Key Learning Objectives
- State Hess's Law: total enthalpy change is independent of the reaction route taken.
- Construct thermochemical cycles using enthalpies of formation (arrows up from elements).
- Construct thermochemical cycles using enthalpies of combustion (arrows down to combustion products).
Topic Summary & Core Notes
Master Hess's Law thermochemical cycles. Calculate reaction enthalpies from standard enthalpies of formation (ΔfH) and combustion (ΔcH) with examiner precision.
Key Rules, Formulas & Definitions
Formation Data Cycle
ΔHr = Σ ΔfH(products) - Σ ΔfH(reactants) [Arrows point UP from constituent elements]
Combustion Data Cycle
ΔHr = Σ ΔcH(reactants) - Σ ΔcH(products) [Arrows point DOWN to combustion products (CO2, H2O)]
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Enthalpy of Reaction from Enthalpies of Formation
Question: Calculate ΔHr for: 2SO2(g) + O2(g) → 2SO3(g). [ΔfH: SO2(g) = -297 kJ/mol, SO3(g) = -396 kJ/mol]
Step-by-Step Solution:
- Step 1: Formula: ΔHr = Σ ΔfH(products) - Σ ΔfH(reactants).
- Step 2: Products sum: 2 × (-396) = -792 kJ/mol.
- Step 3: Reactants sum: 2 × (-297) + 0 (elements have ΔfH = 0) = -594 kJ/mol.
- Step 4: Calculate: ΔHr = -792 - (-594) = -792 + 594 = -198 kJ/mol.
Final Answer: -198 kJ/mol
Senior Examiner Insight: Elements in their standard states (like O2) have ΔfH = 0 by definition.
Worked Example 2
Worked Example 2: Combustion Cycle Reversal
Question: Explain why the formula for calculating ΔHr from combustion data is Σ ΔcH(reactants) - Σ ΔcH(products), reversing the order of formation data.
Step-by-Step Solution:
- Step 1: Cycle route: Reactants burn to combustion products (forward arrow: +Σ ΔcH(reactants)).
- Step 2: Alternate route: Products burn to combustion products; navigating from products backwards reverses the arrow sign (-Σ ΔcH(products)).
- Step 3: Synthesise: Hess's law route equivalence gives ΔHr = Σ ΔcH(reactants) - Σ ΔcH(products).
Final Answer: Because products burn to the common combustion products, traversing the cycle against the product combustion arrows requires subtracting product enthalpies.
Senior Examiner Insight: Remember the mnemonic: Formation = Products minus Reactants; Combustion = Reactants minus Products.
Common Pitfalls & Examiner Warnings
Common Mistake: Mixing up the direction of formation vs combustion arrows
Formation arrows point UP from elements; Combustion arrows point DOWN to oxides.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the standard enthalpy of formation (ΔfH°) of pure liquid bromine, Br2(l)?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) 0 kJ/mol
Detailed Explanation: By definition, the standard enthalpy of formation of an element in its standard state is zero.
Mark Scheme & Scoring Points: 1 mark for 0 kJ/mol.
Question 2 of 3
When constructing a Hess's cycle using enthalpy of COMBUSTION data, which formula gives the reaction enthalpy?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) ΔHr = Σ ΔcH(reactants) - Σ ΔcH(products)
Detailed Explanation: Combustion cycle route arithmetic gives Reactants minus Products.
Mark Scheme & Scoring Points: 1 mark for Reactants minus Products.
Question 3 of 3
Hess's Law is a direct chemical consequence of which fundamental law of physics?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Conservation of Energy (First Law of Thermodynamics)
Detailed Explanation: Because energy is conserved, the enthalpy change between two states is path-independent.
Mark Scheme & Scoring Points: 1 mark for Conservation of Energy.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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