Physical Unit 1.4: Energetics

Question 1

define enthalpy change

Answer 1

heat energy change for a reaction measured at a constant pressure

Question 2

define mean bond enthalpy

Answer 2

enthalpy required to break one mole of a covalent bond
averaged over a range of compounds
with all substances in a gaseous state

Question 3

define standard enthalpy change of formation

Answer 3

enthalpy change when one mole of a substance
is formed from its constituent elements
under standard conditions with all reactants & products in standard states

Question 4

define standard enthalpy of combustion

Answer 4

enthalpy change when one mole of a substance
is completely burned in excess oxygen
under standard conditions

Question 5

what is the standard enthalpy of combustion of O2 & why?

Answer 5

0
O2 cannot burn/react with itself

Question 6

define standard enthalpy of neutralisation

Answer 6

enthalpy change when one mole of water is formed in a reaction
b/w an acid & alkali under standard conditions

Question 7

what is the temperature & pressure in standard conditions?

Answer 7

100kPa (1atm)
298K usually

Question 8

what are the reaction profiles for endothermic & exothermic reactions?

Answer 8

see notes booklet

Question 9

what is the equation for the amount of heat energy released/absorbed?

Answer 9

q=mc.deltat

Question 10

what is the equation for enthalpy change in kJ/mol?

Answer 10

q(kJ) / # moles reacting
think limiting # moles

Question 11

what are the problems with flame calorimetry?

Answer 11

incomplete combustion
heat loss to surroundings
specific heat capacity of copper container not considered
precision of thermometer
evaporation of water

Question 12

what is the equation for % uncertainty?

Answer 12

apparatus uncertainty / reading x 100
if measuring change in temp. or difference in vol. of burette, do 2xapparatus uncertainty bc 2 readings taken

Question 13

how can the accuracy of an enthalpy reaction be improved? (common 6 marker)

Answer 13

1. improve insulation
   insulate beaker or use a polystyrene cup with lid
   to reduce heat loss
2. improve temp. reading
   record temp. for 4 mins before adding the [reactant]
   to get an accurate initial temp.
   record temp. at regular time intervals = every minute
   & plot graph of temp. against time
3. improve analysis of results
   extrapolate the cooling to point of addition of [reactant]
   to get a max. temperature change

Question 14

what is Hess’s law?

Answer 14

the enthalpy change for a reaction is independent of the route taken
a –> c –> b

Question 15

use Hess’s law to calculate enthalpy of formation & combustion

Answer 15

see notes booklet

Question 16

calculate bond enthalpies

Answer 16

see notes booklet
break - make
bond enthalpy + enthalpy of vapourisation