© Boardworks Ltd of 33 KS4 Chemistry Equilibrium

© Boardworks Ltd of 33 Equilibrium Changing conditions The Haber process Summary activities Dynamic equilibrium Contents

© Boardworks Ltd of 33 Equilibrium Changing conditions The Haber process Summary activities Dynamic equilibrium Contents

© Boardworks Ltd of 33 A reversible reaction is where products can, under appropriate conditions, turn back into reactants. There will be a range of conditions over which both the forward and backward reaction will take place, and this can lead to a state of balance with reactants and products present in unchanging amounts. dynamic equilibrium.This is called a dynamic equilibrium. A B A B these combine these decompose Dynamic equilibrium

© Boardworks Ltd of 33 Dynamic equilibrium What is special about the forward and backward reactions during dynamic equilibrium?

© Boardworks Ltd of 33 It is rather like the situation where a man is walking the wrong way along a moving pavement or escalator. Neither have stopped but the man could remain in the same place for ever! The symbol is used to mean dynamic equilibrium. Equilibrium – Equilibrium – because of the unchanging amounts Dynamic – Dynamic – because the reaction is still occurring The man stays in the same place! Dynamic equilibrium Dynamic equilibrium can only take place in a closed system, otherwise products would be able to escape.

© Boardworks Ltd of 33 In reversible reactions, equilibrium means balance but this balance does not have to be at the half-way point. We may have mainly reactants with just a little product, or vice versa. There are two factors that we can change that influence the position of an equilibrium:Temperature Concentration (or pressure in gas reactions) Changing the equilibrium Finding the conditions that gives the most product is really important in industrial chemical reactions. Adding a catalyst speeds up the time it takes to reach equilibrium, but does not change the position of equilibrium.

© Boardworks Ltd of 33 True or false?

© Boardworks Ltd of 33 Equilibrium Changing conditions The Haber process Summary activities Dynamic equilibrium Contents

© Boardworks Ltd of 33 All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. E.g. nitrogen dioxide (NO 2 ) joins to form dinitrogen tetroxide (N 2 O 4 ) exothermically. forward Gets hot going forward (exothermic) backward Gets cold going backward (endothermic) 2NO 2 N 2 O 4 Heating will give more NO 2 in the equilibrium mixture Cooling will give more N 2 O 4 in the equilibrium mixture.. The hotter a reaction is, the more likely it is to go in the endothermic direction. Temperature and equilibrium

© Boardworks Ltd of 33 Gets hot going forward (exothermic) Gets cold going backward (endothermic) 3H 2 + N 2 2NH 3 Which direction is endothermic? In which direction do reactions move when heated? Will heating give more or less NH 3 in the equilibrium mixture? The reaction of nitrogen and hydrogen to form ammonia (NH 3 ) is exothermic. How will temperature affect the composition of the equilibrium mixture? backward less Opposing changes in temperature

© Boardworks Ltd of 33 This applies to gas reactions. Here the rule depends upon the number of gas molecules on each side of the equation Get fewer gas molecules in forward direction Get more gas molecules in backward direction 2NO 2 (g) N 2 O 4 (g) The higher the pressure, the more the reaction moves in the direction with fewer gas molecules. Increasing the pressure will give more N 2 O 4 Decreasing pressure gives more NO 2 at equilibrium. Pressure and equilibrium

© Boardworks Ltd of 33 Look at the reaction of nitrogen and hydrogen to form ammonia. Get fewer gas molecules in forward direction Get more gas molecules in backward direction 3H 2 (g) + N 2 (g) 2NH 3 (g) Increasing the pressure will give more NH 3 Decreasing the pressure give less NH 3 at equilibrium.. Which direction produces less gas molecules. In which direction do reactions move when compressed? Will high pressure give more or less NH 3 in the equilbrium mixture? forward The side that has fewer gas molecules more Opposing changes in pressure

© Boardworks Ltd of 33 This applies to reactions in solution. BiCl 3 (aq) + H 2 O (l) BiOCl (s) + 2HCl (aq) E.g. Bismuth chloride reacts with water to give a white precipitate of bismuth oxychloride. Increasing the concentration of a substance changes the equilibrium in the direction that uses up (decreases) the concentration of the substance added. Adding water will produce more BiOCl solid (to use up the H 2 O). Adding acid (HCl) will result in less BiOCl solid to use up the HCl. Concentration and equilibrium

© Boardworks Ltd of 33 Chlorine gas reacts with iodine chloride (a brown liquid) converting it to iodine trichloride (a yellow solid). ICl (l) + Cl 2 (g) ICl 3 (s) brown pale green yellow What effect will adding more chlorine have on the colour of the mixture in the U-tube? If the U-tube is turned on its side, chlorine gas pours out of the tube. In which way will this change the equilibrium? Produce more ICl 3 and so more yellow solid. Produce less ICl and so more brown liquid. Opposing changes in concentration

© Boardworks Ltd of 33 Dynamic equilibrium and change

© Boardworks Ltd of 33 Equilibrium Changing conditions The Haber process Summary activities Dynamic equilibrium Contents

© Boardworks Ltd of 33 The Haber process

© Boardworks Ltd of 33 1.Is the forward reaction exothermic or endothermic? 2.Will heating the mixture give an equilibrium mixture with more or less ammonia? 3.Are there more gas molecules of reactant or product? 4.Will raising the pressure give an equilibrium mixture with more or less ammonia? 3H 2 (g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol exothermic less reactant more Making ammonia

© Boardworks Ltd of 33 1.What does the graph show about the effect of temperature on the Haber process? 2.Suggest why a temperature of 400 ° C is chosen when a lower temperature gives an equilibrium mixture with greater % conversion to ammonia. 3H 2 (g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol Hint: reaction rates? Reduces % conversion The Haber process and temperature

© Boardworks Ltd of 33 1.What does the graph show about the effect of pressure on the Haber process? 2.Suggest why a pressure of 200 atm is chosen when a higher pressure gives an equilibrium mixture with greater % conversion to ammonia. 3H 2 (g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol Hint: costs? Increases % conversion The Haber process and pressure

© Boardworks Ltd of 33 3H 2 (g) + N 2 (g) 2NH 3 (g) H=-92kJ/mol The Haber compromise 1.The aim of the chemical industry is not to make chemicals. It is to make money! 2.If we use low temperatures it takes ages to reach equilibrium. Its better to get a 40% yield in 2 minutes than an 80% yield in 2 hours! 3.If we use very high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% conversion at 200 atm than 95% conversion at 600 atm. 4.Unchanged reactants can always be recycled. 5.An iron catalyst is used to speed up the reaction, but it speeds up the reaction in both directions.

© Boardworks Ltd of 33 The Haber process

© Boardworks Ltd of 33 Equilibrium Changing conditions The Haber process Summary activities Dynamic equilibrium Contents

© Boardworks Ltd of 33 Stages of the Haber process

© Boardworks Ltd of 33 Anagrams

© Boardworks Ltd of 33 Multiple-choice quiz