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Enthalpy Changes

Chemistry: Cambridge International A Level, topic 1.5.3

Enthalpy Changes

For a given process or reaction, we can measure the enthalpy change.

Definition of enthalpy change

Definition of enthalpy change

  • An enthalpy change is a measure of the heat given out or taken in during a process.
  • When objects are heated, they use energy to expand.
  • Enthalpy takes into account the energy used in the expansion.
Enthalpy vs energy changes

Enthalpy vs energy changes

  • We use enthalpy instead of energy because we cannot easily measure energy changes. This is because objects expand when heated.
  • To measure an energy change, we would have to fix the volume of the object.
  • Enthalpy is much easier to use as it allows for expansion.
  • Enthalpy changes are instead measured under constant pressure.
  • The atmosphere is at a constant pressure, so we require no extra equipment.
Endothermic vs exothermic reactions

Endothermic vs exothermic reactions

  • An exothermic reaction is one which gives out heat energy.
  • An exothermic reaction has a negative enthalpy change.
  • An endothermic reaction is one which takes in heat energy.
  • An endothermic reaction has a positive enthalpy change.
Combustion is exothermic

Combustion is exothermic

  • Combustion is an exothermic process as it gives out heat!
  • E.g. Burning methane:
    • CH4 + 2O2 → CO2 + 2H2O
    • ΔH = −882.00kJmol-1
Thermal decomposition is endothermic

Thermal decomposition is endothermic

  • Thermal decomposition is an endothermic process.
  • E.g. The thermal decomposition of calcium carbonate:
    • CaCO3 → CaO + CO2
    • ΔH = +178.30kJmol-1

Bond Breaking and Making

The basis of chemical reactions is a series of bond breaking and making.

Bond enthalpies

Bond enthalpies

  • During a reaction, some chemical bonds must be broken and made.
  • The energy to break or make a bond is known as the bond enthalpy.
  • Energy is needed to break a bond so it is an endothermic process.
  • Energy is given off when a bond is made so it is an exothermic process.
  • The enthalpy change of a reaction is a sum of the individual bond enthalpies being broken and made during the reaction.
The dominating term

The dominating term

  • During a chemical reaction, we must determine whether the reaction requires more energy to break bonds or to make bonds.
  • This will give an overall reaction enthalpy that is either positive or negative.
Endothermic vs exothermic

Endothermic vs exothermic

  • Since bond breaking is endothermic and bond making is exothermic:
  • More energy required to break bonds in a reaction will lead to an overall endothermic reaction.
  • More energy released making bonds in a reaction will lead to an overall exothermic reaction.

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.
ΔcHθ

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.
ΔfHθ

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.

Test yourself on Enthalpy Changes

  • Types of enthalpy changes

  • Which of the following is true of exothermic reactions?

  • What phase would water be in under standard conditions?

  • Which of the following represents enthalpy change?

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