5.1.3 — Enthalpy Signs & Temperature Changes
Total: 24 marks · 15 Questions · Sections A, B, C · Reveal answers when ready
Exothermic
Endothermic
Section A — The Rules
6 marksState the sign of ΔH for an endothermic reaction.
💡 Hint: Think: the system GAINS energy from surroundings.
State the sign of ΔH for an exothermic reaction.
💡 Hint: Think: the system RELEASES energy to surroundings.
In an endothermic reaction, energy is transferred from the ________ to the ________.
💡 Hint: Where does the energy come FROM and where does it GO?
In an exothermic reaction, energy is transferred from the ________ to the ________.
💡 Hint: The reaction mixture releases heat — where does it go?
A student says: 'An exothermic reaction has a large ΔH.' Identify TWO errors in this statement.
💡 Hint: Think about (1) the sign and (2) the meaning of 'large'.
Section B — Application
8 marksA reaction has a ΔH of −286 kJ/mol. What will happen to the temperature of the reaction mixture?
💡 Hint: Negative ΔH means energy is released.
A reaction has a ΔH of +92 kJ/mol. What will happen to the temperature of the reaction mixture?
💡 Hint: Positive ΔH means energy is absorbed from surroundings.
The combustion of methane has ΔH = −890 kJ/mol. (a) Is this reaction exothermic or endothermic? (b) Explain what the negative sign tells you about energy transfer.
💡 Hint: Negative = energy released. Where does it go?
Thermal decomposition of calcium carbonate has ΔH = +178 kJ/mol. (a) Is this endothermic or exothermic? (b) What happens to the temperature during the reaction?
💡 Hint: Positive sign = energy in.
Two reactions are shown below: Reaction X: ΔH = −572 kJ/mol Reaction Y: ΔH = +131 kJ/mol (a) Which reaction releases energy to the surroundings? (b) Which reaction will cause the temperature of the mixture to decrease?
💡 Hint: Match the sign to the type.
Section C — Definitions
10 marksWhat is meant by the term enthalpy change (ΔH)?
💡 Hint: Think: heat energy, standard conditions, per mole.
Define the term exothermic reaction in terms of enthalpy change.
💡 Hint: Use ΔH sign in your definition.
Define the term endothermic reaction in terms of enthalpy change.
💡 Hint: Use ΔH sign and direction of energy flow.
What are the units of enthalpy change?
💡 Hint: Per mole is important!
Explain the difference between bond breaking and bond forming in terms of energy and their effect on the sign of ΔH. [3 marks]
💡 Hint: One is endo, one is exo — which is which?
🎯 Professor's Final Reminder
The three golden rules: (1) Exothermic → ΔH negative → temperature increases. (2) Endothermic → ΔH positive → temperature decreases. (3) Always state the direction of energy transfer in definitions — "released to surroundings" earns marks, "released" alone does not.
Sign In to Keep Going
You've reached the end of the free preview. Sign in — and upgrade to Plus or Pro — to keep going with Chemistry.
Full Access
Unlock all 320 lessons