Ch9 Enthalpy Flashcards
What is enthalpy (H)?
H = heat energy stored in chemical system (R + P involved in chemical reaction).
What does exothermic mean?
What does endothermic mean?
What does activation energy mean?
Ea = minimum amount of energy required to break bonds in Reactants to allow reaction to occur.
Show an exothermic and endothermic enthalpy profile:
What are the standard (θ) conditions?
Define the enthalpy change of reactions:
Define the enthalpy change of formation:
Define the enthalpy change of combustion:
Define the enthalpy change of neutralisation:
What is the formula to calculate q in enthalpy?
q = mcΔT
divide answer by 1000 to get kJ
What is the formula to calculate enthalpy change- ΔH?
ΔH = q ÷ n
n is no. moles
units = kJmol-1
all values must be + or - .
+ is endo (temp goes down)
- is exo (temp goes up)
Why are experimental values often less negative, or less exothermic, than data book values?
- Heat loss to surroundings (air/apparatus)
- Incomplete combustion
- Evaporation from the wick of spirit burner
- Use of non-standard conditions.
Define: Average Bond Enthalpy
Avg B.E. = avg enthalpy change that takes places when breaking by homolytic fission one mole of given type of bond in gaseous molecule.
What are limitations with using AVERAGE bond enthalpy calculations?
- Actual energy involved in breaking+making individual bonds could be slightly different.
- All species have to be gaseous molecules, so calculation wont give standard enthalpy change.