Enthalpy&measuring change Flashcards
What is enthalpy change?
The heat transferred in a reaction at a constant pressure
What are units of enthalpy change?
kJ mol -1
What are standard conditions?
100kPa
Stated temp usually 298K
What is an exothermic reaction?
energy given out to surroundings, temperature goes up
Is the enthalpy change negative or positive in an exothermic reaction?
Negative
What is an example of an exothermic reaction?
Combustion
What is an endothermic reaction?
energy taken in from surroundings, temperature falls
Is the enthalpy change negative or positive in an endothermic reaction?
Positive
What technique is used to measure enthalpy change?
Calorimetry
How does calorimetry work to measure enthalpy change?
As the fuel burns, it heats the water and you can work out how much energy has been absorbed by the water if you know the mass, the temperature change and the specific heat capacity
What two equations do you need to work out enthalpy change?
q=m x c x Δt
ΔH= q/n
What is mass measured in?
g
What is temperature measured in?
K
What is the specific heat capacity?
4.18 j g-1 K-1
What is ‘q’ in the equation?
Heat lost or gained in joules
How do you convert q from joules into kilojoules?
divide by 1000
What do you divide q by?
the number of moles of the fuel that caused the enthalpy change
How do you calculate the number of moles?
Moles = Mass/Mr
What would you put in front of the answer for enthalpy change if the temperature has gone up?
Negative symbol as it is an exothermic reaction
What would you put in front of the answer for enthalpy change if the temperature has gone down?
Postive symbol as it is an endothermic reaction
What are some experimental problems with calorimetry?
- some heat absorbed by container
- some heat lost to surroundings
What are some experimental problems which only apply to flammable liquid calorimetry?
- Some combustion may be incomplete - less energy given out
- Some flammable liquid may escape by evaporation
