Revision:Energetics

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Energetics

Enthalpy Change

Enthalpy, H, is the amount of heat energy stored in the system. Enthalpy change, ΔH, is the change in enthalpy of the chemicals in the system during a reaction. Exothermic reactions release energy into the surroundings, so the enthalpy of the system decreases (from the first law of thermodynamics - that total energy is conserved) and ΔH is negative. The reverse is true for endothermic reactions, so ΔH is positive.

Enthalpy changes can be measured for many reactions, under standard conditions (Represented by a ^o), which are 298K, 1 atmosphere, and 1 molar for solutions. The standard state is the state of a substance under these conditions. All standard enthalpy changes are with products and elements in standard states, and under standard conditions.

The standard enthalpy change of reaction, ΔH^o_r, is the enthalpy change of a reaction for the number of moles of each substance in the chemical equation. ΔH^o_r should always be used with an equation.

The standard enthalpy change of formation, ΔH^o_f, is the enthalpy change when one mole of a compound is formed from its elements. For an element, this is zero.

The standard enthalpy change of combustion, ΔH^o_c, is the enthalpy change when one mole of a substance reacts completely with oxygen.

The standard enthalpy change of neutralisation, ΔH^o_neut, is the enthalpy change when one mole of water is formed from the neutralisation of an acid and a base:

The standard enthalpy change of atomisation, ΔH^o_at, is the enthalpy change when one mole of gaseous atoms are formed from the element in its standard state:

Hess' Law

Hess' law states that the total enthalpy change of a reaction is the same regardless of the route taken. A Hess' law diagram is similar to a vector triangle (Diagram to be added).

Specific Heat Capacity

The energy change of the surroundings, Q, can be calculated from , where m is the mass of the surroundings, c is the specific heat capacity, and ΔT is the change in temperature of the surroundings.

Bond Enthalpy

Bond enthalpy is the enthalpy change when one mole of bonds in a gaseous substance are broken, with the products experiencing no forces from each other. Bonds between atoms have different strengths in different compounds. The average bond enthalpy gives an idea of the mean strength of the bonds. Breaking bonds requires energy, so is endothermic, while making bonds is exothermic and releases energy. The enthalpy change of a reaction is the sum of the bond enthalpies of all bonds of reactants minus the sum of bond enthalpies of products.

Notes

This topic involves a lot of calculations which don't yet have examples on this page.