pressure not change the colour of the solution? Would the addition of a colourless catalyst change the colour of the solution? Do Now I Do We Do You Do Keywords Counteract Le Chatelier’s Principle Qualitative System Gaseous Equilibrium shift Test The effect of changing conditions on equilibrium Friday, 10 June 2022 Learning Objectives - To be able to: Keywords • Make qualitative predictions about the effect of changes on systems at equilibrium when given appropriate information. • Interpret appropriate given data to predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium. • Interpret appropriate given data to predict the effect of a change in temperature on given reactions at equilibrium. • Interpret appropriate given data to predict the effect of pressure changes on given reactions at equilibrium. Do Now Intro Develop Practice Counteract Le Chatelier’s Principle Qualitative System Gaseous Equilibrium shift Test 4.6.2.3 Equilibrium Explain the term equilibrium and give suitable examples of when it can occur When a reversible reaction occurs in apparatus which prevents the escape of reactants and products – a closed system – equilibrium is reached when the forward and reverse reactions occur at exactly the same rate: At the start of the reaction, there are only reactants, the forward reaction is fast There is now a lower concentration of reactants – the forward reaction slows down At the start of the reaction, there are no products, the reverse reaction cannot start yet The forward reaction has now slowed down to the same rate as the reverse reaction The reverse reaction has now sped up to the same rate as the forward reaction Do Now There is now a higher concentration of products – the reverse reaction speeds up I Do We Do You Do Test 4.6.2.3 Equilibrium Explain the term equilibrium and give suitable examples of when it can occur When a reversible reaction occurs in apparatus which prevents the escape of reactants and products – a closed system – equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. Once the forward and reverse reactions are going at the same rate, there is no longer any net change in the concentration of reactants and products – this is dynamic equilibrium (Note: this is not necessarily at 50% reactants 50% products). Do Now I Do We Do You Do Test 4.6.2.3 Equilibrium Explain the term equilibrium and give suitable examples of when it can occur • This equilibrium has more products than reactants – we say it lies to the right. • This equilibrium has more reactants than products – we say it lies to the left. • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. Do Now I Do We Do You Do Test 4.6.2.4 The effect of changing conditions on equilibrium Make qualitative predictions about the effect of changes on systems at equilibrium • At equilibrium, both forward and backward reactions are still happening, but there is no overall effect – it is a dynamic equilibrium. This means that the concentrations of the reactants and products have reached a balance and do not change. • If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change. • The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle. Do Now I Do We Do You Do Test 4.6.2.5 The effect of changing concentration Predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium • If you change the concentration of either the reactants or products, the system will no longer be at equilibrium. • So the system responds to bring itself back to equilibrium again. • If you increase the concentration of the reactants the system tries to decrease it by making more products. • If you decrease the concentration of the products the system tries to increase it again by reducing the amount of reactants. Do Now I Do We Do You Do Test 4.6.2.5 The effect of changing concentration Predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium Easy to remember: • Increase the concentration, shift is to other side of arrow. • Decrease the concentration, stay on same side of arrow. Do Now I Do We Do You Do Test 4.6.2.5 The effect of changing concentration Predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium • Increasing the concentration of a chemical will make equilibrium shift in the direction that will use it up Increase ammonium chloride NH4Cl (s) ammonia NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts left Increase Hydrated copper sulfate CuSO4.5H2O (s) anhydrous copper sulfate CuSO4 (s) + water 5H2O (l) Equilibrium shifts right Do Now I Do We Do You Do Test 4.6.2.5 The effect of changing concentration Predict the effect of a change in concentration of a reactant or product on given reactions at equilibrium • Decreasing the concentration of a chemical will make equilibrium shift in the direction that will replace it Decrease ammonium chloride NH4Cl (s) ammonia NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts right Decrease Hydrated copper sulfate CuSO4.5H2O (s) anhydrous copper sulfate CuSO4 (s) + water 5H2O (l) Equilibrium shifts left Do Now I Do We Do You Do Test 4.6.2.6 The effect of temperature changes on equilibrium Predict the effect of a change in temperature on given reactions at equilibrium • All reactions are exothermic in one direction and endothermic in the other. • If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat. This means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction. • If you raise the temperature, the equilibrium will move in the endothermic direction to try to decrease it. You’ll get more products for the endothermic reaction and less products for the exothermic reaction. Do Now I Do We Do You Do Test 4.6.2.6 The effect of temperature changes on equilibrium Predict the effect of a change in temperature on given reactions at equilibrium Easy to remember: • Increase the temperature, shift is in the endothermic direction. • Decrease the temperature, shift is in the exothermic direction. Do Now I Do We Do You Do Test 4.6.2.6 The effect of temperature changes on equilibrium Predict the effect of a change in temperature on given reactions at equilibrium • Increasing the temperature favours the endothermic reaction (takes in heat): endothermic ammonium chloride ammonia exothermic NH4Cl (s) NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts right endothermic Hydrated copper sulfate anhydrous copper sulfate exothermic CuSO4.5H2O (s) CuSO4 (s) + nitrogen N2 (g) water 5H2O (l) Equilibrium shifts right exothermic hydrogen 3H2 (g) + ammonia 2NH3 (g) endothermic Equilibrium shifts left Do Now I Do We Do You Do Test 4.6.2.6 The effect of temperature changes on equilibrium Predict the effect of a change in temperature on given reactions at equilibrium • Decreasing the temperature favours the exothermic reaction (takes in heat): endothermic ammonium chloride ammonia exothermic NH4Cl (s) NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts left endothermic Hydrated copper sulfate anhydrous copper sulfate exothermic CuSO4.5H2O (s) CuSO4 (s) + nitrogen N2 (g) water 5H2O (l) Equilibrium shifts left exothermic hydrogen 3H2 (g) + ammonia 2NH3 (g) endothermic Equilibrium shifts right Do Now I Do We Do You Do Test 4.6.2.7 The effect of pressure changes on equilibrium Predict the effect of pressure changes on given reactions at equilibrium • Changing the pressure only affects equilibria involving gases. • If you increase the pressure, the equilibrium tries to reduce it – it moves in the direction where there are fewer molecules of gas. • If you decrease the pressure, the equilibrium tries to increase it – it moves in the direction where there are more molecules of gas. • You must use the balanced symbol equation for a reaction to see which side has more molecules of gas. Do Now I Do We Do You Do Test 4.6.2.7 The effect of pressure changes on equilibrium Predict the effect of pressure changes on given reactions at equilibrium Easy to remember: • Increase the pressure, shift is to fewer molecules. • Decrease the pressure, shift is to more molecules. Do Now I Do We Do You Do Test 4.6.2.7 The effect of pressure changes on equilibrium Predict the effect of pressure changes on given reactions at equilibrium • Increasing the pressure favours the side with the fewest gas moles: ammonium chloride NH4Cl (s) ammonia NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts left hydrogen 3H2 (g) Do Now + nitrogen N2 (g) I Do ammonia 2NH3 (g) We Do Equilibrium shifts right You Do Test 4.6.2.7 The effect of pressure changes on equilibrium Predict the effect of pressure changes on given reactions at equilibrium • Decreasing the pressure favours the side with the most gas moles: ammonium chloride NH4Cl (s) ammonia NH3 (g) + hydrogen chloride HCl (g) Equilibrium shifts right NB: if you have the same number of molecules of gas on each side, a change in pressure will have no effect on the position of the equlibrium hydrogen 3H2 (g) Do Now + nitrogen N2 (g) I Do ammonia 2NH3 (g) We Do Equilibrium shifts left You Do Test Answer the following questions in full sentences in your exercise books. 1. What is Le Chatelier’s Principle? 2. What three factors can be changed in a system at equilibrium? 3. If the concentration of a reactant is increased what will happen to the products of the reaction? 4. What will happen to the amount of product in an exothermic reaction at equilibrium if the temperature is increased? 5. What will happen to the amount of product in an endothermic reaction at equilibrium if the temperature is decreased? 6. What would happen to the position of equilibrium in a gaseous reaction if the pressure is increased? 7. What will happen if we increase the temperature of the reaction below? Explain why you think this. exothermic N2(g) + 3H2(g) 2NH3(g) 8. Explain what will happen if we decrease the pressure in the reaction above. Do Now I Do We Do You Do Test Knowledge Organizer