1.4.1

Enthalpy Changes

Test yourself

Enthalpy Changes

For a given process or reaction, we can measure the 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

• 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

• 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 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 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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.