Chemistry 3

When fuels and foods react with oxygen they are exothermic. The amount of energy released when a substance is burned can be measured using a calorimeter.

A simple calorimeter is some water in a glass beaker, when a substance is burned and heats the water the temperature rise depends on the amount of energy released. To calculate the energy released we can use the equation:

Mass of the water (g) x specific heat capacity of water (j/g*c) x temp change (*c)

To reduce energy transfers to or from the surroundings in a reaction we can use a insulated cup. 

Energy level diagrams show the energy changes during chemical reactions Breaking bonds is endothermic because energy is taken in from the surroundings. Forming bonds is exothermic because energy is released into the surroundings.

The minimum energy needed for the reaction to happen is the activation energy. Catalysts increases the rate of the reaction by reducing activation energy.               

In a chemical reaction energy is needed to break bonds in the reactants and energy is released when new bonds are formed in the products. The energy needed to break the bond between two atoms is called bond energy.

An equal amount of energy is released when the bond forms between two atoms so we can use bond energies to calculate the overall energy change for a reaction.

To do this we need to work out:

- the total amount of energy needed to break all of the reactant bonds

- the total amount of energy released in making of of the product bonds

- the difference between the two totals (energy released - energy needed)