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Energy Changes in Chemistry Flashcards
Free GCSE and A Level Chemistry Cards
From bond energy calculations to Hess's law and reaction profiles, these free Chemistry flashcards cover the energy changes content you need for GCSE and A Level exams.
Question
What is an exothermic reaction?
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Answer
A reaction that releases energy to the surroundings, causing a temperature rise. The products have lower energy than the reactants. Examples: combustion, neutralisation, respiration.
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Question
What is an endothermic reaction?
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Answer
A reaction that absorbs energy from the surroundings, causing a temperature fall. The products have higher energy than the reactants. Examples: thermal decomposition, photosynthesis, dissolving ammonium nitrate.
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Question
What is enthalpy change (ΔH)?
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Answer
The heat energy exchanged at constant pressure. ΔH is negative for exothermic reactions (energy released) and positive for endothermic reactions (energy absorbed). Units: kJ/mol.
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Question
What is the standard enthalpy of combustion?
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Answer
The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions (298 K, 100 kPa). Always exothermic (ΔH negative).
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Question
What is the standard enthalpy of formation?
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Answer
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. ΔHf of an element in its standard state = 0.
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Question
What is Hess's Law?
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Answer
The total enthalpy change for a reaction is independent of the route taken — only depends on the initial and final states. Allows calculation of ΔH values that are difficult to measure directly.
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Question
How do you calculate ΔH using bond enthalpies?
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Answer
ΔH = Σ(bonds broken) − Σ(bonds formed). Breaking bonds requires energy; forming bonds releases energy. This gives an estimate because average bond enthalpies are used.
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Question
What is a calorimeter and how is it used?
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Answer
A device to measure heat changes in reactions. Q = mcΔT, where m = mass of water, c = 4.18 J/g/°C (specific heat capacity of water), ΔT = temperature change.
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Question
What is activation energy?
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Answer
The minimum energy colliding particles must have for a reaction to proceed — represents the energy needed to break bonds in reactants and start the reaction.
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Question
What does an energy profile diagram show?
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Answer
The energy of reactants and products along the reaction coordinate. Shows the activation energy (peak) and ΔH (difference between reactants and products energy levels).
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