3.2.1 Enthalpy Changes Flashcards
What is enthalpy?
Heat change in a reaction under standard conditions
What are standard conditions⦵?
100kPa
298K
Endothermic: +/- ΔH?
+
Exothermic: +/- ΔH?
-
Standard enthalpy change of reaction? ΔHr⦵
ΔH of a reaction under ⦵
Standard enthalpy change of formation? ΔHf⦵
ΔH when 1 mole of a compound formed from its elements in their standard states, under ⦵
Standard enthalpy change of neutralisation? ΔHn⦵
ΔH when an acid and alkali react to form 1 mole of water, under ⦵
Standard enthalpy change of combustion? ΔHc⦵
ΔH when 1 mole of a substance is completely burned in 02 to make CO2 and H20, under ⦵
Bond broken, exo or endo?
Endothermic
(BENDO)MEX
Bond made, exo or endo?
Exothermic
BENDO(MEX)
What is mean bond enthalpy?
Mean average to make/ break a bond
Why do we have mean bond enthalpies?
Because each bond has a slightly different enthalpy; easier
How do you calc. enthalpy change?
Total energy to break bonds – Total energy released from forming bonds
What is calorimetry?
Practical technique; work out enthalpy of combustion/ neutralisation
How can a calorimetry experiment give you the enthalpy change of a fuel?
q of calorimetry (kJ) / moles of fuel
What is the equation for calorimetry?
q = mcΔT q = energy (J) m = mass of solution (g) c = SHC of water (4.18) ΔT = Δ in temp (K)
How can a calorimetry experiment give you the enthalpy change of a neutralisation?
q of calorimetry (kJ) / moles of acid or base
What is Bond dissociation enthalpy/ Average bond enthalpy?
Energy required to break the bonds in 1 mole of gas
What is Hess’ law?
Total enthalpy change of reaction is independent of the route taken
What 2 Hess cycles do you need to know?
Formation & combustion
In a Hess cycle what do you do if you go against the arrows?
Reverse the +/- sign of the ΔH
How do you check your Hess cycle is right?
Do a complete cycle = 0
In a Hess cycle do you multiply the ΔH by the number of moles? E.g. 2O2
Yes
What is the unit for enthalpy change?
kJmol-1
