Thermochemical equations are balanced chemical equations that include the enthalpy change (ΔH), which represents the heat absorbed or released during the reaction. The enthalpy change is positive for endothermic reactions (heat absorbed) and negative for exothermic reactions (heat released)....
Manipulating Thermochemical Equations
Thermochemical equations can be manipulated in a similar way to regular chemical equations. Here are the key rules:
- Multiplying by a constant: Multiplying a thermochemical equation by a constant multiplies the enthalpy change by the same constant. For example, if a reaction has ΔH = -50 kJ/mol, multiplying the equation by 2 will result in ΔH = -100 kJ/mol.
- Reversing the equation: Reversing a thermochemical equation changes the sign of the enthalpy change. If a reaction has ΔH = -50 kJ/mol, reversing the equation will result in ΔH = +50 kJ/mol.
Combining Reactions to Determine Enthalpy Change
When combining multiple thermochemical equations, the enthalpy change of the overall reaction is the sum of the enthalpy changes of the individual reactions. This is based on Hess's Law, which states that the enthalpy change of a reaction is independent of the pathway taken.
Applying the Concepts: An Example
Consider the following reactions:
Reaction 1: NaOH(s) → Na+(aq) + OH−(aq) ΔH1
Reaction 2: NaOH(s) + H+(aq) + Cl−(aq) → Na+(aq) + Cl−(aq) + H2O(l) ΔH2
Reaction 3: Na+(aq) + OH−(aq) + H+(aq) + Cl−(aq) → Na+(aq) + Cl−(aq) + H2O(l) ΔH3
We can manipulate Reactions 1 and 2 to obtain Reaction 3. Here's the process:
- Reverse Reaction 1: This cancels out Na+(aq) and OH−(aq) on the product side of Reaction 1 and introduces them on the reactant side of Reaction 3.
- Add the modified Reaction 1 to Reaction 2: This cancels out NaOH(s) and leaves the desired products and reactants for Reaction 3.
The resulting equation after combining Reactions 1 and 2 is Reaction 3. By manipulating Reactions 1 and 2, we can determine the relationship between their enthalpy changes and the enthalpy change of Reaction 3:
ΔH3 = -ΔH1 + ΔH2
Conclusion
Understanding how to manipulate and combine thermochemical equations is essential for calculating the enthalpy change of complex reactions. By applying these principles, we can predict the heat absorbed or released in a given reaction and gain valuable insights into the energy changes involved in chemical processes.