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Enthalpy Calculations Worksheet
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Enthalpy Calculations Worksheet
A student team is checking energy data before presenting its findings. Use the balanced equations and enthalpy values supplied in each question. Show your working, include units, and keep the sign convention clear: negative ΔH means energy is released; positive ΔH means energy is absorbed. Use a calculator if helpful.
Part A · Energy, amount and mass
Treat each ΔH value as applying to the reaction exactly as written.
1.Hydrogen fuel. H₂(g) + ½O₂(g) → H₂O(l), ΔH = −286 kJ per mole of H₂O formed. A fuel cell releases 1,430 kJ. How many moles of water form? State the energy change with its sign.
2.Methane for a burner. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔH = −890 kJ mol⁻¹ CH₄. A 10.0 g methane sample burns completely. Given M(CH₄) = 16.04 g mol⁻¹, calculate the heat change in kJ. Is heat absorbed or released?
3.Making lime. CaCO₃(s) → CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹ CaCO₃. A kiln decomposes 50.0 g CaCO₃. Given M(CaCO₃) = 100.09 g mol⁻¹, find the energy change. Include the sign and identify the process as endothermic or exothermic.
4.Check the reaction scale. 2Na(s) + Cl₂(g) → 2NaCl(s), ΔH = −821 kJ for the equation as written. Calculate the enthalpy change per mole of NaCl formed, in kJ mol⁻¹. Explain how the coefficient 2 affects your calculation.
5.Carbon in a furnace. C(s) + O₂(g) → CO₂(g), ΔH = −394 kJ mol⁻¹ C. A sample releases 1,576 kJ when burned completely. Given M(C) = 12.01 g mol⁻¹, calculate the mass of carbon that reacted.
Part B · Interpret and troubleshoot
A careful chemist checks whether the number, sign and units make sense—not just the arithmetic.
6.Sign check. A reaction has ΔH = +65 kJ mol⁻¹. Choose the correct interpretation.
- It releases 65 kJ per mole; it is exothermic.
- It absorbs 65 kJ per mole; it is endothermic.
- It releases 65 kJ in total, whatever the amount.
- The sign tells us the reaction rate.
7.Spot the unit error. For 0.250 mol of a reaction with ΔH = −120 kJ mol⁻¹, a student writes: “ΔH = −120 × 0.250 = −30 mol.” Identify the error and write the correct result with units and meaning.
8.Reverse the reaction. A reaction written A → B has ΔH = −75 kJ mol⁻¹. What is ΔH when the reaction is reversed, B → A? Explain the sign change.
9.Scale up a data set. For 2H₂(g) + O₂(g) → 2H₂O(l), ΔH = −572 kJ as written. What is the energy change when 0.750 mol H₂ reacts completely?
10.Explain the method. In two or three sentences, describe how you would calculate the energy change when given a reactant mass and a ΔH value per mole. Include what you do with the sign.
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