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Starter: In the back of your book. List 5 chemical changes. List 4 physical changes. List 3 exothermic reactions. List 2 endothermic reactions. List one way that you can tell a reaction is exothermic. Objectives. To develop scientific vocabulary.
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Starter: In the back of your book . . . • List 5 chemical changes. • List 4 physical changes. • List 3 exothermic reactions. • List 2 endothermic reactions. • List one way that you can tell a reaction is exothermic.
Objectives • To develop scientific vocabulary. • To understand how exothermic and endothermic reactions can be shown on graphs. • To be able to complete bond calculations.
Making water • What is the type of reaction? Chemical change. • Is the reaction exothermic or endothermic? Exothermic. • What is the word equation for the reaction? Hydrogen + Oxygen Water • What is the balanced symbol equation for the reaction? 2H2(g) + O2(g) 2H2O(l) http://www.youtube.com/watch?v=kknU6cpKWL0
H–H H + H H2 2H Stage 1 Energy input H = +436 kJ/mol
O + O Stage 1 O=O Energy output ΔH = –498 kJ/mol
O + O + H + H + H + H H-O-H H-O-H Stage 2 Energy output ΔH = –486 kJ/mol
Reactants have more chemical energy. reactants Some of this is lost as heat which spreads out into the room. Energy / (kJ) Products now have less chemical energy than reactants. products Progress of reaction (time) Energy level diagram – exothermic reaction
Energy level diagram - water Energy
Energy level diagram – endothermic reaction • Have a go at sketching what shape of graph you would expect – and why do you think that? Energy
This is how much energy is taken in products This is positive because the products have more energy than the reactants. H=+ Energy / kJ) reactants Progress of reaction Energy level diagram – endothermic reaction
The reactants Bonds are broken. This takes energy. This is endothermic. The products. Bonds are made. This gives out energy. This is exothermic. What is happening in the reactions? ΔH reaction = Energy of bonds made – energy of bonds broken.
Calculations for water 2H2(g) + O2(g) 2H2O(l)
Activity H H O O O O H H O O H H H H O O • Hydrogen peroxide decomposes as shown: • Calculate energy for bond breaking. • Calculate the energy from bond making • What is the value of H for the reaction shown
Answer H H O O O O H H O O H H H H O O Bond breaking. (endothermic) 4(O-H) + 2(O-O) =1856+292 = +2148kJ Bond forming: (exothermic) 4(O-H) + 1(O=O) =1856+498 = -2354kJ H = +2148 – 2354 = -206kJ (Exothermic)