Endothermic and Exothermic Reactions C2.5 GCSE Chemistry

Energy Transfer When chemical reactions occur energy is transferred to or from the surroundings

Exothermic Reactions Exothermic reactions increase in temperature Examples include: –Burning reactions including the combustion of fuels. –Self heating cans (e.g. for coffee) –Reaction of acids with metals. –Neutralisation –Hand Warmers

Acivity Say whether these processes are exothermic. 1.Charcoal burning 2.A candle burning. 3.A kettle boiling 4.Ice melting 5.A firework exploding

Say whether these processes are exothermic. 1.Charcoal burning Yes 2.A candle burning Yes 3.A kettle boiling No 4.Ice melting No 5.A firework explodingYes Answers in You have to put heat in for boiling and melting. out You get heat out from all the other processes

Explaining Exothermic Reactions Heat energy is given out. This energy has come from chemical energy in the starting materials (reactants). The reactants convert chemical energy to heat energy. The temperature rises. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. The reaction products have less chemical energy than the reactants had to start with.

Energy Level Diagram – Exothermic Reactions Energy / kJ) Progress of reaction (time) reactants The reactants have more chemical energy. Some of this is lost as heat which spreads out into the room. products The products now have less chemical energy than reactants.

∆H represents the energy change. It is negative for exothermic reactions Exothermic reactions give out energy. There is a temperature rise and  H is negative. products Energy / kJ) Progress of reaction reactants  H is negative

Endothermic Reactions Endothermic reactions cause a decrease in temperature. Examples: Thermal decompositions Sports injury packs Cold packs (from the nurse) Endothermic chemical reactions are relatively rare.

Explaining endothermic reactions Extra energy is needed in order for endothermic reactions to occur. The reactants gain energy. This comes from the substances used in the reaction and the reaction gets cold. Overall the chemicals have gained energy. Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

Energy Level Diagram – Endothermic Reactions products Energy / kJ) Progress of reaction reactants  H=+ This is positive because the products have more energy than the reactants.

Endothermic reactions take in energy. There is a temperature drop and  H is positive. products Energy / kJ Progress of reaction reactants  H=+ ∆H represents the energy change. It is positive for exothermic reactions

Reversible Reactions If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction. The same amount of energy is transferred in each case. hydrated copper sulphate anhydrous copper sulphate CuSO 4.5H20 CuSO 4 + 5H 2 O water Heat