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4.1.4Reactivity 3 — What are the mechanisms of chemical change?

Standard Enthalpy Change — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 12 of 12 Unit quiz

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 6Standard Conditions

4.1.4 Standard Enthalpy Change

Standard values can be compared because everyone uses the same reference conditions.

Pressure
  • Standard pressure is 100 kPa, which is the value used in IB data and in the definition of standard states.
Temperature
  • Standard enthalpy values are normally quoted at 298 K,
    • although the temperature is stated because it is not part of the definition of the standard state.
Concentration
  • Solutions are taken to be at a concentration of 1 mol dm⁻³ when a standard enthalpy change involves dissolved species.
Standard state
  • Each substance is in its most stable physical form under those conditions, such as O₂(g), C(graphite) and Br₂(l).

Part 2 of 6Defined Enthalpy Changes

Defined Enthalpy Changes
Standard enthalpy of combustion
  • ΔHc° is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
Standard enthalpy of formation
  • ΔHf° is the enthalpy change when one mole of a compound forms from its elements in their standard states.
Elements
  • The standard enthalpy of formation of an element in its standard state is zero, because nothing has to change to make it.
Units
  • Both are given in kJ mol⁻¹, meaning per mole of the substance named in the definition.
QuantityEquation formExample
ΔHc°1 mol fuel + O₂ → CO₂ + H₂OC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
ΔHf°Elements → 1 mol compound2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)

Part 3 of 6Writing the Right Equation

1
Decide which enthalpy change is wanted and read the definition to see what must appear on each side of the equation.
2
For formation, put the elements in their standard states on the left and exactly one mole of the compound on the right.
3
For combustion, put exactly one mole of the fuel on the left with excess oxygen, and the fully oxidised products CO₂(g) and H₂O(l) on the right.
4
Use fractions in front of formulae if necessary to keep the stated amount at one mole.
5
Include state symbols for every species since the enthalpy change depends on the physical states.

Part 4 of 6Using the Values

Using the Values
Heat from a mass
  • Convert mass to moles with n = m/M and then multiply by the molar enthalpy change to get the energy transferred.
Comparing fuels
  • Standard combustion values allow fuels to be compared per mole, or per gram after dividing by the molar mass.
Sign
  • Combustion and many formation reactions have negative ΔH, but some compounds, such as ethyne, have a positive ΔHf°.
Why states matter
  • Water as a liquid releases more energy on formation than water as a gas, so state symbols change the value.

Part 5 of 6Worked Example

Question

  • The standard enthalpy of combustion of ethanol is −1367 kJ mol⁻¹.
  • How much energy is released when 2.30 g of ethanol burns?
  • (Mr = 46.08)

  1. Amount of ethanol = 2.30 ÷ 46.08 = 0.0499 mol.
  2. Energy released = 0.0499 × 1367 = 68.2 kJ.
  3. Write the answer as a positive amount of energy released.

Answer About 68.2 kJ of energy is released.

Part 6 of 6Exam Checklist & Tips

4.1.4 – Key Points for the Exam

  • Standard conditions A pressure of 100 kPa, a stated temperature (usually 298 K) and 1 mol dm⁻³ for solutions.
  • Standard enthalpy of combustion The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
  • Standard enthalpy of formation The enthalpy change when one mole of a compound forms from its elements in their standard states.
  • Standard state The most stable form of a substance at 100 kPa and the stated temperature.

Common Mistakes

  • Writing two moles of product in a formation equation when the definition needs exactly one mole.
  • Leaving out state symbols or giving H₂O(g) when the combustion definition uses liquid water.
  • Assuming that the enthalpy of formation of every element is zero, rather than only for elements in their standard states.
  • Giving ΔHc° the wrong sign by treating combustion as endothermic.

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