3.2.1 Enthalpy Changes Flashcards
What does system mean in a chemical reaction?
- The atoms and bonds involved in the chemical reaction
Explain the law of conservation.
- The amount of energy in an isolated system remains the same
- Energy cannot be destroyed or created
- It can only be transferred from one form to another
What energy change is breaking bonds associated with?
- Energy is taken in to break bonds -> endothermic reaction
What energy change is making bonds associated with?
- Energy is released to make bonds -> exothermic reaction
What is an endothermic reaction?
- A reaction with an overall positive enthalpy change (+ΔH)
- Enthalpy of products > enthalpy of reactants
What is an exothermic reaction?
- A reaction with an overall negative enthalpy change (-ΔH)
- Enthalpy of products < enthalpy of reactants
Draw an enthalpy change diagram for an endothermic and exothermic reaction.
- Endothermic reaction:
Enthalpy of products > enthalpy of reactants (curve goes upwards for activation energy) - Exothermic reaction:
Enthalpy of products < enthalpy of reactants (curve goes downwards for ΔH)
What does activation energy mean?
- The minimum energy required for a reaction to take place
Which way does the arrow for activation energy point on an enthalpy profile diagram?
- Always points upwards
What are the standard conditions?
- 100 kPa
- 298 K
What does ‘in standard state’ mean?
- The state an element/compound exists at in standard conditions (100 kPa, 298 K)
Define enthalpy change of formation.
- The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
Give an example of an equation representing standard enthalpy of formation.
- H₂ (g) + ½O₂ (g) -> H₂O (l)
Define enthalpy change of combustion.
- The energy change that takes place when 1 mole of a substance is completely combusted
Give an example of an equation representing standard enthalpy of combustion.
- C (s) + O₂ (g) -> CO₂ (g)
Define enthalpy change of neutralisation.
- The energy change that takes place when 1 mole of water is formed from a neutralisation reaction
What does enthalpy change of reaction mean?
- The energy change associated with a given reaction
How can you calculate enthalpy change from experimental data?
- Use the equation Q = mcΔT
- m = mass of the substance being heated (usually water)
- c = specific heat capacity (SHC of water = 4.18 Jg⁻¹K⁻¹)
- ΔT = change in temperature
Draw a simple calorimeter.
- Thermometer to measure temperature change of water
- Beaker containing known mass of water
- Gauze and tripod
- Spirit burner (measure change in mass to find moles burned)
- Heatproof mat
What are the advantages of using a bomb calorimeter?
- Minimises heat loss
- Uses pure oxygen to ensure complete combustion
Why might experimental methods for enthalpy determination not be accurate?
- Heat is lost to surroundings
- Conditions are not standard
- Reaction may not go to completion
What does average bond enthalpy mean?
- The mean energy required to break 1 mole of bonds in gaseous molecules
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
- Bond enthalpies are an average across different molecules
- Standard enthalpy values apply to specific molecules and are therefore more accurate
How to calculate enthalpy change of reaction using average bond enthalpies?
- ΔH = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)
