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A-level Chemistry: 3.1.4 Energetics Part 2

Chemistry17 CardsCreated 2 months ago

This deck covers key concepts in energetics and calorimetry, including Hess's Law, bond enthalpy, and methods to calculate enthalpy changes.

Describe how you can use a graph to find an accurate temperature change

During experiment, record temp at regular intervals
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Key Terms

Term
Definition
Describe how you can use a graph to find an accurate temperature change
During experiment, record temp at regular intervals
State what equation (& the units) you can use to calculate the enthaply change in calorimetry
State the equation used to calculate heat energy given out per mole
ΔH = q / moles of reaction
Define Hess's Law
Total enthalpy change of a reaction is independent of the route taken
What is Hess's law used for?
Working out enthaply changes that you can't find directly by doing an experiment
When working out enthalpies changes of formation, what data do you need?
ΔfH⦵ for all reactants and products that are compounds

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TermDefinition
Describe how you can use a graph to find an accurate temperature change
During experiment, record temp at regular intervals
State what equation (& the units) you can use to calculate the enthaply change in calorimetry
State the equation used to calculate heat energy given out per mole
ΔH = q / moles of reaction
Define Hess's Law
Total enthalpy change of a reaction is independent of the route taken
What is Hess's law used for?
Working out enthaply changes that you can't find directly by doing an experiment
When working out enthalpies changes of formation, what data do you need?
ΔfH⦵ for all reactants and products that are compounds
Value of ΔfH⦵ for all elements = ____
0 (∵ element's being 'formed' from element = no change)
What is bond enthalpy?
Energy required to break bonds
Define mean bond enthalpy
Enthalpy heat (energy) change required to break a covalent bond on average across different compounds

Draw the bonds between CO2

Draw the bonds between NH3

Draw the bonds between water

Draw the bonds between oxygen

Draw the bonds between nitrogen

Why are enthaply change values calculated from mean bond enthalpies less accurate than using Hess's Law?
∵ use average values, enthalpy changes calculated using mean bond enthalpies aren't exact

Using calorimetry to find the enthalpy of combustion of a flammable liquid Suggest 3 general improvements you can do to reduce errors due to heat loss

  • Put a draught shield around the apparatus

  • Put a lid on the cup

  • Reduce the distance between the flame and the beaker

  • Use a copper calorimeter rather than a pyrex beaker

Explain why the bond enthalpy of a Cl-Cl bond is greater than that of a Br-Br bond (2)

  • Bond is shorter or bonding pair closer to nucleus

    • (Cl is a smaller atom)

  • So attraction (between nucleus and) (to) bond pair is stronger