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Energetics I

Enthalpy changes, standard definitions and calorimetry

  • 3 things to know
  • Papers 1 and 3

Overview

Enthalpy change and standard conditions, exothermic and endothermic signs and enthalpy level diagrams, the standard enthalpy changes of reaction, formation, combustion and neutralisation, using Q = mcΔT to find ΔH in kJ mol⁻¹, and the heat losses and assumptions that limit calorimetry.

What you need to know

Answer each one before you reveal it.

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Process

How does calorimetry measure an enthalpy change?

Numbers

State the standard conditions used for enthalpy changes.

Key term

What is an enthalpy change?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is an enthalpy change?

    Model answer

    Heat taken in or given out while the pressure is held constant.

  2. 2

    State the standard conditions used for enthalpy changes.

    Model answer

    A pressure of 100 kPa and a stated temperature, usually 298 K.

  3. 3

    What is the sign of the enthalpy change for an exothermic reaction?

    Model answer

    Negative, because heat energy is released to the surroundings.

  4. 4

    What is the sign of the enthalpy change for an endothermic reaction?

    Model answer

    Positive, because heat energy is taken in from the surroundings.

  5. 5

    On an enthalpy level diagram, where are the products of an exothermic reaction relative to the reactants?

    Model answer

    Below the reactants, at lower enthalpy.

  6. 6

    On an enthalpy level diagram, where are the products of an endothermic reaction relative to the reactants?

    Model answer

    Above the reactants, at higher enthalpy.

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