3.2.1: Enthalpy changes

Question 1

ΔH for exothermic reactions is

Answer 1

negative

Question 2

ΔH for endothermic reactions is

Answer 2

positive

Question 3

Enthalpy change (ΔH) is

Answer 3

the energy change that occurs when a reaction is carried out at a constant pressure, (under standard conditions)

Question 4

What are standard conditions?

Answer 4

• 298K
• 1 atm or 100kPa
• 1 moldm⁻³

Question 5

How to calculate enthalpy change

Answer 5

ΔH = ΣΔH(products - reactants)

i.e.

(make - break)

Question 6

Standard enthalpy change of reaction(ΔHr)

Answer 6

The enthalpy change when a chemical reaction occurs in the molar quantities given in the chemical equation, under standard conditions.

Question 7

Standard enthalpy change of formation (ΔHf)

Answer 7

the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states, under standard conditions.

Question 8

Standard enthalpy change of combustion (ΔHc)

Answer 8

the enthalpy change that occurs when 1 mole of a substance is completely burnt in excess oxygen under standard conditions.

Question 9

Standard enthalpy change of atomisation (ΔHat)

Answer 9

the enthalpy change that occurs on production of 1 mole of atoms in the gas phase from an element in its standard state.

Question 10

Equation for energy transferred

Answer 10

Q = mcΔT

Q = energy transferred, J/kJ
m = mass of water heated, kg
c = SHC J/kg
ΔT = change in temp, K

Question 11

ΔH from Q = mcΔT

Answer 11

ΔH = Q/no. of moles of fuel burnt

Question 12

-Q=

Answer 12

-Q = n x ΔHc

Question 13

Average bond enthalpy

Answer 13

Breaking of 1 mole of bonds in gaseous molecules

Question 14

Enthalpy change of neutralisation

Answer 14

the enthalpy change that occurs when 1 mol of water is formed from a neutralisation reaction.

Question 15

Activation energy

Answer 15

The minimum energy required for a reaction to take place.