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.
Give the calculation, and describe exothermic + endothermic reactions in terms of bond enthalpy, i.e. energy released and energy required… What is happening to the bonds in reactants, what is happening to bonds in products.
Describe the bond breaking/making in bond enthalpy reactions?
What does Hess’ law state?
What is the Hess Cycle for standard enthalpy changes of formation?
What is the Hess Cycle for standard enthalpy changes of combustion?
