Endothermic and exothermic reactions Year 9 Science Seaview High School

Not always a reaction Placing two chemicals together doesn’t mean there will be a reaction. For example, hydrogen gas and oxygen gas will react violently. However, if they are kept in cool conditions in a secure container, they can be stored together indefinitely. It takes energy to start the reaction.

Reactions that need energy Energy must be provided for the reaction to take place. The energy breaks the bonds in the reactant and allows the atoms to form new bonds in the product. We include the energy source in the equation as shown below: Hydrogen + oxygen heat water

Reactions that give energy Chemical reactions can provide energy as the new bonds form. When fuels like petrol are burned, energy is released to keep your car moving. In this chemical reaction, fuel reacts with oxygen as it burns to produce carbon dioxide, water and some other products.

Exothermic reactions When there is less energy stored in the bonds of the products than there was in the bonds of the reactants, energy is released by the reaction. Chemical reactions that release energy are said to be ‘exothermic’ reactions. We can determine whether a reaction is exothermic or not by comparing the temperature before and after the reaction.

Endothermic reactions When there is more energy stored in the bonds of the products than there was in the bonds of the reactants, energy is absorbed by the reaction. Chemical reactions that absorb energy are said to be ‘endothermic’ reactions. We can determine whether a reaction is endothermic or not by comparing the temperature before and after the reaction.

Releasing heat Portable hand warmers, such as those pictured below, use exothermic reactions to release heat. These commonly contain iron, water, salt and sawdust. Shaking the packet causes the iron to mix with oxygen forming iron oxide with some energy released as heat. Iron + oxygen  iron oxide

Cold as ice First aid kits often contain instant cold packs. These cold packs are plastic bags filled with ammonium nitrate or ammonium chloride and a smaller inner bag filled with water. Squeezing the bag causes the water bag to break initiating the reaction. Ammonium + water  ammonium chloride chloride in solution Not a chemical reaction as no new substance formed. Bonds are broken.

Exothermic reactions save lives! The inflation of airbags are a result of an explosive exothermic reaction. The products are sodium and a rapid release of nitrogen gas. The crash sensor provides electrical energy to the reaction. sodium azide (NaN3)  sodium + nitrogen