Chemical Energy - Enthalpy Flashcards
What is the enthalpy of combustion?
The energy released when 1 mole of a substance burns completely in oxygen.
What are the units for enthalpy of combustion?
kJmol-1
Why could the result of the enthalpy of combustion be a lot lower than the actual value?
- Heat was lost to the surroundings
- Incomplete combustion
- Some fuel evaporated before it burned, so the mass used is wrong.
What apparatus is used to accurately measure the enthalpy of combustion?
A CALORIMETER.
How is a calorimeter better?
- All the heat given off goes into heating the water, no heat is lost.
- Pure oxygen is used so no incomplete combustion
- It is a sealed system so no evaporation of fuel is possible.
What is enthalpy of solution?
The energy change when 1 mole of a substance is dissolved completely in water.
Why was a polystyrene cup used to insulate the mixture? How could it be made better?
To minimise heat loss to the surroundings or the heat being absorbed. Adding a lid would make it better.
What is Hess’s law?
The enthalpy change for a chemical reaction is independent of the route taken. This means the enthalpy change for the overall process will be identical regardless of how many steps are taken.
What could be an error in Hess’s Law?
Heat being lost to the surroundings
What is bond enthalpy?
The measure of the strength of a bond (in gas phase reactions)
What is molar bond enthalpy?
The energy required to break 1 mole of the bonds
What is the mean bond enthalpy?
The average bond strength for that bond type in different compounds
Bond breaking must be?
ENDOTHERMIC. the Delta H will be +
Bond forming will be?
EXOTHERMIC. the Delta H will be -
