Energy changes

The precise value of an enthalpy change depends on; 1) The number of moles. 2) Temperature 3) Pressure 4) The physical states of reactants. In order to compare reactions standard enthalpy changes are used, represented by the symbols; ΔH Θ ( Δ = difference Θ = standard )

Standard conditions ΔH Θ is defined as the enthalpy change when molar quantities react under standard conditions. Standard conditions are defined as; A temperature of 298 K (25 o C) A pressure of 101 kPa (1atm) All substances in their natural physical states at 298 K and 101kPa. Eg; Water must be liquid, not steam.

ΔH Θ f Standard enthalpy of formation The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions. Eg; C (s) + 2H 2 (g) → CH 4 (g) As the elements are in their standard states, and one mole is being produced ΔH for this reaction is; ΔH r = ΔH Θ f CH 4

ΔH Θ c Standard enthalpy of combustion The enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions. Eg; CH 4 + 2O 2 → CO 2 + 2H 2 O One mole is being completely burnt so; ΔH r = ΔH Θ c CH 4 NB; If combustion was incomplete, due to a lack of oxygen, carbon monoxide or carbon would be produced instead.

ΔH Θ n Standard enthalpy of neutralisation The enthalpy change when one mole of water is formed when an acid is neutralised by a base to form an infinitely dilute solution. Eg; HCl + NaOH → NaCl + HOH As one mole of water is produced this represents; ΔH r = ΔH Θ n

NB H 2 SO 4 + 2NaOH → Na 2 SO 4 + 2HOH Two moles of water are produced. ΔH r = 2ΔH Θ n ½H 2 SO 4 + NaOH→ ½Na 2 SO 4 + HOH Now one mole of water is produced, so the enthalpy change is; ΔH r = ΔH Θ n

Write equations to represent the standard neutralisation enthalpies of; Hydrochloric acid and calcium hydroxide. HCl + ½Ca(OH) 2 → ½ CaCl 2 + H 2 O Phosphoric acid and sodium hydroxide ⅓H 3 PO 4 + NaOH → ⅓ Na 3 PO 4 + H 2 O Aqueous ammonia and sulphuric acid. ½ H 2 SO 4 + NH 4 OH → ½ (NH 4 ) 2 SO 4 + H 2 O

ΔH Θ a the enthalpy of atomisation This is the enthalpy change when one mole of gaseous atoms are formed from the element at stp. Eg; C (s) → C (g) One mole of gaseous atoms are being formed so this represents ΔH r = ΔH Θ a C

NB For H 2(g) → 2H (g) ΔH r is not ΔH Θ a H In this case 2 moles of gaseous atoms are produced so; ΔH r = 2ΔH Θ a H But for ½H 2(g) → H (g) one mole of gaseous atoms are produced so; ΔH r = ΔH Θ a H

NB; The state of the element does not matter. ½Cl 2 (g) → Cl (g) ½Br 2 (l) → Br (g) ½I 2 (s) → I (g) In each case ΔH Θ r = ΔH Θ a

Write an equations to represent ΔH Θ a for; Mercury. Aluminium Fluorine Phosphorous Hg (l) → Hg (g) Al (s) → Al (g) ½F 2(g) → F (g) ¼P 4 (s) → P (g)