Presentation on theme: "12.5 Do Chemical Reactions Always Release Energy?"— Presentation transcript:
1 12.5 Do Chemical Reactions Always Release Energy? 14/04/201712.5 Do Chemical Reactions Always Release Energy?Heolddu Comprehensive School
2 Rates of Reaction – A reminder 14/04/2017Chemical 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 concentrationTheir surface areaWhether or not a catalyst is used
3 Endothermic and exothermic reactions 14/04/2017Step 1: Energy must be SUPPLIED to break bonds:EnergyStep 2: Energy is RELEASED when new bonds are made:EnergyA reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC
4 Energy level diagrams Energy level Activation energy 14/04/2017Energy levelActivation energyUsing a catalyst might lower the activation energyEnergy given out by reactionReaction progress
5 Exothermic vs endothermic: 14/04/2017EXOTHERMIC – 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 Examples of Energy Profile Diagrams 14/04/2017Very 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 14/04/2017A 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.
8 The reverse reaction can be used as a test for water Reversible Reactions14/04/2017Some chemical reactions are reversible. In other words, they can go in either direction:A+BCDe.g. Ammonium chlorideAmmonia + hydrogen chlorideNH4ClNH3 + HClIf 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+ WaterCuSO4 + H2OCuSO4.5H2OThe reverse reaction can be used as a test for water
9 Endothermic reactions Reversible Reactions14/04/2017When 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+BCDEndothermic reactionsIncreased temperature:Decreased temperature:Exothermic reactionsIncreased temperature:Decreased temperature:A+BCDA+BCDMore productsLess productsA+BCDA+BCDH Tier onlyH Tier onlyLess productsMore products
10 Reversible reactions and effect of temperature 14/04/2017Reversible reactions and effect of temperatureIf 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+BCDExothermicEndothermic
11 Equilibrium in reactions involving gases 14/04/2017In 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 + 3H NH3There are 4 molecules on the left and 2 on the rightTherefore an increase in pressure would shift this reaction to the right – more ammonia is made!
12 Making Ammonia Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 14/04/2017Fritz 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.Nitrogen + hydrogen AmmoniaN2 + 3H NH3To produce ammonia from nitrogen and hydrogen you have to use three conditions:High pressure450O CIron catalystMixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.NitrogenHydrogenRecycled H2 and N2
13 Ammonia + nitric acid Ammonium nitrate Uses of Ammonia14/04/2017Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid:Water and oxygenNitrogen monoxideAmmonia gasOxygenHot platinum catalystNitrogen monoxideNitric acidCooledMore ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen).Ammonia + nitric acid Ammonium nitrateNH3 + HNO NH4NO3The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.
14 Higher Tier Only-Haber Process:The economics 14/04/2017A while ago we looked at reversible reactions:A+BCDEndothermic, increased temperatureA+BCDExothermic, increase temperatureNitrogen + hydrogen AmmoniaN2 + 3H NH3EndothermicExothermic1) 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 fasterIf pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation
15 Haber Process Summary14/04/2017A low temperature increases the yield of ammonia but is too slowA high temperature improves the rate of reaction but decreases the yield too muchA high pressure increases the yield of ammonia but costs a lot of moneyTo compromise all of these factors, these conditions are used:200 atm pressure450O CIron catalystRecycled H2 and N2NitrogenHydrogenMixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.
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