Applications of Hess’s Law (Oxford AQA International A Level Chemistry)

Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Hess's Law Calculations

  • Hess’s Law states that:

"The total enthalpy change in a chemical reaction is independent of the route taken."

  • This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

Illustration of Hess's Law

Chemical Energetics Hess Cycles, downloadable AS & A Level Chemistry revision notes
According to Hess’ Law, the enthalpy change of the direct route, going from reactants (A+B) to product (C), is equal to the enthalpy change of the indirect routes
  • Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using calorimetry, such as:

3C (s) + 4H2 (g) → C3H8 (g)

  • ΔHf (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions

Calculating ΔHf from ΔHc using Hess’s Law energy cycles

  • It can be difficult to find the enthalpy change of formation of compounds experimentally

  • However, many enthalpy changes of combustion can be measured experimentally so these can be used to find the enthalpy of formation

  • To do this, we follow these steps:

    1. Write the equation for the formation of the compound

    2. Write the combustion products below the equation

    3. Draw downward pointing arrows from each substance to its combustion products

    4. Write values on the arrows and multiply by the number of moles

    5. In a cycle, go from the reactants to the products, changing the sign of the value if the arrow points in the opposite direction

Worked Example

Using the data provided, calculate the standard enthalpy change of formation, ΔHf, of propanone.

3C (s)  +     3H2 (g)   +      ½ O2 (g)               CH3COCH3 (l)

Substance

C (s)

H2 (g)

CH3COCH3 (l)

HCө / kJ mol–1

-394

-286

-1821

Answer:

  • Step 1: Write the balanced equation

wk eg step 1
  • Step 2:Write the combustion products below the equation

wk eg step 2
  • Step 3: Draw downward pointing arrows from each substance to its combustion product

wk eg step 3
  • Step 4: Write the appropriate values on the arrows and multiply by the number of moles

worked example step 4
  • Step 5: In a cycle, go from the reactants to the products, changing the sign of the value if the arrow points in the opposite direction

worked example step 5

ΔHfө = -1182 - 858 + 1821 = -219 kJ mol-1

  • The sign on -1821 needs reversing as the cycle goes in the opposite direction to the arrow pointing to the combustion products

Calculating ΔHr  from ΔHf using Hess’s Law energy cycles

  • Knowing the enthalpy change of formation, ΔHf, allows us to determine the overall enthalpy change of a reaction, ΔHr

  • To do this, we follow these steps:

    1. Write the equation for the reaction

    2. Write the elements with the correct number of moles and state symbols underneath

    3. Draw upwards pointing arrows to each compound

    4. Write the appropriate values on the arrows and multiply by the number of moles

    5. In a cycle, go from the reactants to the products, changing the sign of the value if the arrow points in the opposite direction

Worked Example

Use the information in the table to calculate the enthalpy change for this reaction:

NH4NO3 (s)  +  ½C (s)  →  N2 (g)  +  2H2O (g)  +  ½CO2 (g)

Substance

C (s)

N2 (g)

H2O(g)

CO2 (g)

NH4NO3 (s)

Hfө / kJ mol–1

0

0

–242

–394

–365

Answer:

 Step 1: Write the balanced equation

wk eg step 1

Step 2: Write the elements with the correct number of moles and state symbols underneath

wk eg step 2

Step 3: Draw upwards pointing arrows to each compound

wk eg step3

Step 4: Write the appropriate values on the arrows and multiply by the number of moles

wk eg step 4

Step 5: In a cycle, go from the reactants to the products, changing the sign of the value if the arrow points in the opposite direction

wk eg step 5

ΔHrө = +365 - 484 - 197 = -316 kJ mol-1

  • The sign on -365 needs reversing as the cycle goes in the opposite direction to the arrow pointing upwards

  • There is no need to draw arrows from the elements to carbon and nitrogen as ΔHfө is 0 for elements

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.