Energy changes in reactions Exothermic and Endothermic – what do they mean?

What’s the difference between these two reactions?

HEAT OUT HEAT IN

Exothermic Reactions Exothermic reactions increase in temperature. HEAT EXITS the reaction so it gets hotter on the outside

Endothermic Reactions Endothermic reactions decrease in temperature. HEAT Enters the reaction so it gets colder on the outside.

Endothermic or exothermic Liquid Temperature (°C) Solid Min or Max temp (°C) Endothermic or exothermic Hydrochloric acid Magnesium Water Ammonium nitrate Citric acid solution Sodium hydrogen carbonate Copper sulfate solution Zinc

Summary Q’s Name an example of an exothermic reaction Name an example of an endothermic reaction What happens to the energy in an endothermic reaction? How do you know? What happens to the energy in an exothermic reaction? How do you know?

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.

Exothermic Reactions Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. 25o C 45o C Chemical energy becomes heat energy. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings. It follows that reaction products have less chemical energy than the reactants had to start with.

Say whether these processes are exothermic. Activity Say whether these processes are exothermic. Charcoal burning A candle burning. A kettle boiling Ice melting A firework exploding yes yes no no yes You have to put heat in for boiling and melting. You get heat out from all the other processes

Endothermic Reactions Endothermic reactions cause a decrease in temperature. Endothermic chemical reactions are relatively rare. A few reactions that give off gases are highly endothermic - get very cold. Dissolving salts in water is another process that is often endothermic.

Endothermic Reactions Endothermic reactions cause a decrease in temperature. Ammonium nitrate Water Heat energy taken in as the mixture returns back to room temp. Cools Starts 25°C Cools to 5°C Returns to 25°C

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.

Endothermic Reactions The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. 25o C 5o C 25o C The reactants gain energy. This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy.

Summary Energy is absorbed to break bonds. Bond- breaking is an endothermic process. Energy is released when new bonds form. Bond- making is an exothermic process. Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form. If more heat energy is released when making the bonds than was taken in when they broke, the reaction is exothermic.

Endothermic or exothermic Liquid Temperature (°C) Solid Min or Max temp (°C) Endothermic or exothermic Hydrochloric acid Magnesium Water Ammonium nitrate Citric acid solution Sodium hydrogen carbonate Copper sulfate solution Zinc What is happening in an exothermic reaction? What is happening in an endothermic reaction?