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Energetics
This topic uses energy transfer data to calculate and explain enthalpy changes.
18
Objectives
10
Flashcards
10
Questions
90 min
Study time
AqaA LevelChemistryPhysical chemistry
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18 objective pages available
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 (H)Exothermic reactionStandard Enthalpy Change (ΔH°)Activation Energy (Ea)Standard ConditionsCalorimetryEnthalpyCalorimeterEnthalpy change (ΔH)Exothermic ReactionEndothermic ReactionStandard enthalpy of formation (ΔHf°)
Exam tips
- Always state conditions: When discussing enthalpy changes, always specify that the reaction occurs at constant pressure.
- Always check ΔH signs: When determining if a reaction is exothermic or endothermic, always check the sign of ΔH.
Common mistakes
- Confusing exothermic and endothermic reactions: Remember that a negative enthalpy change indicates an exothermic reaction, while a positive value indicates an endothermic reaction.
- Confusing exothermic and endothermic reactions: Remember that exothermic reactions release heat and have a negative ΔH, while endothermic reactions absorb heat and have a positive ΔH.
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