Determining Enthalpy Changes


There are several different ways to determine enthalpy changes: experimentally, using average bond enthalpies or using Heiss’s law.

Determining Enthalpy Change Experimentally
There are two equations that are used to determine enthalpy change.

 

Equation 1


Energy change (q) = mcT


Where:
m = Mass of water in grams (where 1cm3 = 1g)
c = Specific Heat Capacity of Water 4.18 Jg-1K-1
T = Change in Temperature in Kelvin

Remember: 0°C = 273 K and a change in temperature of 1°C = a change in Kelvin of 1°.
The specific heat capacity of water is normally given in the exam

Note: Energy change can also be referred to as energy transferred, heat evolved, energy released during the reaction or energy released and absorbed by the solution.

 

 

Equation 2


H = q/n


Where:
H = Enthalpy Change
q = Energy Change
n = Moles of reactant that is not in excess

Note: The enthalpy change can be referred to as the enthalpy change per mole.


Example - Exothermic Reaction

0.585g of C2H6 reacted with O2 to form CO2 + H2O. The reaction occurred under a can containing 100 cm3 of water and there was a temperature rise of 9°C. How much energy would be obtained from one mole of C2H6?

Step 1: Write the equation

C2H6 (l) + 3½O2(g) 2CO2(g) + 3H2O (l)

Step 2: Calculate how much energy was transferred to the water

q=mcT
q=100 x 4.18 x 9 = 3762 J

Step 3: How much energy was obtained from one mole of ethane

Moles = 0.585 / ((12x2) + 6) = 0.0195 mol
H = q/n = 3762/0.0195 = -192900 JMol-1 = -192.9 kJmol-1


Don’t forget the positive or negative symbol. Positive for an endothermic reaction and negative for an exothermic reaction.


Hess’s Law
The total enthalpy change for a chemical reaction is independent of the route taken.


There are two main questions you will be asked relating to Hess’s Law – the first is determining the enthalpy change of formation from enthalpy change of combustion and the second is determining the enthalpy change of a reaction e.g. combustion from the enthalpy change of formation.

Hess’s law may be used if the values of a reaction cannot be measured directly, for example carbon and hydrogen won’t normally react under standard conditions, however it is possible to calculate the enthalpy change using data for formation or combustion.

Determining the enthalpy change of formation from enthalpy change of combustion

Data: Using Hc

Standard Diagram

For this style of question to gain the marks in the exam you must draw a cycle. This style of question uses
Hc for cycle.


Hf + Hc1 = Hc2 + Hc3

Hf = (Hc2 + Hc3) - Hc1


Enthalpy change of reaction from enthalpy changes of formation

Data: Using Hc

Standard Diagram

For this style of question to gain the marks in the exam you don’t need to draw a cycle. This style of question uses Hf for formula.


Enthalpy change of reaction from average bond enthalpies
This method uses data for the average bond enthalpies and therefore may result in an answer that is slightly inaccurate. For example the energy of a C-O bond in CH3OH is slightly different that in CH3COOH.

Hr = + [Energy to break bonds] – [Energy to make bonds]