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Using ∆Hf° to Find Enthalpy Changes (DP IB Chemistry: SL)

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Using ∆Hf° to Find Enthalpy Changes

  • 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

Enthalpy of formation and Hess cycles Part 1, downloadable IB Chemistry revision notes

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 and Hess cycles Part 2, downloadable IB Chemistry revision notes
  • Try the following worked example:

Worked example

Given the data:5.1.8 Worked Example Table, downloadable IB Chemistry revision notesCalculate 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 Tip

In Paper 1, Enthalpy of Formation data will given in the question. For Paper 2, you may need to refer to Section 12 of the Data Booklet where you will find Thermodynamic Data for Selected Compounds

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Stewart

Author: Stewart

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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 Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.