Enthalipes of combustion

Enthalpy of combustion (ΔHc): The enthalpy change when 1 mole of

a substance is burned completely in oxygen.

Enthalpy of solution (ΔHaq): The enthalpy change when 1 mole of

a substance is dissolved in water until there is no further

temperature change.

Enthalpy of neutralisation (ΔHn) The enthalpy change when 1

mole of water is formed by an acid being neutralised by a base.

Calculating Enthalpy Changes- step 1

Measure the energy transferred (Eh) to a known mass of water (solution), by measuring

the temperature change and then using the equation. (Revision from National 5 Chemistry)

Eh = c mΔT

where c = specific heat capacity of water (4.18 kJKg-1oC-1)

m = mass of water (solution) heated in kilograms Kg

ΔT = Temperature change (oC)

Calculating Enthalpy Changes- step 2

Calculate the enthalpy change for 1 mole (ΔH) using the triangle

Eh : Energy transferred in experiment

ΔH : Energy transferred per mole

n : number of moles of substance used

in energy transfer experiment.

Worked Example:

Question: Calculate the enthalpy of combustion when 1.1g of methanol burned, the

energy produced raised the temperature of 100cm3 of water by 57°C

Step 1: Calculate Eh

C= 4.18kJKg-1°C-1

m = 100g = 0.1kg (remember this is the mass of

water in kg)

ΔT = 57°C

Eh = cmΔT

=4.18x0.1x57

= 23.826 KJ

Step 2: Calculate the number of moles

m = 1.1g

GFM of methanol = 32g

n=m/gfm = 1.1/32

n=0.034moles

Step 3: Calculate enthalpy change (ΔH)

Eh =23.826kJ

n=0.034moles

ΔH=Eh/n

= 23.826/0.034

= 700.76kJmol-1

Common mistakes

The mass of ethanol is used when calculating Eh.

Remember when calculating Eh we use the mass of

water.

People forget to change the mass of water from

grams into kilograms (divide by 1000)

People get Eh and ΔH mixed up. The easiest way

to distinguish between the two is to look at the

units. Eh is always kJ whereas ΔH is always

kJmol-1.

Enthalpy of combustion

The enthalpy of combustion is the heat energy

given out when 1 mole of fuel burns completely in

oxygen.

The enthalpy of combustion of ethanol can be

represented by the equation

C2H5OH + 3O2 🡪 2CO2 + 3H2O

Enthalpy of combustion

The heat energy released when alcohols burn can be measured

The enthalpy of combustion of a substance is the amount of energy

given out when one mole of a substance burns in excess oxygen.

Precautions to Reduce Heat Loss

Surround the beaker with insulation or use a heat

shield to minimise heat loss to the surroundings

Have the flame as close to the beaker as possible so

that the flame is heating the beaker contents and

not the surrounding air.

Measurements

volume of water heated = 100ml

mass of water heated = 100 g = 0.1 kg

Calculation

Eh = cmΔt

Eh = 4.18 x 0.1 x 10

Eh = 4.18kJ

Using the triangle we see

ΔH = Eh / n

(from 1) Eh = 4.18kJ

n = number of moles of ethanol burned

In experiment mass of ethanol burned = 0.19 g

GFM ethanol (C2H5OH) = 46 g

number of moles = m / GFM

=0.19/46 = 0.004moles

Therefore as Δ H = Eh / n

=4.18/0.004

= 1045kJmol-1

NOTE: reaction is exothermic (it got

hot)therefore: ΔH is -1045kJmol-1

Errors

The data book value for combustion of ethanol is ΔH = -1367kJmol-1

Suggest reasons why the value from your experiment was different.

energy lost to the surroundings

incomplete combustion

loss of ethanol to evaporation

uneven temperature distribution

Sources of inaccuracy

Heat loss to surroundings

Ignore heat rise of calorimeter

Incomplete combustion

Possible loss of fuel by evaporation from wick