Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.7 ΔS_system calculations-Study Notes - New Syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.7 ΔS_system calculations- Study Notes- New syllabus
Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.7 ΔS_system calculations- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.
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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics
12.7 Calculation of Entropy Change of the System (\( \mathrm{\Delta S_{system}} \))
The entropy change of the system for a reaction can be calculated using standard entropy values of reactants and products. This allows the change in disorder during a reaction to be quantified and used to predict feasibility when combined with other factors.
Entropy Change of the System
\( \mathrm{\Delta S_{system} = \sum S^\circ (products) – \sum S^\circ (reactants)} \)
Where:
- \( \mathrm{S^\circ} \) = standard entropy value (usually in \( \mathrm{J\ mol^{-1}K^{-1}} \))
- Values must be multiplied by their respective stoichiometric coefficients.
Method
- Write the balanced chemical equation.
- Identify the \( \mathrm{S^\circ} \) values for all reactants and products.
- Multiply each value by its coefficient in the equation.
- Sum the entropy values for products.
- Sum the entropy values for reactants.
- Subtract: products − reactants.
Interpretation
- \( \mathrm{\Delta S_{system} > 0} \): increase in disorder (more randomness).
- \( \mathrm{\Delta S_{system} < 0} \): decrease in disorder (more order).
- Often linked to changes in state or number of gas molecules.
Key Considerations
- Ensure all entropy values are in consistent units.
- Include physical states (s, l, g, aq) as they affect entropy values.
- Pay attention to coefficients in balanced equations.
Example 1:
Calculate \( \mathrm{\Delta S_{system}} \) for the reaction:
\( \mathrm{2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)} \)
Given: \( \mathrm{S^\circ(SO_2) = 248,\ S^\circ(O_2) = 205,\ S^\circ(SO_3) = 257\ J\ mol^{-1}K^{-1}} \)
▶️ Answer/Explanation
Sum of products: \( \mathrm{2 \times 257 = 514} \)
Sum of reactants: \( \mathrm{(2 \times 248) + 205 = 496 + 205 = 701} \)
\( \mathrm{\Delta S_{system} = 514 – 701 = -187\ J\ mol^{-1}K^{-1}} \)
The negative value indicates a decrease in disorder, consistent with a reduction in moles of gas.
Example 2:
Calculate \( \mathrm{\Delta S_{system}} \) for the reaction:
\( \mathrm{N_2O_4(g) \rightarrow 2NO_2(g)} \)
Given: \( \mathrm{S^\circ(N_2O_4) = 304,\ S^\circ(NO_2) = 240\ J\ mol^{-1}K^{-1}} \)
▶️ Answer/Explanation
Sum of products: \( \mathrm{2 \times 240 = 480} \)
Sum of reactants: \( \mathrm{304} \)
\( \mathrm{\Delta S_{system} = 480 – 304 = +176\ J\ mol^{-1}K^{-1}} \)
The positive value indicates an increase in disorder due to an increase in the number of gas molecules.