How is the enthalpy change, ΔH calculated?
ΔH = H(products) - H(reactions)
What type of reaction is a negative ΔH?
Exothermic.
What type of reaction is a positive ΔH?
Endothermic.
Use ‘Enthalpy’ card to test knowledge of the enthalpy profile diagram of an exothermic reaction.
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Use ‘Enthalpy’ card to test knowledge of the enthalpy profile diagram of an endothermic reaction.
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Define activation energy.
The minimum energy required for a reaction to take place.
Use ‘Enthalpy’ card to test knowledge of the activation energy on an enthalpy profile diagram of an exothermic reaction.
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Use ‘Enthalpy’ card to test knowledge of the activation energy on an enthalpy profile diagram of an endothermic reaction.
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What are standard conditions?
- 100 kPa.
- 298K.
What is the standard concentration for solutions?
1 mol dm^-3.
Define the term enthalpy change of formation.
The enthalpy change when 1 mole of substance is formed
from its constituent elements.
Define the term enthalpy change of combustion.
The enthalpy change for the complete combustion
of 1 mol of a substance.
Define the term enthalpy change of neutralisation.
The formation of 1 mol of water from neutralisation.
What is average bond enthalpy?
The breaking of 1 mol of bonds in gaseous molecules.
Is bond breaking exothermic or endothermic?
- Energy is required to break bonds.
- Bond breaking is endothermic.
Is bond making exothermic or endothermic?
- Energy is released when bonds form.
- Bond making is exothermic.
Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic.
- Bond breaking absorbs energy.
- Bond making releases energy.
- More energy released than absorbed.
Suggest three reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values.
- Heat released to the surroundings.
- Incomplete combustion.
- Non-standard conditions.
Why do bond enthalpies have positive values?
Bond breaking is endothermic and so energy has to be put in to break a bond.
In ∆Hfθ, what are the conditions indicated by the symbol θ?
- Standard conditions.
- 100 kPa.
- 298K.
State one important manufacturing process in which hydrogen is used
The production of ammonia.
What is meant by the term exothermic?
A reaction that releases heat.
