Standard Enthalpies

ObjectivesRecall the definition of standard enthalpy changes of reaction, formation, combustion

Edexcel AS Chemistry p79

Standard EnthalpiesWhen we compare enthalpy

changes of reactions we use standard conditions, - known temperatures, pressures, masses and concentrations of reactants or products, in order to make it a fair comparison.KEY IDEAS

The standard conditions are:•A pressure of 1 atmosphere•A temperature of 298K (25oC) •Reactants and products in physical states, normal for the above conditions. •A concentration of 1.0 mol dm-3 for solutions.

The o sign indicates standard conditions

Task

You have 10 minutes to produce a 2 minute presentation on one of the standard enthalpies.

• You should find out it’s symbol• Explain what it is• Give some examples with word and

symbol equations

Standard enthalpy change of reaction

ΔHor

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions in their standard states

Eg. 2C3H8(g) + 10O2(g) 8H2O(l) + 6CO2(g)

Standard enthalpy change of formation

ΔHof

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.

Eg. 2C(s) + 3H2(g) + ½O2(g) C2H5OH(l)

Standard Enthalpy Change of Formation

Standard enthalpy change of combustion

ΔHoc

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

Eg. CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Combustion Reaction Demonstration

Exam QuestionMethanol, CH3OH(l), when blended with petrol, can

be used as a fuel. ΔHoc[CH3OH] = -726kJmol-1

a) Write an equation, including state symbols, for the standard enthalpy change of combustion of methanol (2)

b) Write an equation, including state symbols. For the standard enthalpy change of formation of methanol (2)

c) Liquid petroleum gas is a fuel that contains propane, C3H8. Explain why the following equation does not represent a standard enthalpy change of combustion.(1)

2C3H8(g) + 10O2(g) 8H2O(l) + 6CO2(g) ΔHr=-4113kJmol-1

Answers

a)CH3OH(l) + 1½O2(g) CO2(g) + 2H2O(l)

b)C(s) + 2H2(g) + ½O2(g) CH3OH(l)

c) Only one mole of C3H8 should be shown according to the definition of enthalpy of combustion