Exothermic and Endothermic Reactions

Exothermic Reactions Examples include: Burning reactions including the combustion of fuels. Detonation of explosives. Reaction of acids with metals. Exothermic reactions increase in temperature. Magnesium reacting with acid Thermit reaction

Exothermic Reactions Magnesium + Hydrochloric acid 25o C 45o C Heat energy given out Gets hot Hydrochloric acid

Exothermic Reactions If heat is given out this energy must have come from chemical energy in the starting materials (reactants). 45o C 25o C Reactants convert chemical energy to heat energy. The temperature rises.

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

Energy of reactants and products in Exothermic Reactions H is how much energy is given out Energy / kJ Progress of reaction reactants products H=negative H is given a negative sign for exothermic reactions

Definition of an Exothermic Reaction 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. Heat energy Is used up during the reaction so the contents in tube feel cold Ammonium nitrate Water Cools Starts 25°C Cools to 5°C

Explaining Endothermic Reactions Extra energy is needed in order for endothermic reactions to occur. This comes from the thermal energy of the reaction mixture which consequently gets colder. 5o C 25o C Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

Energy of Reactants and Products in an Endothermic Process This is how much energy is taken in products Energy / kJ) Progress of reaction reactants H=+ H is given a positive sign for endothermic reactions

Definition of Endothermic Reactions Endothermic reactions take in energy. There is a temperature drop and H is positive. products Energy / kJ Progress of reaction reactants H=+

Are these endothermic or exothermic? Activity Are these endothermic or exothermic? A red glow spread throughout the mixture and the temperature rose. The mixture bubbled vigorously but the temperature dropped 150C. Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space. The decaying grass in the compost maker was considerably above the outside temperature. exo endo exo exo

Activation energy Energy is given out as new bonds form Chemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken. This means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur. Activation energy needed to break existing bonds New bonds form Reactants Old bonds start to break

Explaining Activation Energy Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started. Energy in chemicals Reaction Activation energy Break old bonds Energy taken in Form new bonds Energy given out

Activation Energy and Exothermic Reactions reactants H= - Energy / kJ) products Progress of reaction

Activation Energy and Endothermic Reactions products H=+ Energy / kJ) reactants Progress of reaction

Activity Use this diagram to explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps. ActivationEnergy Petrol + oxygen Energy / kJ) Carbon dioxide + water Progress of reaction

Answer The reaction is exothermic but requires the Activation energy to be provided before the reaction can get underway. This is necessary to break some of the bonds in the oxygen or petrol before new bonds can start forming. ActivationEnergy Petrol + oxygen Energy / kJ) Carbon dioxide + water Progress of reaction

Catalysts For chemical reactions to occur: Existing bonds have to begin breaking so that new ones can be formed. The molecules have to collide in such a way that the reacting parts of the molecules are brought together. Catalysts can help with either or both of these processes. A catalyst is a substance that speeds up a reaction without getting used up in the process.

Effect of a Catalyst on Activation Energy

Inside car engines some of the nitrogen and oxygen from the air combine to form poisonous nitrogen oxide. Inside the exhaust system a catalyst encourages decomposition back into nitrogen and oxygen. Copy the energy profile for the uncatalysed reaction and draw in new lines showing how the presence of a catalyst will alter the profile. 2NO2 N2 + 2O2