​​Enthalpy changes and reaction profiles

In a nutshell

Enthalpy changes are key to understanding the transfer of heat energy during a reaction. In this summary, you will learn about exothermic and endothermic reactions, and how these relate to enthalpy. Reaction profile diagrams allow you to determine if a reaction is exothermic or endothermic and reveal key parts of the reaction.

Enthalpy

​​Enthalpy is a thermodynamic measure of the heat energy in a system and the surroundings. The more enthalpy (energy) a substance has, the more unstable it is.

Enthalpy change

Enthalpy change, $$\varDelta H$$ refers to the heat energy change from breaking and making of bonds during a reaction. This occurs at constant pressure and the units are $$kJ\ mol^{-1}$$.

Standard conditions of enthalpy change

Standard enthalpy change, $$\varDelta H^\theta$$ refers to the enthalpy change of a reaction measured with elements in the standard states, at standard pressure and a specific temperature. The standard pressure is $$100\ kPa$$​ and the specified temperature is typically measured at $$298\ K$$. 

Note: Standard pressure can be referred to in units of $$100\ kPa$$ or the equivalent of $$1\ atm$$. 

​These standard conditions are important to use as both temperature and pressure impact enthalpy changes. Temperature can be converted from $$^oC$$ to $$K$$

by adding $$273$$.

Example

To convert the boiling point of water from $$^o C$$ to $$K$$​:​

​$$100\ ^oC +273= 373\ K$$​

​​

Exothermic and endothermic reactions

In both endothermic and exothermic reactions, energy must be absorbed by the reactants to break the bonds in each molecule. Then, in the formation of new bonds by the products, energy is released. 

If the released energy is greater than the one absorbed, $$\varDelta H$$ is negative and the temperature of the surroundings increases.

Example

The combustion of coal is exothermic as heat is released from burning a fuel.

$$C_{(s)} + O_2\ _{(g)} \rightarrow CO_2\ _{(g)} +heat$$

If the absorbed energy is greater than the one released, $$\varDelta H$$ is positive and the temperature of the surroundings decreases.

Example

The melting of ice is endothermic as heat is absorbed.

​$$H_2O _{(s)} + heat \rightarrow H_2O _{(l)}$$​​

Properties of exothermic and endothermic reactions

Reaction profile diagrams

Reaction profile diagrams show the enthalpy change, $$\varDelta H$$ in a reaction by displaying the relative enthalpies of the reactants and products. 

​Activation energy, $$E_a$$​ refers to the amount of energy required to overcome the energy barrier to start the chemical reaction. This is the energy required to break the chemical bonds of the reactants.  

The reaction profile diagram of an exothermic reaction can be seen below.

​Exothermic reactions start with unstable (higher enthalpy) reactants and forms stable (lower enthalpy) products resulting in a negative $$\varDelta H$$​. Exothermic reactions have a small $$E_a$$​​ as less energy is needed to  overcome the energy barrier of the higher enthalpy reactants to form lower enthalpy products.​

The reaction profile diagram of an endothermic reaction can be seen below.

Endothermic reactions start with stable (lower enthalpy) reactants and forms unstable (higher enthalpy) products, resulting in a positive $$\varDelta H$$. Endothermic reactions have a large $$E_a$$ as more energy is needed to overcome the energy barrier of the lower enthalpy reactants to form higher enthalpy products. 

Note: $$-\varDelta H$$ arrows point downwards for exothermic reactions and $$+\varDelta H$$ point upwards for endothermic reactions.