Understanding Hess's Law is pivotal for anyone studying thermodynamics or physical chemistry. This law provides a useful approach to calculating the heat of reaction for processes that are difficult to measure directly. For students grappling with complex enthalpy problems, this guide simplifies these concepts with clear examples and explanations, helping you master Hess's Law calculations with ease.
What is Hess’s Law?
Hess’s Law states that the total enthalpy change in a chemical reaction is independent of the pathway taken between the initial and final states. This principle allows for the summing of enthalpy changes from multiple steps to determine the overall change for a given chemical transformation.
💡 Note: Enthalpy (H) represents the sum of the internal energy of a system and the product of pressure and volume.
Hess’s Law Applications
- Indirect Measurement: Calculating enthalpy changes for reactions that are impractical or impossible to measure experimentally.
- Summing Enthalpies: Adding or subtracting enthalpies of several reactions to find an unknown enthalpy change.
- Standard Molar Enthalpy: Useful for determining standard enthalpy changes for formation, combustion, and solution processes.
Hess’s Law Calculations: Step-by-Step
- Identify the overall reaction: Write down the target chemical equation.
- List the given reactions: These are reactions with known enthalpy changes.
- Manipulate the given reactions: Adjust these equations by reversing, doubling, or halving as necessary to match the overall reaction:
- Reversing a reaction changes the sign of ΔH.
- Multiplying or dividing the reaction by any factor multiplies or divides ΔH by the same factor.
- Sum the adjusted reactions: Combine the manipulated reactions so that intermediates cancel out, leaving the desired overall reaction.
- Add the enthalpy changes: Sum the ΔH values for each step to find the overall enthalpy change.
Example Problem:
Let’s find ΔH for the combustion of glucose:
C₆H₁₂O₆(s) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l)
We have the following reactions:
| Reaction | ΔH (kJ) |
|---|---|
| C(graphite) + O₂(g) → CO₂(g) | -393.5 |
| H₂(g) + ½ O₂(g) → H₂O(l) | -285.8 |
| 6 C(graphite) + 6 H₂(g) + 3 O₂(g) → C₆H₁₂O₆(s) | -1274.4 |
Here's how we proceed:
- Target reaction: C₆H₁₂O₆(s) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l)
- Manipulate given reactions:
- Multiply 1 by 6: 6 C(graphite) + 6 O₂(g) → 6 CO₂(g), ΔH = 6(-393.5) = -2361.0 kJ
- Multiply 2 by 6: 6 H₂(g) + 3 O₂(g) → 6 H₂O(l), ΔH = 6(-285.8) = -1714.8 kJ
- Reverse 3: C₆H₁₂O₆(s) → 6 C(graphite) + 6 H₂(g) + 3 O₂(g), ΔH = 1274.4 kJ
- Sum the reactions:
C₆H₁₂O₆(s) + 6 C(graphite) + 6 H₂(g) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l) + 6 C(graphite) + 6 H₂(g)
Net reaction: C₆H₁₂O₆(s) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l)
- Sum ΔH values: -2361.0 kJ + (-1714.8 kJ) + 1274.4 kJ = -2801.4 kJ
Here, we've deduced the combustion enthalpy of glucose using Hess's Law, with an answer of -2801.4 kJ.
📘 Note: Always ensure that the given reactions lead to the overall reaction when summed correctly.
In the final recap, learning Hess’s Law proves beneficial for calculating complex enthalpy changes, especially when direct measurement is challenging. By understanding the principles and following the step-by-step guide, you can tackle any Hess’s Law problem with confidence. This law not only simplifies calculations but also helps you see the connections between different chemical reactions in a broader context.
What is the purpose of Hess’s Law?
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Hess’s Law allows chemists to calculate the enthalpy change for reactions that cannot be directly measured, by summing known enthalpy changes of other reactions.
Can Hess’s Law be applied to reactions with different pressures or temperatures?
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Yes, Hess’s Law applies to state functions like enthalpy, which are independent of the path taken. Adjustments for pressure and temperature can be made using thermodynamic principles.
Why must intermediates cancel out in Hess’s Law calculations?
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To achieve the target reaction, the substances produced in one step must be consumed in another, ensuring that only the initial and final reactants and products remain in the overall equation.
