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05/05/2015 12.5 Do Chemical Reactions Always Release Energy? Heolddu Comprehensive School.

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Presentation on theme: "05/05/2015 12.5 Do Chemical Reactions Always Release Energy? Heolddu Comprehensive School."— Presentation transcript:

1 05/05/ Do Chemical Reactions Always Release Energy? Heolddu Comprehensive School

2 05/05/2015 Rates of Reaction – A reminder Chemical reactions occur when different atoms or molecules collide: For the reaction to happen the particles must have a certain amount of energy – this is called the ACTIVATION ENERGY. The rate at which the reaction happens depends on four things: 1)The temperature of the reactants, 2)Their concentration 3)Their surface area 4)Whether or not a catalyst is used

3 05/05/2015 Endothermic and exothermic reactions Step 1: Energy must be SUPPLIED to break bonds: Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC Energy

4 05/05/2015 Energy level diagrams Energy level Reaction progress Activation energy Energy given out by reaction Using a catalyst might lower the activation energy

5 05/05/2015 Exothermic vs endothermic: EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration) ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)

6 05/05/2015 Examples of Energy Profile Diagrams Very endothermic reaction with a big activation energy. Very exothermic reaction with a small activation energy. Moderately endothermic reaction with moderately high activation energy. Moderately exothermic reaction with a moderately high activation energy.

7 05/05/2015 A small activation energy reaction with no net energy change. (Possible if the total energy absorbed by the reactants in bond breaking equals the energy released by bonds forming in the products) Energy level diagram for an exothermic chemical reaction without showing the activation energy. Energy level diagram for an endothermic chemical reaction without showing the activation energy.

8 05/05/2015 Reversible Reactions Some chemical reactions are reversible. In other words, they can go in either direction: A+BC+D NH 4 ClNH 3 + HCl e.g. Ammonium chlorideAmmonia + hydrogen chloride If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction? For example, consider copper sulphate: Hydrated copper sulphate (blue) Anhydrous copper sulphate (white) + Heat+ Water CuSO 4 + H 2 OCuSO 4.5H 2 O The reverse reaction can be used as a test for water

9 05/05/2015 Reversible Reactions When a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactants are added or taken away) an EQUILIBRIUM is achieved – in other words, the reaction goes at the same rate in both directions: A+BC+D Endothermic reactions Increased temperature: Decreased temperature: A+BC+D A+BC+D More products Less products Exothermic reactions Increased temperature: Decreased temperature: A+BC+D Less products More products A+BC+D H Tier only

10 05/05/2015 Reversible reactions and effect of temperature If the temperature is raised, the yield from the __________ reaction increases and the yield from the ________ reaction decreases. If the temperature is lowered, the yield from the endothermic reaction _______ and the yield from the exothermic reaction ________. A+BC+D Exothermic Endothermic

11 Equilibrium in reactions involving gases In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction. 05/05/2015 N 2 + 3H 2 2NH 3 There are 4 molecules on the left and 2 on the right Therefore an increase in pressure would shift this reaction to the right – more ammonia is made!

12 05/05/2015 Making Ammonia Nitrogen + hydrogen Ammonia N 2 + 3H 2 2NH 3 High pressure 450 O C Iron catalyst Recycled H 2 and N 2 Nitrogen Hydrogen Mixture of NH 3, H 2 and N 2. This is cooled causing NH 3 to liquefy. Fritz Haber, Guten Tag. My name is Fritz Haber and I won the Nobel Prize for chemistry. I am going to tell you how to use a reversible reaction to produce ammonia, a very important chemical. This is called the Haber Process. To produce ammonia from nitrogen and hydrogen you have to use three conditions:

13 05/05/2015 Uses of Ammonia Nitrogen monoxide Hot platinum catalyst Ammonia gas Oxygen Cooled Water and oxygen Nitrogen monoxide Nitric acid Ammonia + nitric acid Ammonium nitrate NH 3 + HNO 3 NH 4 NO 3 Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid: More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen). The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.

14 05/05/2015 Higher Tier Only-Haber Process:The economics A while ago we looked at reversible reactions: A+BC+D Endothermic, increased temperature A+BC+D Exothermic, increase temperature ExothermicEndothermic 1) If temperature was DECREASED the amount of ammonia formed would __________... 2)However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster 3)If pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation Nitrogen + hydrogen Ammonia N 2 + 3H 2 2NH 3

15 05/05/2015 Haber Process Summary 200 atm pressure 450 O C Iron catalyst Recycled H 2 and N 2 Nitrogen Hydrogen Mixture of NH 3, H 2 and N 2. This is cooled causing NH 3 to liquefy. To compromise all of these factors, these conditions are used: A low temperature increases the yield of ammonia but is too slow A high temperature improves the rate of reaction but decreases the yield too much A high pressure increases the yield of ammonia but costs a lot of money


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