Energy changes in chemical reactions

When the head of a match is struck, energy is released. Some of this energy heats the surroundings and some is released as light.

Energy is released in chemical reactions when the reactants have more chemical energy than the products.

The chemical energy of a substance is the potential energy stored within it. This stored chemical potential energy is called the heat content or enthalpy of the substance and is given the symbol H.

System and surroundings System – the collection of atoms or molecules involved in a chemical reaction. Surroundings – anything around, or in contact with the system eg air, or the solvent.

System and surroundings If the enthalpy of a system decreases during a chemical reaction, then that same amount of energy must be released to the surroundings. This will cause the temperature of the surroundings to increase.

System and surroundings Note that the enthalpy is potential energy in the system, but it is transferred to the surroundings as heat.

What will happen to the temperature of the surroundings in each of these cases?

Endothermic reaction  H positive heat absorbed from surroundings, causing a drop in the temperature of the surroundings Exothermic reaction  H negative heat released to the surroundings, causing an increase in the temperature of the surroundings

Ba(OH) 2 (s) + 2 NH 4 SCN (s)  Ba(SCN) 2(s) + 2 NH 3(g) + 2 H 2 O (l) This reaction is endothermic. The system gains enthalpy, and the surroundings lose heat.

The change in enthalpy can be shown in the equation; or written after the equation;