AL Chemistry Summer Project Standard enthalpy changes Group 6 (produced by Chen William and Lam Yu Wing)

What is enthalpy? Enthalpy (H,or heat content) is the amount of heat energy possessed by substances. It is the sum of internal energy (U) and the product of the pressure and volume of a system.(H=U+PV)

Enthalpy change, ΔH As the enthalpy of a system cannot be measured but changes in enthalpy can be measured experimentally. Enthalpy change in a process then corresponds to the heat change of the system when the process occurs at constant pressure: ΔH= H products － H reactants

Endothermic reactions Endothermic reactions are those in which heat is added, or flows, into the system, the heat change of the process has a positive value.(ΔH > 0)

Exothermic reactions Exothermic reactions are those in which heat is withdrawn, or flows, from the system, the heat change of the process has a negative value.(ΔH < 0)

Standard enthalpy change Standard enthalpy of a reaction (ΔH Θ 298 or ΔH Θ ) is the enthalpy change when the molar quantities of reactants as stated in the balance equation completely react together to give the products under standard conditions (298 K,1 atm.)

Standard Condition 298k Gas: 1 atmosphere Solution: 1 mol dm -3 Solid or liquid: normal physical state at 298 K and 1 atmosphere

Types of standard enthalpy of reaction There are four types of reaction: The standard enthalpy change of neutralization The standard enthalpy change of formation The standard enthalpy change of solution The standard enthalpy change of combustion

Standard enthalpy change of neutralization Standard enthalpy change of neutralization(  H n Θ ) is the enthalpy change when 1 mole of water is formed from the neutralization under standard conditions. e.g. H + (aq) + OH - (aq)  H 2 O(l)  H n Θ = kJ mol -1

Standard enthalpy change of formation Standard enthalpy change of formation(  H f  ) is the enthalpy change when 1 mole of the compound is formed from its elements in their standard states at 298K and 1 atmosphere. 2C(s) + 3H 2 (g) + ½ O 2 (g)  C 2 H 5 OH(l)  H f Θ = kJ mol -1

Standard enthalpy change of formation  H f Θ of all elements are 0, e.g.  H f Θ [O 2 (g)] = 0. For elements with allotropes, the most stable allotrope is used, e.g. graphite is the most stable allotrope of carbon then  H f Θ [graphite] = 0 but  H f Θ [diamond] = +1.9 kJ mol -1. Standard enthalpy change of formation (  H f Θ ) is a measure of energetic stability, more –ve =more energetcially stable with respect to the element.

Standard enthalpy change of solution Standard enthalpy change of solution (  H s Θ ) is the enthalpy change when 1 mol of a substance is completely dissolved in a infinite amount of solvent under standard conditions. e.g.CuSO 4 (s) + water  Cu 2+ (aq) + SO 4 2- (aq)  H s Θ = -57 kJ mol -1 CuSO 4. 5H 2 O(s) + water  Cu 2+ (aq) + SO 4 2- (aq)  H s Θ = +10 KJ mol -1

Standard enthalpy change of combustion Standard enthalpy of combustion (  H c Θ )is the enthalpy change when 1 mole of it is burnt completely in excess oxygen in the standard condition. e.g. C(graphite) + O 2 (g)  CO 2 (g)  H c Θ = kJ mol -1 CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H c Θ = kJ mol -1

EXPERIMENTAL DETERMINATION OF ENTHALPY CHANGES BY SIMPLE CALORIMETRY Enthalpy change of neutralization Enthalpy change of combustion

Enthalpy change of neutralization Heat evolved = [(M acid +M base )S product + C]*  T  H neu Θ = - heat evolved/no. of moles of water formed Specific heat capacity (usually taken as 4.18 J g - 1 K -1 for dilute solution Heat capacity of apparatus NOTE that if  H is -ve

Precaution