2.1.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Using Hess Cycles — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 13 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6Hess's Law
If two routes lead from the same reactants to the same products, the enthalpy changes must be equal.
Statement
- Statement The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
Reason
- Reason It follows from the conservation of energy, since enthalpy is a state function.
Use
- Use It lets chemists find ΔH for reactions that are too slow, too dangerous or impossible to measure directly.
Cycle
- Cycle Draw a triangle with the target reaction on top and two alternative routes joined by the same elements or products.
Section 2 of 6Using Formation Data
Using Formation Data
Route
- Route Reactants break down into elements and the elements then form the products.
Formula
- Formula ΔrH = ΣΔfH(products) − ΣΔfH(reactants), multiplying each term by its coefficient.
Elements
- Elements Elements in their standard states have ΔfH of zero, so they can be left out.
Example
- Example For CH₄ + 2O₂ → CO₂ + 2H₂O(l), use −394 and −286 for products and −75 for CH₄.
Section 3 of 6Using Combustion Data
1
Draw the reactants and products at the top, with the combustion products at the bottom of the cycle.
2
Both reactants and products can be burned to the same final substances, such as CO₂ and H₂O.
3
The arrows from reactants to combustion products give ΔcH values.
4
ΔrH = ΣΔcH(reactants) − ΣΔcH(products), in the opposite way to formation data.
5
For example, ΔfH of propane from carbon and hydrogen is 3(−394) + 4(−286) − (−2219) ≈ −107 kJ mol⁻¹.
Section 4 of 6Avoiding Mistakes
Avoiding Mistakes
Check signs
- Check signs Reversing an arrow changes the sign of that enthalpy change.
Multiply
- Multiply Multiply each value by the number of moles in the balanced equation.
State symbols
- State symbols Use the correct enthalpy of formation for water, which depends on whether it is liquid or gas.
Sanity check
- Sanity check An answer for a combustion or formation of a stable compound is usually of the order of hundreds of kJ mol⁻¹.
| Substance | ΔfH° / kJ mol⁻¹ |
|---|---|
| CO₂(g) | −394 |
| H₂O(l) | −286 |
| CH₄(g) | −75 |
Section 5 of 6Worked Example
Question Calculate ΔrH for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), given ΔfH° of CH₄ = −75, CO₂ = −394 and H₂O(l) = −286 kJ mol⁻¹.
- Sum of products = (−394) + 2(−286) = −966 kJ mol⁻¹.
- Sum of reactants = (−75) + 2(0) = −75 kJ mol⁻¹.
- ΔrH = −966 − (−75) = −891 kJ mol⁻¹.
Answer ΔrH = −891 kJ mol⁻¹
Section 6 of 6Exam Checklist & Tips
2.1.4 – Key Points for the Exam
- Hess's law The enthalpy change of a reaction is the same whatever route is taken.
- State function A property that depends only on the current state of the system and not on how it got there.
- Enthalpy cycle A diagram that links a reaction to alternative routes with known enthalpy changes.
- Σ The symbol for the sum of terms.
- Formation data Enthalpies of formation used to calculate the enthalpy change of a reaction.
Common Mistakes
- Subtracting reactants from products when using combustion data.
- Forgetting to multiply by the stoichiometric coefficient.
- Including the formation enthalpy of an element that is not zero.
- Mixing H₂O(l) and H₂O(g) data in one calculation.
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