Acids and Bases
Equilibrium Constants
Free-Energy & Entropy
Further Enthalpy
Hess's Law
Rate of Reaction
Redox Equilibria
Redox Reactions

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Enthalpy Change

What is Enthalpy?

Usually there is energy change involved in a chemical reaction somehow. In chemistry this change in heat energy is represented as enthalpy change using the following symbol:

delta H

So that this figure is the same everywhere and fair, it is taken at standard conditions which are 298K and 100 kPa pressure. Here are two reactions, each with a different enthalpy change.

some reactions

For the first rection, the enthalpy change is negative this is because the reaction is exothermic meaning it gives out heat. The reason it is negative is energy is leaving the reaction.

The second reaction has a positive enthalypy change, this is because it is endothermic and heat is entering the system.

Enthalpy of Reaction, Combustion, and Formation

These are three different types of situation that an energy change may take place. On the top image x is used to show where a letter is written to indicate what type of enthalpy is shown: r for reaction and so on.

The easiest one to get your head around is standard enthalpy of reaction this is merely the energy change of a reaction in the quantities that are expressed.

The standard enthalpy of combustion is the enthalpy change when 1 mole of substance is completely burned in excess oxygen under standard conditions, all reactants in their standard states. The enthalpies of combustion can be determined using a calorimeter which is explained below; it uses the symbol c.

The standard enthalpy of formation is the enthalpy change when 1 mole of compound is produced from its elements in standard conditions, all products and reactants in the standard state. The standard state is important, it means the way that the element is, at standard conditions (see above). So you would have H2(g) NOT H on its own. Therefore you sometimes need to do fractions of elements, as with the standard enthalpy of formation of ethanol:

C (s) + 2H2 (g) + ½O2 (g) ® CH3OH (l)


This is a method we can use to find out what the enthalpy changes of reactions are. Typically you will do this at school using some sort of alcohol heating up some water. To do it correctly, you would have it in a closed system so no heat escapes, but this is not usually possible in a high school lab. Once the water has been heated up you must use the following equation.

calorimetry equation

A bit more about the specific heat capacity this is the energy required to raise the temperature of one gram of substance by 1 kelvin (or celcius depending on what you're using). It is measured in Jg-1K-1 and water has a specific heat capacity of 4.18 Jg-1K-1.

For example, if 1g of ethanol is burned to heat 100g of water, raising its temperature by 42K, you would do the following calculations:

calorimetry calculations