Calculate Enthalpy Changes Using ΔHf⦵ (HL) (DP IB Chemistry) : Revision Note

Alexandra Brennan

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Calculate Enthalpy Changes Using ΔHf⦵

  • Standard Enthalpy of Formation is defined as

“The enthalpy change when one mole of a compound is formed from its elements under standard conditions”

  • We can use enthalpy of formation of substances to find an unknown enthalpy change using a Hess cycle

  • In this type of cycle the elements are always placed at the bottom of the diagram:

hess's law cycle for enthalpy of formation

Enthalpy changes using enthalpy of formation

  • In this cycle the arrows will always be pointing upwards because the definition of the enthalpy of formation must go from elements to compounds

  • This means the Hess's Law calculation of ΔH will always be in the same arrangement

Enthalpy of formation equation
  • Try the following worked example:

Worked Example

Given the data:

Substance

B2H6 (g)

B2O3 (g)

H2O (g)

Hf /kJmol-1

+31.4

-1270

-242

Calculate the enthalpy of combustion of gaseous diborane given that it burns according to the following equation:

B2H6 (g) + 3O2 (g)  B2O3 (s) + 3H2O (g)

Answer:

  • Step 1: Find the sum of the enthalpies of combustion of the products

ΔHf = + ( -1270) + ( - 242 x 3) = - 1996 kJ

  • Step 2: Find the sum of the enthalpies of combustion of the reactants

ΔHf = + (+31.4) + 0  = + 31.4 kJ

There is no enthalpy of formation for oxygen as ΔHf of elements by definition is zero

  • Step 3: Calculate the enthalpy change

ΔH = ΔHf products  - ΔHf reactants  = - 1996 - (+ 31.4) = -2027.4 kJ

Examiner Tips and Tricks

Enthalpy of  formation data are given to you in the data booklet.

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

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.

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