The theory behind enthalpy changes

DH = -xkJmol-1

An endothermic enthalpy change is always given a positive value, as the energy is gained by the system from the surroundings.

DH = + ykJmol-1.

Standard enthalpy changes: standard conditions

If we are to compare the enthalpy changes of a various reactions we must use standard conditions, such as known temperatures, pressures, amounts and concentrations of reactants or products.

The standard conditions are:

A pressure of 100kilopascals (102kPa)

A temperature of 298K (25oC)

Reactants and products in physical states, normal for the above conditions.

A concentration of 1.0mol dm-3 for solutions.

The o sign indicates standard conditions.

Standard enthalpy change of reaction - DHor

The standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation for the reaction, react under standard conditions to give the products in their standard states.

Standard enthalpy change of formation - DHof

The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions; both compound and elements are in standard states

Standard enthalpy change of combustion - DHoc

The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.

Bond breaking and bond making

For a chemical reaction to occur bonds must break before new bonds can be made. When bonds break energy is absorbed (endothermic). When bonds form, energy is released (exothermic).

If the energy absorbed whilst making bonds is greater than the energy transferred to the surroundings as bonds are made, then an endothermic reaction will occur

Whereas, if the energy released on bond formation is greater than that absorbed through breaking bonds then an exothermic reaction is observed.

Bond energy: This is the amount of energy required to break a covalent bond, it indicates the strength of a bond. Values are always quoted as bond energy per mole, E.

Example:

O2(g) (r) 2O(g)

DH = +498kJmol-1

Values are always positive, as they refer to bond breaking which is endothermic. They are average values as a particular bond energy will depend upon the molecule the bond is held within, hence average values are taken.

The following measurements are taken:

. Mass of cold water (g)

2. Temperature rise of the water (K)

3. The loss of mass of the fuel (g)

We know that it takes 4.2J of energy to raise the temperature of 1g of water by 1K. This is called the specific heat capacity of water, c, and has a value of 4.2Jg-1K-1.

Hence, energy transferred can be calculated using:

Energy transfer = mcDT (joules)

If one mole of the fuel has a mass of M grams, then:

Enthalpy transfer = m x 4.2 x T x M/y

where y is mass loss of fuel.

Enthalpy changes of reactions

The apparatus required is different to that used in measurements of combustion, since we are not burning a fuel.