The enthalpy change (\( \Delta H \)) for a reaction can be calculated using the formula:
\[ \Delta H = H_{\text{products}} - H_{\text{reactants}} \]
Where:
- \( H_{\text{products}} \) is the total enthalpy of the products.
- \( H_{\text{reactants}} \) is the total enthalpy of the reactants.
Example
Consider a hypothetical reaction:
\[ A + B \rightarrow C + D \]
Let's say:
- The enthalpy of reactants \( H_{\text{reactants}} = 150 , \text{kJ/mol} \).
- The enthalpy of products \( H_{\text{products}} = 100 , \text{kJ/mol} \).
Using the formula: \[ \Delta H = H_{\text{products}} - H_{\text{reactants}} = 100 , \text{kJ/mol} - 150 , \text{kJ/mol} = -50 , \text{kJ/mol} \]
This shows that the reaction is exothermic, since the enthalpy change is negative.
Energy Diagram
An energy diagram visually represents the energy changes during a chemical reaction. Here’s a simple conceptual representation of an energy diagram for the above reaction:
Energy
|
| ∆H = -50 kJ/mol
| C + D
| *
| *
| * Reactants
| * A + B
| *
| *
| ___________ *_____________________
| Reactants
| Products
|
|-------------------------------------> Reaction Coordinate
In this diagram:
- The y-axis represents energy.
- The x-axis represents the progress of the reaction.
- The starting point (Reactants) has higher energy compared to the endpoint (Products), indicating that energy was released in the form of heat, making the reaction exothermic.
Note
You can sketch this diagram using graphing software or graph paper to properly illustrate the energy levels and include labels for reactants and products.