Energy Changes

If the products of a reaction store more energy than the reactants, they must have taken in the difference in energy from the surroundings (endothermic). If the store is less, they have given off the energy which was transferred to the surroundings (exothermic).
The overall amount of energy is not changed but transferred, energy is always conserved.

Exothermic - transfers to surroundings, usually by heat. Causes a rise in temperature.
Examples: burning fuels (combustion), neutralisation reactions (acid + alkali), most oxidisation reactions.
Everyday uses: Some hand warmers use the exothermic oxidation of iron in the air (with a salt catalyst) to release energy. Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals and bases.

Endothermic - takes energy from the surroundings and results in a fall in temperature.
Examples: The reaction between citric acid and sodium hydrocarbonate. Thermal decomposition, e.g. Heating calcium carbonate causes decomposition into calcium oxide and carbon dioxide.
Everyday uses: Some sports injury packs where the chemical reaction causes the pack to become instantly cooler without having to put it in the freezer.

Method:

1) Put 25cm^3 of 0.25mol/dm^3 of hydrochloric acid and sodium hydroxide in seperate beakers.
2) Place the beakers in a water bath set to 25*c until they are both 25*c.
3) Add the HCl followed by the NaOH to a polystyrene cup with a lid.
4) Take the temperature every 30 seconds and record the highest.
5) Repeat steps 1-4 using 0.5 mol/dm^3 and then 1 mol of hydrochloric acid.

Theory - This method tests the effects of acid concentrate on the enrergy released in a nuetralisation reaction between hydrochloric acid and sodium hydroxide but the theory can also works for reactions between metals and acids, or carbonates and acids. As well as the temperature change, it also investigates how different variables have on the amount of energy transferred.

Problems - Energy will be lost to the surroundings. The can be prevented by putting the polystyrene cup ina beaker of cotton wool to give it more insulation and by putting a lid on the cup to prevent evaporation.

During chemical reactions bonds are broken to create new ones. Energy must be supplied to break these bonds, therfore it is endothermic. When new bonds are formed, energy is released so it is a exothermic reaction.
E.g. In exothermic reactions the energy used to form the bonds is greater than the energy to break them.

How to calculate an energy change:
1) Balance the equation.
2) Draw out the molecules on each side of the arrow.
3) Calulate the number of each molecule.
4) calculate the bond energy for each bond.
5) Decide whether it is exothermic or endothermic.

• if the enegy for the reactants is higher it is exothermic.
• if the energy for the products is higher it is endothermic.

Reaction profiles show the relative energies of the reactants and products in a reaction and how the energy changes over the course of a reaction.