3.1.4 Energetics

Question 1

Enthalpy change (∆ H )

Answer 1

The heat energy change measured under conditions of constant pressure.

Question 2

What are the standard conditions

Answer 2

100 kPa
298 K

Question 3

Standard enthalpy of combustion

Answer 3

The enthalpy change that occurs when one mole of a compound is burned completely in oxygen, under standard states and conditions

Question 4

Standard enthalpy of formation

Answer 4

The enthalpy change when one mole of substance is produced from its constituent elements under standard states an conditions

Question 5

Energy change equation + units

Answer 5

q=mcΔT
q - J
m - g
c - J kg−1 K−1
T - K

Question 6

Mean bond enthalpy

Answer 6

The enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules

Question 7

Why do mean bond enthalpy values differ to values calculated by Hess’ law

Answer 7

Hess’ law finds the exact value for that compound. Values in data books are averaged across many different compounds

Question 8

Enthalpy change calculation + units

Answer 8

ΔH = q/n
ΔH - Jmol-1
q - J
n - mol

Question 9

What does Hess’s law state

Answer 9

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