Hess's Law is a fundamental principle in thermochemistry that states: the enthalpy change of a reaction is independent of the pathway taken, meaning it's the same whether the reaction occurs in one step or multiple steps. This law allows us to calculate the enthalpy change of a reaction that cannot be measured directly by using the enthalpy changes of known reactions.
Here's how Hess's Law is used:
1. Identify the target reaction: Determine the balanced chemical equation for the reaction whose enthalpy change you want to calculate.
2. Find known reactions: Find reactions with known enthalpy changes (usually given in tables or data sheets) that involve the reactants and products of the target reaction.
3. Manipulate the known reactions: You may need to:
* Reverse a known reaction to match the direction of the target reaction. Remember, reversing a reaction changes the sign of its enthalpy change.
* Multiply a known reaction by a factor to match the stoichiometric coefficients of the target reaction. Remember, multiplying a reaction by a factor also multiplies its enthalpy change by that factor.
4. Combine the manipulated reactions: Add the manipulated reactions together. The enthalpy change of the combined reaction will be the sum of the enthalpy changes of the individual reactions.
5. Ensure cancellation: Make sure all species that appear on both sides of the combined reactions cancel out, leaving only the reactants and products of the target reaction.
6. Calculate the enthalpy change: The enthalpy change of the target reaction is the sum of the enthalpy changes of the manipulated reactions.
Example:
Let's calculate the enthalpy change of the following reaction:
```
CO(g) + 1/2 O2(g) -> CO2(g)
```
We know the following reactions and their enthalpy changes:
```
C(s) + O2(g) -> CO2(g) ΔH = -393.5 kJ/mol
C(s) + 1/2 O2(g) -> CO(g) ΔH = -110.5 kJ/mol
```
To get the target reaction, we can:
* Reverse the second reaction:
```
CO(g) -> C(s) + 1/2 O2(g) ΔH = +110.5 kJ/mol
```
* Add the reversed second reaction to the first reaction:
```
C(s) + O2(g) -> CO2(g) ΔH = -393.5 kJ/mol
CO(g) -> C(s) + 1/2 O2(g) ΔH = +110.5 kJ/mol
---------------------------
CO(g) + 1/2 O2(g) -> CO2(g) ΔH = -283 kJ/mol
```
Therefore, the enthalpy change of the target reaction is -283 kJ/mol.
Key Points:
* Hess's Law is a powerful tool for calculating enthalpy changes of reactions that are difficult or impossible to measure directly.
* It relies on the principle that enthalpy changes are state functions, meaning they depend only on the initial and final states of the system, not the pathway taken.
* Carefully manipulate known reactions to match the target reaction and pay attention to the signs and coefficients of enthalpy changes.
By using Hess's Law, you can predict enthalpy changes for a wide range of reactions using known thermochemical data.
