energetics [P1] PAPER 1+2

Question 1

define the term mean bond enthalpy

Answer 1

enthalpy needed in breaking one mole of (covalent) bonds (in gaseous state), averaged (for that type of bond) over a range of compounds

Question 2

define standard enthalpy of formation

Answer 2

The enthalpy change at constant pressure when 1 mole of a product is formed from its constituent elements with all reactants and products in their standard states under standard conditions

Question 3

define enthalpy change (ΔH)

Answer 3

heat energy change at constant pressure

Question 4

what does standard enthalpy change refer to?

Answer 4

standard conditions, ie 100kPa, 298K, 1moldm-3

Question 5

define standard enthalpy of combustion

Answer 5

the enthalpy change at constant pressure when 1 mole of a substance is completely burned in oxygen with all reactants and products in their standard states under standard conditions

Question 6

how is heat change, q, calculated?

Answer 6

q = mcΔT

• q = heat change (J)
• m = mass of substance undergoing the temperature change (g)
• c = specific heat capacity (given)
• ΔT = temperature change

Question 7

how is ΔH calculated? (using q)

Answer 7

• q = mcΔT → convert to kJ (divide by 1000)
• ΔH = q (kJ) /mol
• mol = the moles of the limiting reagent
• make sure ΔH is negative if the reaction is exothermic

Question 8

state Hess’ Law

Answer 8

the enthalpy change at constant pressure in a reaction is independent of the route, and depends only on the initial and final states

ΔH1 = ΔH2 + ΔH3
(works like vectors)

Question 9

the standard enthalpy of formation of an element is ___

Answer 9

zero

Question 10

Hess cycle, given you have enthalpies of formation

Answer 10

ΔH = (Σ ΔH products) – (Σ ΔH reactants)

ΔH1 = ΔH3 - ΔH2 [diagram]

⚠️ make sure everything is multiplied according to the numbers in front of each substance in the balanced equation, e.g. you’re given the enthalpy of formation of CO2 → from BALANCED equation - 3CO2 → multiply enthalpy by 3, THEN use it in the above equation

Question 11

Hess cycle, given you have enthalpies of combustion

Answer 11

ΔH = (Σ ΔH reactants) – (Σ ΔH products)

ΔH1 = ΔH2 - ΔH3 [diagram]

⚠️ make sure everything is multiplied according to the numbers in front of each substance in the balanced equation

Question 12

what is calorimetry used for?

Answer 12

• experimental method for finding enthalpy changes by measuring temperature change over time
• data can be extrapolated to find ΔT from the graph drawn from obtained results

Question 13

how to work out ΔH, given you have mean bond enthalpies?

Answer 13

ΔH = (break + vaporisation) – (make + vaporisation)
ΔH1 = ΔH2 - ΔH3
• ΔH2 is the sum of all the bond enthalpies (AND enthalpy of vaporisation if a reactant isn’t in a gaseous state) required to BREAK all the bonds of the reactants
• ΔH3 is the sun of all the bond enthalpies required to MAKE all the products

⚠️ make sure everything is multiplied according to the numbers in front of each substance in the balanced equation and that everything must be a gas

Question 14

Why may the enthalpy change of a reaction calculated using Hess’ Law be similar to the enthalpy change for the same reaction calculated using mean bond enthalpies?

Answer 14

• Same reaction
• Same reactants and products
• Same equation
• Same number and types of bonds broken and formed

Question 15

Why may the enthalpy change of a reaction calculated using Hess’ Law be different to the enthalpy change for the same reaction calculated using mean bond enthalpies?

Answer 15

The mean bond enthalpies differ from the actual bond enthalpies of the substances formed/broken in the reaction

Question 16

Give one reason why the bond enthalpy calculated for the __ bond in ___ is different from the mean bond enthalpy quoted in a data book.

Answer 16

Data book value is derived from a number of different compounds, not just different ___ molecules

Question 17

Suggest one reason, other than the use of mean bond enthalpies, why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained through a calculation.

Answer 17

• Alkane is not gaseous
• Equation relates to gaseous alkanes
• It takes energy to convert it into a gas
• The substances are gaseous in calculations using bond enthalpies

Question 18

State why the value for the standard enthalpy of formation of CO2(g) is the same as the value for the standard enthalpy of combustion of carbon.

Answer 18

These two enthalpy changes are for the same reaction

Question 19

State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.

Answer 19

pressure isn’t constant in a bomb calorimeter