12.5 Do Chemical Reactions Always Release Energy? 14/04/2017 12.5 Do Chemical Reactions Always Release Energy? Heolddu Comprehensive School

Rates of Reaction – A reminder 14/04/2017 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: The temperature of the reactants, Their concentration Their surface area Whether or not a catalyst is used

Endothermic and exothermic reactions 14/04/2017 Step 1: Energy must be SUPPLIED to break bonds: Energy Step 2: Energy is RELEASED when new bonds are made: Energy 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 level diagrams Energy level Activation energy 14/04/2017 Energy level Activation energy Using a catalyst might lower the activation energy Energy given out by reaction Reaction progress

Exothermic vs endothermic: 14/04/2017 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)

Examples of Energy Profile Diagrams 14/04/2017 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.

14/04/2017 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 endothermic chemical reaction without showing the activation energy. Energy level diagram for an exothermic chemical reaction without showing the activation energy.

The reverse reaction can be used as a test for water Reversible Reactions 14/04/2017 Some chemical reactions are reversible. In other words, they can go in either direction: A + B C D e.g. Ammonium chloride Ammonia + hydrogen chloride NH4Cl NH3 + HCl 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 CuSO4 + H2O CuSO4.5H2O The reverse reaction can be used as a test for water

Endothermic reactions Reversible Reactions 14/04/2017 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 + B C D Endothermic reactions Increased temperature: Decreased temperature: Exothermic reactions Increased temperature: Decreased temperature: A + B C D A + B C D More products Less products A + B C D A + B C D H Tier only H Tier only Less products More products

Reversible reactions and effect of temperature 14/04/2017 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 + B C D Exothermic Endothermic

Equilibrium in reactions involving gases 14/04/2017 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. N2 + 3H2 2NH3 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!

Making Ammonia Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 14/04/2017 Fritz Haber, 1868-1934 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. Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 To produce ammonia from nitrogen and hydrogen you have to use three conditions: High pressure 450O C Iron catalyst Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy. Nitrogen Hydrogen Recycled H2 and N2

Ammonia + nitric acid Ammonium nitrate Uses of Ammonia 14/04/2017 Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid: Water and oxygen Nitrogen monoxide Ammonia gas Oxygen Hot platinum catalyst Nitrogen monoxide Nitric acid Cooled More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen). Ammonia + nitric acid Ammonium nitrate NH3 + HNO3 NH4NO3 The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.

Higher Tier Only-Haber Process:The economics 14/04/2017 A while ago we looked at reversible reactions: A + B C D Endothermic, increased temperature A + B C D Exothermic, increase temperature Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 Endothermic Exothermic 1) If temperature was DECREASED the amount of ammonia formed would __________... However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster If pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation

Haber Process Summary 14/04/2017 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 To compromise all of these factors, these conditions are used: 200 atm pressure 450O C Iron catalyst Recycled H2 and N2 Nitrogen Hydrogen Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.