Chemistry • Year 11 • Module 4 • Lesson 1

Enthalpy & Energy Profile Diagrams

Apply enthalpy concepts to real data graphs, Australian case studies, and energy profile diagram interpretation.

Apply • Data & Reasoning

1. Interpret an energy profile diagram — LNG combustion at Karratha

The graph below shows a stylised energy profile diagram for the combustion of methane (the main component of liquefied natural gas processed at the North West Shelf LNG facility near Karratha, Western Australia). Use the graph to answer the questions. 8 marks

Figure 1.1. Energy profile diagram for the combustion of methane. E_a and ΔH values are illustrative. Adapted from standard thermochemical data; context: North West Shelf LNG, Karratha, WA.

1.1 Is the combustion of methane exothermic or endothermic? Justify using both the sign of ΔH and the relative positions of reactants and products on the diagram. 2 marks

1.2 Using the diagram, calculate the activation energy for the reverse reaction (decomposition of CO₂ and H₂O back to CH₄ and O₂). Show your working. 2 marks

1.3 A catalyst is added to the reaction. On the diagram above, draw a dashed line to represent the catalysed energy profile. State what changes and what stays the same. 2 marks

1.4 Natural gas combustion at Karratha releases energy that is exported worldwide as LNG. Write the thermochemical equation for 2 moles of methane combusting. State the new ΔH value. 2 marks

Stuck? Use the formula E_a(rev) = E_a(fwd) − ΔH from Worked Example 2 in Lesson 1.

2. Interpret thermochemical data — sports medicine cold packs in Australia

Sports trainers at two Australian football clubs tested different salt solutions for use in cold packs. The table below shows the enthalpy of dissolution measured at 25°C and 100 kPa for four salts. 6 marks

Salt	Dissolution equation	ΔH (kJ mol⁻¹)	Surroundings temperature change
Ammonium nitrate (NH₄NO₃)	NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq)	+25.7	?
Ammonium chloride (NH₄Cl)	NH₄Cl(s) → NH₄⁺(aq) + Cl⁻(aq)	+14.8	?
Calcium chloride (CaCl₂)	CaCl₂(s) → Ca²⁺(aq) + 2Cl⁻(aq)	−81.3	?
Sodium hydroxide (NaOH)	NaOH(s) → Na⁺(aq) + OH⁻(aq)	−44.5	?

2.1 Fill in the “Surroundings temperature change” column with either “decreases” or “increases” for each salt. 2 marks

2.2 Which salt would be most effective in a cold pack designed to cool an injured ankle rapidly? Justify using the ΔH values. 2 marks

2.3 A trainer accidentally uses CaCl₂ instead of NH₄NO₃. Predict the effect on the injured player’s ankle and explain using ΔH. 2 marks

Stuck? Connect the sign of ΔH to the direction of heat flow between system and surroundings (Card 02).

3. Cause-and-effect chain — photosynthesis and enthalpy

Complete the cause-and-effect chain for photosynthesis. Each “cause” box is filled in; write what each cause leads to in the “effect” box. Then answer the follow-up question. 5 marks

Cause (given)	→	Effect (you write)
Photosynthesis: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g)	→
ΔH for photosynthesis is positive (+2803 kJ mol⁻¹)	→
Glucose (C₆H₁₂O₆) is produced at higher enthalpy than reactants	→
The reverse of photosynthesis is combustion of glucose	→

3.5 Overall outcome: What does the sign of ΔH for photosynthesis (+2803 kJ mol⁻¹) tell you about the source of energy for this reaction, and how does this relate to the energy stored in glucose? 1 mark

Stuck? Recall that reversing an equation flips the sign of ΔH (Card 03), and that ΔH is a state function.

4. Compare exothermic vs endothermic reactions

Complete the table below. For each feature, write the correct answer for both reaction types. 8 marks (1 per cell)

Feature	Exothermic reaction	Endothermic reaction
Sign of ΔH
Direction of heat flow
Relative energy of products vs reactants
Direction of ΔH arrow on energy profile diagram
Effect on surroundings temperature
Australian real-world example
Thermochemical equation sign convention
ΔH for reverse reaction compared to forward

Stuck? Use the comparison table in Card 02 and the Australian examples from the lesson (hand warmers, cold packs, LNG combustion).

Answers — Do not peek before attempting

Q1.1 — Exo/endo classification (2 marks)

Exothermic [1]. ΔH = −890 kJ mol⁻¹ (negative sign), meaning energy is released to the surroundings. On the diagram, the products (CO₂ + 2H₂O) sit at a lower enthalpy level than the reactants (CH₄ + 2O₂), confirming that H(products) < H(reactants) [1].

Q1.2 — E_a for the reverse reaction (2 marks)

E_a(rev) = E_a(fwd) − ΔH [1].
E_a(rev) = 420 − (−890) = 420 + 890 = 1310 kJ mol⁻¹ [1].
Reasoning: The products sit 890 kJ mol⁻¹ below the reactants, so starting from the products and climbing to the same peak requires a much larger energy input.

Q1.3 — Catalysed pathway (2 marks)

A catalyst lowers the activation energy, so the dashed line should show a lower peak (transition state) than the original curve [1]. The reactant and product enthalpy levels, and therefore ΔH, remain unchanged — only E_a decreases [1].

Q1.4 — Thermochemical equation for 2 mol CH₄ (2 marks)

2CH₄(g) + 4O₂(g) → 2CO₂(g) + 4H₂O(l) ΔH = −1780 kJ mol⁻¹ [1 for correct equation, 1 for ΔH doubled].
Rule: Scaling the equation by a factor of 2 scales ΔH by the same factor.

Q2.1 — Temperature change column

NH₄NO₃: decreases (endothermic, +ΔH). • NH₄Cl: decreases (endothermic, +ΔH). • CaCl₂: increases (exothermic, −ΔH). • NaOH: increases (exothermic, −ΔH). [½ mark per row, 2 marks total]

Q2.2 — Best cold pack salt (2 marks)

Ammonium nitrate (NH₄NO₃) with ΔH = +25.7 kJ mol⁻¹ [1]. It absorbs the most heat per mole from the surroundings (largest positive ΔH among the endothermic salts), meaning the pack will cool the most [1].

Q2.3 — Trainer uses CaCl₂ by mistake (2 marks)

CaCl₂ has ΔH = −81.3 kJ mol⁻¹ (exothermic) [1]. The pack would become hot, potentially burning or scalding the player’s ankle rather than cooling and reducing swelling [1].

Q3 — Cause-and-effect chain

Row 1: Photosynthesis is an endothermic reaction (ΔH > 0); it absorbs energy from the surroundings (light).

Row 2: Energy flows from the surroundings (sunlight) into the system; the surroundings “cool” (light energy is consumed).

Row 3: Energy from sunlight is stored as chemical potential energy in the bonds of glucose molecules.

Row 4: The combustion of glucose has ΔH = −2803 kJ mol⁻¹ (exothermic); the sign is flipped from photosynthesis.

3.5 Overall: A positive ΔH means energy must be absorbed (from sunlight) to drive the reaction. That energy is stored in the chemical bonds of glucose — photosynthesis converts light energy into chemical potential energy.

Q4 — Comparison table

Feature	Exothermic	Endothermic
Sign of ΔH	Negative (< 0)	Positive (> 0)
Direction of heat flow	System → surroundings	Surroundings → system
Products vs reactants energy	Products lower	Products higher
ΔH arrow on diagram	Downward (reactants → products)	Upward (reactants → products)
Surroundings temperature	Increases (warms)	Decreases (cools)
Australian example	LNG combustion (Karratha) / hand warmer	Cold pack (NH₄NO₃) / photosynthesis
Sign convention	ΔH stated as negative number	ΔH stated as positive number
ΔH of reverse reaction	Equal magnitude, opposite sign (+)	Equal magnitude, opposite sign (−)

Award 1 mark per correctly completed row pair (0.5 per cell). Accept equivalent phrasing.