9701 · 5.1
Enthalpy change, ΔH flashcards
Revision flashcards for Cambridge 9701 Enthalpy change, ΔH (syllabus 5.1). Flip, recall, then mark a real past-paper question.
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What is enthalpy change (ΔH)?
The heat energy transferred in a chemical reaction at constant pressure. Its units are typically kilojoules per mole (kJ mol⁻¹).
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What is an exothermic reaction?
A reaction that releases heat energy into the surroundings, causing the temperature of the surroundings to increase. ΔH for an exothermic reaction is negative.
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What is an endothermic reaction?
A reaction that absorbs heat energy from the surroundings, causing the temperature of the surroundings to decrease. ΔH for an endothermic reaction is positive.
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On an enthalpy profile diagram for an endothermic reaction, are the products higher or lower than the reactants?
Higher. The system has absorbed energy from the surroundings, so the products have a higher enthalpy than the reactants.
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What are 'standard conditions' for enthalpy changes?
A pressure of 100 kPa and a specified temperature, usually 298 K (25 °C). For solutions, the concentration is 1 mol dm⁻³.
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What does the symbol ΔH⦵ represent?
Standard enthalpy change. The '⦵' (plimsoll) symbol signifies that the measurement was made under standard conditions.
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State the equation used in calorimetry to calculate heat energy transferred.
q = mcΔT, where q is heat energy (J), m is the mass of the substance being heated/cooled (g), c is the specific heat capacity (J g⁻¹ K⁻¹), and ΔT is the change in temperature (K or °C).
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What is the value of the specific heat capacity (c) of water?
4.18 J g⁻¹ K⁻¹. This value is provided in the exam data booklet but is very useful to know.
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A calorimetry experiment gives a ΔH value that is less exothermic than the data booklet value. Give two reasons why.
1. Heat loss to the surroundings (the most common reason). 2. Incomplete combustion or reaction. 3. Evaporation of fuel from the wick (for combustion experiments).
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How do you convert the calculated heat energy, q (in Joules), to the molar enthalpy change, ΔH (in kJ mol⁻¹)?
1. Divide q by 1000 to get kJ. 2. Divide by the number of moles (n) of the limiting reactant. 3. Add the correct sign (+ for endothermic, - for exothermic).
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Why is a polystyrene cup used as a calorimeter for reactions in solution?
Polystyrene is a good thermal insulator. It minimises heat exchange (loss or gain) with the surroundings, making the measured temperature change more accurate.