Chemical change — WJEC A-Level Chemistry
In summary: Chemical change is a key topic in WJEC A-Level Chemistry. Key exam tip: Always include state symbols in balanced equations unless otherwise specified
Exam Tips for Chemical change
- Always include state symbols in balanced equations unless otherwise specified
- When calculating Kc, ensure all concentrations are at equilibrium, not initial values
- Use the correct number of significant figures based on the least accurate piece of data provided
- When describing the effect of a catalyst, explicitly state it provides an alternative route with lower activation energy
- Clearly label axes and units when plotting graphs for rate or enthalpy determinations
Common Mistakes
- Confusing the effect of temperature on the rate of reaction with its effect on the equilibrium constant (Kc)
- Incorrectly applying Le Chatelier’s principle to pressure changes in reactions where the total moles of gas are equal on both sides
- Failing to use the correct units for Kc or rate constants
- Misinterpreting the Boltzmann distribution curve when temperature is increased
- Errors in sign convention when calculating enthalpy changes from experimental data
Marking Points
- Application of Le Chatelier’s principle to predict shifts in equilibrium position
- Calculation of equilibrium constants (Kc) from concentration data
- Use of Hess’s Law cycles to calculate enthalpy changes
- Interpretation of reaction profiles and activation energy
- Explanation of rate changes using collision theory and Boltzmann distribution
- Calculation of reaction rates from experimental data
- Understanding the role of catalysts in providing alternative reaction routes
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