AS-Level Chemistry Chapter 6 Chemical Energetics Standard Enthalpy Changes

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Standard Enthalpy Changes

The standard condition

1) To make comparison of enthalpy changes a fair comparison, same conditions must be used. These are called the standard conditions:

- A pressure of 100 kPa(approximately atmospheric pressure).
- A temperature of 298 K or 25 °C.
- Where solutions are involved, a concentration of 1.0 mol dm⁻³.
- Every substance involved must be in its normal physical state at 100 kPa and 298 K. For example, water is in liquid, not ice or steam.
- If allotropes are involved, the allotrope which is more energetically stable is used. For example, for carbon, graphite is chosen over diamond because graphite is energetically more stable than diamond.

2) Standard enthalpy changes can be represented by the symbol ΔH°. This refers that the reaction is carried out under standard conditions. For example:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ; ΔH° = -890.3 kJ mol⁻¹

Various enthalpy changes

1) Enthalpy changes are described according to the type of reaction. Examples:

- standard enthalpy change of reaction, ΔH°r (in general)
- standard enthalpy change of formation, ΔH°f
- standard enthalpy change of combustion, ΔH°c
- standard enthalpy change of neutralisation, ΔH°n
- standard enthalpy change of atomisation, ΔH°at
- standard enthalpy change of solution, ΔH°sol
- standard enthalpy change of hydration, ΔH°hyd

Standard enthalpy change of reaction, ΔH°r

1) Standard enthalpy change of reaction, ΔH°r is the enthalpy change when the amount of reactants shown in the equation react to give products under standard condition. The reactants and products must be in their standard states.

2H2(g) + O2(g) → 2H2O(l) ; ΔH°r = -576 kJ mol⁻¹

2) The equation shows that 576 kJ of energy is released when two moles of hydrogen react with one mole of oxygen to give two moles of water.

3) This is a theoretical reaction, it does not happen in practice. The enthalpy change can be found by applying Hess' law.

Standard enthalpy change of formation, ΔH°f

1) Standard enthalpy change of formation, ΔH°f is the enthalpy change when one mole of a compound is formed from its elements under standard condition. The reactants and products must be in their standard states.

2Fe(s) + 1½O2(g) → Fe2O3(s) ; ΔH°f [ Fe2O3(s) ] = -824.5 kJ mol⁻¹

2) By definition, the standard enthalpy change of formation of an element is zero.

3) The standard enthalpy change of formation can be exothermic or endothermic.

Standard enthalpy change of combustion, ΔH°c

1) Standard enthalpy change of combustion, ΔH°c is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. The reactants and products must be in their standard states.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ; ΔH°c [ CH4(g) ] = -890.3 kJ mol⁻¹

2) The standard enthalpy change of combustion is always exothermic.

Standard enthalpy change of neutralization, ΔH°n

1) Standard enthalpy change of neutralization, ΔH°n is the enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) ; ΔH°n = -57.1 kJ mol⁻¹

2) For any acid-alkali reaction, the ionic equation is:

H⁺(aq) + OH⁻(aq) → H2O(l)

3) The enthalpy change of neutralization between strong acids and strong bases is a constant(-57.1 kJ mol⁻¹). This is because all strong acids and strong bases dissociate completely in water to form aqueous ions.

4) So, neutralization between strong acids and strong bases involves the same reaction, that is, H⁺ reacts with OH⁻ to form H2O. The other ions present are just simply spectator ions. They do not take part in the reaction. Hence, the heat released is the same.

5) However, the enthalpy change of neutralization between sulfuric acid and sodium hydroxide is more exothermic(-66.8 kJ mol⁻¹) than expected. This is because the enthalpy change of dilution of sufuric acid is significant. When sulfuric acid is added to sodium hydroxide, the acid is diluted in the process and heat is released.

6) The enthalpy change of neutralization involving weak acids or weak bases is less than 57.1 kJ mol⁻¹. For example:

CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l) ;

ΔH°n = -56.1 kJ mol⁻¹

NaOH(aq) + HCN(aq) → NaCN(aq) + H2O(l) ; ΔH°n = -11.7 kJ mol⁻¹

7) This is because weak acids and weak bases only partially dissociated in water. For example:

CH3COOH(aq) 􂾮 CH3COO⁻(aq) + H⁺(aq) ; ΔH° = positive

8) On addition of strong base such as NaOH, the OH⁻ ions react with H⁺ from the dissociation of CH3COOH. The removal of H⁺ ions causes the position of equilibrium to shift to the right and more CH3COOH molecules dissociate. However the dissociation of CH3COOH is an endothermic process, hence, some energy is absorbed and the enthalpy change overall is less negative.

9) The standard enthalpy change of neutralization is always exothermic.

Standard enthalpy change of atomization, ΔH°at

1) Standard enthalpy change of atomization, ΔH°at is the enthalpy change when one mole of gases atoms is formed from its element under standard conditions.

½H2(g) → H(g) ; ΔH°at [ ½H2 ] = +218 kJ mol⁻¹

Na(s) → Na(g) ; ΔH°at [ Na ] = +107 kJ mol⁻¹

2) By definition, the standard enthalpy change of atomization of the noble gases is zero because all of them exist as monoatomic gases at standard conditions.

3) The standard enthalpy change of atomization of diatomic gases(example: O2, Cl2, N2 and F2) is equal to half the value of their bond energies.

4) The standard enthalpy change of atomization is always endothermic.

Standard enthalpy change of solution, ΔH°sol

1) Standard enthalpy change of solution, ΔH°sol is the enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions.

NaCl(s) + aq → NaCl(aq) ; ΔH°sol = +6.0 kJ mol⁻¹

NaOH(s) + aq → NaOH(aq) ; ΔH°sol = -44.5 kJ mol⁻¹

2) An infinitely dilute solution is one which does not produce any further enthalpy change when more solvent is added.

3) The standard enthalpy change of solution can be exothermic or endothermic.

Standard enthalpy change of hydration, ΔH°hyd

1) Standard enthalpy change of hydration, ΔH°hyd is the enthalpy change when one mole of gases ions dissolves in water to form hydrated ions of infinite dilution under standard conditions.

Ca²⁺(g) + aq → Ca²⁺(aq) ; ΔH°hyd = -1650 kJ mol⁻¹

2) Water is a polar molecule, this means that it has a negative end and a positive end. The negative end of the water molecule will be attracted to the cations while the positive end of the water molecule will be attracted to the anions.

3) The attraction set up is called the ion-dipole forces.

4) The standard enthalpy change of hydration is always exothermic.