P1: 4 Energetics

Question 1

What is enthalpy change?

Answer 1

The heat energy change measured under conditions of constant pressure.

Question 2

What are standard enthalpy changes?

Answer 2

Enthalpy changes under standard conditions.
e.g. 100 kPa and a stated temperature.
ΔH⦵

Question 3

What is standard enthalpy of combustion?

Answer 3

ΔcH⦵
The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.

Question 4

What is standard enthalpy of formation?

Answer 4

ΔfH⦵
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states.

Question 5

What is equation for heat change?

Answer 5

q = mcΔt

q - heat (enthalpy) change (J)
m - mass (g)
c - specific heat capacity (J/g/K)
Δt - temperature change (K)

Question 6

What is the equation for enthalpy change per mole?

Answer 6

ΔH = q/moles

Question 7

What is Hess’s law?

Answer 7

The enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 8

What is mean bond enthalpy?

Answer 8

The energy required to break a covalent bond in a molecule averaged over a range of compounds.

Question 9

What is the equation for enthalpy change using mean bond enthalpies?

Answer 9

Enthalpy change = sum of bond enthalpies for bonds broken - sum of bond enthalpies for bonds made

Question 10

Why do values using mean bond enthalpies differ from those determined using Hess’s law?

Answer 10

Mean bond enthalpies are not exact/ less accurate because they are averages from a range of different values.