1.4.3

Enthalpy Changes: Standard Conditions

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Standard Conditions

For many measurements, we can record them under standard conditions. This is true for enthalpy changes - we call it the standard enthalpy change.

Symbols

Symbols

  • Enthalpy is given the symbol H.
  • Enthalpy changes are given the symbol ΔH.
    • A negative enthalpy change (pictured) is one which gives out heat.
    • A positive enthalpy change is one which takes in heat.
Standard conditions

Standard conditions

  • You'll hear the term 'standard conditions' a lot in chemistry. These conditions refer to:
    • A pressure of 1 bar or 100 kPa.
    • A temperature of 298K.
    • A substance's most stable state at 298K and 1 bar pressure.
      • E.g. For water, it is liquid.
      • E.g. For carbon, it is graphite.
Standard enthalpy changes

Standard enthalpy changes

  • A standard enthalpy change is an enthalpy change carried out under standard conditions.
  • There are two you need to know;
    • Standard enthalpy of combustion (ΔcHθ).
    • Standard enthalpy of formation (ΔfHθ).
  • The superscript θ signifies that it is a standard enthalpy change.
&Delta;<sub>c</sub>H<sup>&theta;</sup>

ΔcHθ

  • The standard enthalpy change of combustion is defined as:
    • The enthalpy change when one mole of a substance in its standard state burns completely in oxygen under standard conditions of 298K and 1 bar pressure.
&Delta;<sub>f</sub>H<sup>&theta;</sup>

ΔfHθ

  • The standard enthalpy change of formation is defined as:
    • The enthalpy change when one mole of a substance is formed in its standard state from the pure elements in their standard states under standard conditions of 298K and 1 bar pressure.
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1

Physical Chemistry

1.1

Atomic Structure

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1.2

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Energetics

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A-A* (AO3/4) - Organic 2

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Practice questions on Enthalpy Changes: Standard Conditions

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  1. 1
    Standard conditions:Fill in the list
  2. 2
    What phase would water be in under standard conditions?Multiple choice
  3. 3
    Which of the following represents enthalpy change?Multiple choice
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Bond Breaking and Making

Calorimetry