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Energetics
This topic uses energy transfer data to calculate and explain enthalpy changes.
18
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91
Flashcards
89
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90 min
Study time
AQAA LevelChemistryPhysical chemistry
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Enthalpy change4 objectives
- Define enthalpy change under constant pressure.
- Distinguish exothermic and endothermic reactions using enthalpy change signs.
- Interpret reaction profile diagrams.
- Define standard enthalpy changes for formation, combustion and neutralisation where appropriate.
Calorimetry and enthalpy measurements5 objectives
- Use q = mcΔT to calculate heat energy transferred.
- Convert calorimetry data into molar enthalpy changes.
- Use appropriate units and significant figures in enthalpy calculations.
- Explain practical limitations and heat loss in calorimetry.
- Required practical: measure an enthalpy change.
Applications of Hess's law5 objectives
- State Hess's law.
- Use Hess's law to calculate reaction enthalpy changes.
- Calculate enthalpy changes from enthalpies of combustion.
- Calculate enthalpy changes from enthalpies of formation.
- Construct and interpret enthalpy cycles.
Bond enthalpies4 objectives
- Define mean bond enthalpy.
- Calculate approximate enthalpy changes from mean bond enthalpies.
- Distinguish bonds broken from bonds formed in bond enthalpy calculations.
- Explain why mean bond enthalpy estimates differ from values obtained using Hess's law.
Key terms
enthalpy changeconstant pressureexothermic reactionendothermic reactionreaction profile diagramactivation energystandard enthalpy change of formationstandard enthalpy change of combustionheat energy transferredspecific heat capacitymolar enthalpy changecalorimetry
Exam tips
- Understanding Enthalpy Change: Explain clearly by define enthalpy change as the heat energy change at constant pressure during a reaction. Link the point to Enthalpy change, then state the evidence, calculation, mechanism, observation, or conclusion required by the question.
- Understanding Reaction Types: Explain clearly by clearly distinguish between exothermic and endothermic reactions by focusing on their enthalpy change signs. Link the point to Enthalpy change, then state the evidence, calculation, mechanism, observation, or conclusion required by the question.
Common mistakes
- Misunderstanding Enthalpy Change Definition: To correctly define enthalpy change, remember that it represents the heat content of a system at constant pressure. Use the formula ΔH = q_p, where ΔH is the enthalpy change and q_p is the heat transferred at constant pressure. Ensure to clarify that this definition applies specifically under constant pressure conditions.
- Confusing Exothermic and Endothermic Reactions: Exothermic reactions release energy to the surroundings, resulting in a negative enthalpy change (ΔH < 0), while endothermic reactions absorb energy, leading to a positive enthalpy change (ΔH > 0). To distinguish between them, remember that exothermic reactions feel hot (like combustion), whereas endothermic reactions feel cold (like photosynthesis).
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