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An exothermic reaction transfers energy to the surroundings and has a negative enthalpy change; an endothermic reaction has a positive change. A standard enthalpy of formation forms one mole of a compound from its elements in their standard states under standard conditions. Standard combustion burns one mole completely in oxygen. Always define the reaction and amount: reversing a reaction reverses the sign, and multiplying all stoichiometric coefficients multiplies the stated enthalpy change.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Calculate molar enthalpy changes from calorimetry
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Is covalent bond breaking endothermic or exothermic?
Endothermic. Separating bonded atoms requires energy.
After watching 2: What happens to ΔH when the entire reaction is reversed?
Its sign reverses. Initial and final states exchange places.
What you'll be able to do
An exothermic reaction transfers energy to the surroundings and has a negative enthalpy change; an endothermic reaction has a positive change. A standard enthalpy of formation forms one mole of a compound from its elements in their standard states under standard conditions. Standard combustion burns one mole completely in oxygen. Always define the reaction and amount: reversing a reaction reverses the sign, and multiplying all stoichiometric coefficients multiplies the stated enthalpy change.
Calorimetry estimates heat using . Use the mass of the heated solution, not just one reactant, and consistent heat-capacity units. Divide by reacting moles and apply the opposite sign to obtain molar reaction enthalpy. Heat loss, heat absorbed by apparatus, evaporation and incomplete combustion can change the result. Repeated readings reduce random uncertainty, but simply repeating the same heat-loss arrangement does not remove a systematic bias. Extrapolating a cooling curve can estimate a corrected temperature rise.
Hess’s law follows conservation of energy: enthalpy change depends on initial and final states. With formation enthalpies, subtract the sum for reactants from the sum for products, using coefficients. With combustion enthalpies, a cycle often reverses that subtraction; draw the arrows to avoid a memorised-sign error. Bond breaking requires energy and bond making releases it. Mean bond enthalpies give an approximate gaseous reaction enthalpy from energy in minus energy out, because average bond values vary with molecular environment.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Formation enthalpies total −600 kJ mol⁻¹ for products and −250 for reactants. Calculate the reaction enthalpy.
−350 kJ mol⁻¹ for the balanced reaction.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.