3.1.4 Energetics

Mean bond enthalpy values may be different because the data is:

From a range of compounds In the gaseous state.

We now know two ways of calculating H values:

using Hess's Law with ΔHc⦵ and ΔHf⦵ using mean bond enthalpies

Hess' Law can be used to calculate many enthalpy changes which cannot be measured directly, often using data on enthalpies of formation and combustion. Give two reasons why it might not be possible to measure an enthalpy change of a particular reaction:

1. The reaction does not occur under standard conditions. 2. Activation energy could be too high 3. Rate of reaction could be too slow

Standard conditions:

298K and 100kPa Standard states are the states that elements and compounds are found in under standard conditions.

C3H6 (g) + 9/2O2 (g) →

3CO2 (g) + 3H2O (l)

C8H18 (l) + 25/2O2 (g) →

8CO2 (g) + 9H2O (l)

Reactions in Solution Experimental details:

A polystyrene cup is often used as the container as it partly insulates the reaction to reduce heat loss to the surroundings. The beaker can be placed into a larger beaker or a lid added: again to reduce heat loss to the surroundings.

Assumption:

All solutions have a density of 1 gcm-3 and a specific heat capacity of 4.18 Jg-1 K-1.

Endothermic reaction

An endothermic reaction is one where heat energy is taken into a system from the surroundings; ΔH is positive (the measured temperature decreases).

Exothermic reaction

An exothermic reaction is one where heat energy is given out from a system to the surroundings; ΔH is negative (the measured temperature increases).

C (s) + 3/2H2 (g) + 1/2Br2 (l) →

CH3Br (g)

3 steps to any calorimetry calculation:

Calculate q (in J then kJ) Calculate n ΔH = q ÷ n (including sign)

This type of enthalpy measurement can often be lower than expected (from databook values):

Due to incomplete combustion of the fuel. It is also likely that some evaporation of the fuel will occur due to its high volatility which will also reduce the experimental enthalpy change.

Calorimetry values are smaller because:

Energy is lost to the surroundings. The specific heat capacity of the equipment is different to that of water. Incomplete combustion occurred.

Which of these do you think will give the more accurate value? Hess's cycles

Explain why: For Hess's cycles the data is specific to that particular reaction. Mean bond enthalpies are average values and may not be accurate for the specific bonds present in the given reaction.

Calorimetry

Measures the energy change in a reaction usually by heating a volume of water which is then converted to an enthalpy change.

1/2N2 (g) + 3/2H2 (g) →

NH3 (g)

Combustion Reactions Experimental details:

The container of water should be placed as close as possible to the flame to minimise heat loss to the surroundings. Ideally, all the heat produced by the fuel would be used to heat the water, not the surroundings e.g. air/ container/ thermometer etc. A lid can be added, again to reduce heat loss to the surroundings.

Hess Law values are the most accurate, although the reaction has been completed under standard conditions because:

The data is specific to the compounds in the calculation.

Hess' Law states that:

The enthalpy change for a chemical reaction is independent of the route taken.

Enthalpy change

The enthalpy change of a reaction is the change in heat energy measured at constant pressure. Symbol: ΔH Units: kJmol-1

Mean bond enthalpies

The mean bond enthalpy is the average energy required to separate completely the atoms in one mole of covalent bonds in the gas phase. The average energy needed to break one mole of bonds from a range of compounds in the gaseous state.

Hess Law calculations are useful for enthalpy changes which cannot be measured directly, if:

The reaction does not proceed under standard conditions; Other products may be formed; The rate of reaction is too slow; The activation energy is too high.

Standard Enthalpy of Combustion, ΔHϴc

The standard enthalpy of combustion (ΔHϴc) is the enthalpy change when one mole of a substance is burned in excess oxygen with all reactants and products in their standard states under standard conditions.

Standard enthalpy of formation, ΔHfϴ

The standard enthalpy of formation (ΔHfϴ) is the enthalpy change when one mole of a compound is formed from its elements where all reactants and products are in their standard states under standard conditions.

Heat energy of reaction:

q (in J) = mcΔT

Mean bond enthalpies ΔH =

sum of bond energies in reactants - sum of bond energies in products.

ΔHfϴ of elements=

zero

Enthalpy change:

ΔH = q in kJ / moles