Endothermic & Exothermic Reactions

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Enthalpy Changes

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

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.

An exothermic reaction is one which gives out heat energy.
An exothermic reaction has a negative enthalpy change.
- Energy is transferred from the reaction system to the surroundings.
An endothermic reaction is one which takes in heat energy.
An endothermic reaction has a positive enthalpy change.
- Energy is transferred from the surroundings to the reaction system.

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

The formation of a solution may be viewed as a stepwise process:
1. Energy is consumed to overcome solute-solute and solvent-solvent attractions (endothermic processes).
2. Energy is released when solute-solvent attractions are established (an exothermic process called solvation).
The relative magnitudes of the energy changes determine whether the dissolution process overall will release or absorb energy.
Solutions may not form if the energy required to separate solute and solvent species is much greater than the energy released by solvation.

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

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.

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.

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.

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.