Exothermic reactions
- reactions which gives out energy
- the surroundings gain energy
- bonds are formed between reactants
- Reactants → Products + energy
Examples of Exothermic reactions
- Change of states eg when gas condenses water or water freezes to solid
- Combustion reactions eg burning hydrogen in air
- Dissolving anhydrous salts/acids in water eg sodium carbonate, copper(11)sulphate and concentrated acids in water eg HCl
- Neutralization. When acid and alkali, H+ ions from acid combines with OH- from alkali forming water
- Metal displacement eg reaction of Mg with Copper (11)sulphate
Endothermic reactions
- Reactions which takes heat
- the chemicals gain energy from the surroundings
- bonds breaking
- Reactants + heat → Products
Examples of Endothermic reactions
- Change of states eg when solid melts to water and boils to steam
- Photolysis eg reaction of light sensitive silver chloride in camera reel in light
- Decomposition by heat eg CaCO3
- When acids react with bicarbonate, HCO3
Energy profile diagrams
Enthalpy Change : The amount of energy given out or absorbed during a chemical reation is called enthalpy change, measured in kilojoules. When bonds are made, heat is given out and its exothermic ie enthalpy change is negative and when bonds are broken, heat energy is absorbed, its endothermic, enthalpy change is positive
Activation energy it is the minimum energy needed to start a reaction.