Enthalpy Change

Enthalpy Change

Image

All chemical reactions involve a transfer of energy.

Chemists call this energy change as the enthalpy change of the reaction.

Exothermic reactions have a negative enthalpy change, that is they transfer energy to their surroundings.

Endothermic reactions have a positive enthalpy change, that is they take in energy from their surroundings.

The standard enthalpy change of combustion is the enthalpy change when 1 mole of a compound is burnt completely in oxygen under standard conditions (298K and 100kPa), all reactants and products being in their standard state.

The standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (298K and 100kPa), all reactants and products being in their standard state.

Enthalpy of formation

- is the enthalpy change that occurs when one mole of compound in its standard state if formed from its element in their standard states under standard conditions.

Ionisation enthalpy

- of an element Is the enthalpy change that accompanies the removal of 1 electron in each atom of one mole of gaseous atoms

Enthalpy of atomisation

- of an element is the enthalpy change that occurs when 1 mole of gaseous atoms are formed from the element in its standard state.

Bond dissociation enthalpy

- the enthalpy required to break and separate one mole of bond so that the gaseous atoms exert no force on each other.

Electron affinity

- of an element is the enthalpy change that accompanies the addition of one electron to each atom in one mole of gaseous atoms.

Lattice enthalpy

- of an ionic compound is the enthalpy change which accompanies the formation of one mole of ionic compound from its constituent gaseous ions.

You will need to be able to use the quantities in energy cycle calculations, by applying Hess’s Law.

Remember it doesn’t matter which route a chemical change takes the net energy change stays the same.

Calorimetry

The enthalpy change can be calculated from the temperature change in a reaction using the equation:

q = mcΔT

q is the enthalpy change (J), m is the mass (g) c is the specific heat capacity J g-1K-1, ΔT is the temperature change in K.

Example: excess of magnesium was added to 100cm³ of 2M CuSO₄ solution. The temperature increased by 45K. The specific heat capacity of water is 4.2 J g^-1K^-1

Find q using the above equation q = 100 x 4.2 x 45 =18810J

Find the amount in moles that reacted 2X100/1000 =0.2

Scale the enthalpy change for one mole of CuSO⁴ 18810/0.2 = 94050J =94 kJ mol^-1