ASChemistryAQARevision NotesPhysical ChemistryEnergeticsApplications of Hess’s Law

Applications of Hess’s Law (AQA AS Chemistry): Revision Note

Exam code: 7404

Stewart Hird

Last updated

Hess's Law Calculations

  • You must make sure that you can apply Hess' Law effectively and calculate enthalpy changes in different situations

  • Remember - it is the data that is important

  • Check whether the data you have been given is formation data or combustion data, and then complete the cycle or calculation according to that

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

Calculating average bond energies using Hess’s cycles

  • Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy

  • This can be done using enthalpy changes of atomisation and combustion or formation

  • The enthalpy change of atomisation (ΔHatꝋ ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.

    • Eg. ΔHatꝋ [H2] relates to the equation:

½ H2(g) → H(g)

Worked Example

Calculating average C-H bond energy

Calculate the average bond energy of the C-H bond using the relevant ΔHfꝋ and ΔHatꝋ values in the table below:

Answer

Step 1: Write down the equation for the dissociation of methane at the top

Chemical Energetics Step 1 - Calculating average C-H bond energy, downloadable AS & A Level Chemistry revision notes

Step 2: Write down the elements at the bottom

Chemical Energetics Step 2 - Calculating average C-H bond energy, downloadable AS & A Level Chemistry revision notes

Step 3: Draw all arrows in the correct direction

Chemical Energetics Step 3 - Calculating average C-H bond energy, downloadable AS & A Level Chemistry revision notes

Step 4: Apply Hess’s Law

ΔH = ΔHθat + ΔHθf

ΔH = ((+717.7) + (4 x (+218)) - (-74.8)

ΔH = +1664.5 kJ mol-1

Step 5: Since there are 4 C-H bonds in methane:

Average bond enthalpy (C-H) = fraction numerator plus 1664.5 over denominator 4 end fraction

Average bond enthalpy (C-H) = +416.1 kJ mol-1

Examiner Tips and Tricks

Remember: Take into account the number of moles of each reactant and product.

For example, there are two moles of NaHCO3(s) so the ΔHf value is multiplied by 2.

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Stewart Hird

Author: Stewart Hird

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