Back to AI Flashcard MakerChemistry /A-level Chemistry: 3.1.4 Energetics Part 1
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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| 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 |
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 |
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