Hess's Law:
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.
How it's used to measure enthalpy:
1. Identify Known Enthalpy Changes: Find reactions with known enthalpy changes (ΔH) that can be combined to form the desired reaction. These reactions are often found in tables of standard enthalpy of formation.
2. Manipulate Equations: You might need to:
* Reverse reactions: If a reaction is reversed, the sign of ΔH is flipped.
* Multiply reactions by a constant: If a reaction is multiplied by a factor, ΔH is also multiplied by that factor.
3. Combine Reactions: Add the manipulated equations together, ensuring that any common species on both sides of the equations cancel out.
4. Calculate Overall Enthalpy Change: The enthalpy change of the desired reaction (ΔH) is the sum of the enthalpy changes of the individual reactions used to form it.
Example:
Let's say we want to find the enthalpy change for the following reaction:
C(s) + O2(g) → CO2(g) (Desired Reaction)
We know the following reactions and their enthalpy changes:
1. C(s) + 1/2 O2(g) → CO(g) ΔH = -110.5 kJ/mol
2. CO(g) + 1/2 O2(g) → CO2(g) ΔH = -283.0 kJ/mol
To get the desired reaction, we can:
1. Leave Reaction 2 as is.
2. Multiply Reaction 1 by 2.
Now, combining the equations gives:
2C(s) + O2(g) → 2CO(g) ΔH = -221.0 kJ/mol
2CO(g) + O2(g) → 2CO2(g) ΔH = -566.0 kJ/mol
C(s) + O2(g) → CO2(g) ΔH = -787.0 kJ/mol
Therefore, the enthalpy change of the desired reaction is -787.0 kJ/mol.
Key Points:
* Hess's Law is a powerful tool for calculating enthalpy changes, even for reactions that are difficult or impossible to measure directly.
* It relies on the principle of conservation of energy, meaning that the total enthalpy change for a reaction is the same regardless of the pathway taken.
* Understanding enthalpy of formation tables and how to manipulate chemical equations is crucial for using Hess's Law.
