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A-level Chemistry: 3.1.4 Energetics Part 1

Chemistry22 CardsCreated 2 months ago

This deck covers key concepts in energetics, including definitions, standard conditions, and calorimetry, as outlined in A-level Chemistry curriculum.

Define Enthalpy

Heat content in a substance
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Key Terms

Term
Definition
Define Enthalpy
Heat content in a substance
Define Enthalpy Change (ΔH)
Change in heat energy (of a reaction) at a constant pressure
State the units for enthalpy change (ΔH)
kJ mol-1
What is meant by this symbol ⦵?
Means substances were in their standard states and measurement was under standard conditions
Name the standard conditions
1 atm (100 kPa) 298 K
Exothermic: ΔH is ______
negative

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TermDefinition
Define Enthalpy
Heat content in a substance
Define Enthalpy Change (ΔH)
Change in heat energy (of a reaction) at a constant pressure
State the units for enthalpy change (ΔH)
kJ mol-1
What is meant by this symbol ⦵?
Means substances were in their standard states and measurement was under standard conditions
Name the standard conditions
1 atm (100 kPa) 298 K
Exothermic: ΔH is ______
negative
Endothermic: ΔH is ______
positive
Give 2 examples of exothermic reactions
Neutralisation Combustion
Give 2 examples of endothermic reactions
Thermal decomposition Photosynthesis

Sketch a graph to show an exothermic reaction

Sketch a graph to show an endothermic reaction

Why is bond breaking endothermic (ΔH = positive)?
Need energy to break bonds
Why is bond making exothermic (ΔH = negative)?
Energy released when bonds formed
If more energy is needed to break bonds than is released when bonds made, ΔH is _____
positive (If it’s less = ΔH is negative)
Define standard enthalpy of formation (ΔfH⦵)
Is the enthalpy change when 1 mole of a compound is formed from its constituent elements under standard states & standard condition
Define standard enthalpy of combustion (ΔcH⦵)
Is the enthalpy change when 1 mole of a substance is burnt completely in oxygen, under standards states & standard conditions
Standard enthalpy of formation equation K2Cr2O7(l)
2K(s) + 2Cr(s) + 7/2O2(g) → K2Cr2O7(l)
Standard enthalpy of combustion equation CH4(g)
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
How do we use calorimetry to find how much heat is given by reaction?
By measuring temperature change
Describe how you can find the enthalpy of combustion of a flammable liquid using calorimetry
Burn flammable liquid inside apparatus (e.g. calorimeter) Then can work out heat energy that’s been absorbed by water (heat given by fuel as it burns = be absorbed by water)
Why is it hard to get an accurate result when using calorimetry? Name 3 reasons
Heat is always lost to the surroundings Combustion may be incomplete (less energy given out) Flammable liquids often volatile = lose some fuel to evaporation

Describe how you can use calorimetry to calculate the enthalpy change for a neutralisation reaction

  1. Add known volume of acid to insulated container (e.g. polystyrene cup) & measure temperature

  2. Add known volume of alkali and record temp. of mixture at regular intervals over period of time

    1. Stir solution to make sure it’s evenly heated

  3. Find temp change of experiment

    1. Use it to calculate enthalpy change of reaction