module 3: enthalpy changes

Question 1

breaking bonds

Answer 1

endothermic

Question 2

forming bonds

Answer 2

exothermic (releases energy)

Question 3

enthalpy

Answer 3

total energy of a system

Question 4

standard conditions

Answer 4

pressure (gases) 101 kPa (1atm)
concentration (solutions) 1moldm-3
stated temperature 25 ° (298K)

Question 5

exothermic

Answer 5

temperature of surroundings increases
chemical energy to thermal energy
-ve enthalpy change

Question 6

standard enthalpy change of reaction definition

Answer 6

molar quantities as stated in a chemical equation react together under standard conditions

Question 7

standard enthalpy change of reaction equation

Answer 7

ΔrHꝊ= ΣEꝊ(bonds broken) - ΣEꝊ(bonds formed

Question 8

standard enthalpy change of formation equation

Answer 8

ΔrHꝊ= ΣΔfHꝊ(products)- ΣΔfHꝊ(reactants)-

Question 9

standard enthalpy change of combustion equation

Answer 9

ΔrHꝊ=ΣΔcHꝊ(reactants)-ΣΔcHꝊ(products)

Question 10

standard enthalpy change of combustion

Answer 10

1 mole of a substance completely burned in O2 under standard conditions

Question 11

standard enthalpy change of formation

Answer 11

1 mole of substance formed from its constituent elements in their standard states at s.t.p

Question 12

bond enthalpy

Answer 12

average energy required to break 1 mole of covalent bonds in gaseous molecules

Question 13

Hess’ Law

Answer 13

the enthalpy change is independent of the path taken, provided the initial and final conditions are the same

use known enthalpy changes, enthalpy change cannot be measured directly

Question 14

q= mcΔT

Answer 14

q= heat transferred to or from mass of water J
c= specific heat capacity of water (4.18 KJmol-1)

Question 15

ΔrH=q/n

Answer 15

measured in KJmol-1

Question 16

Hess’ cycle calculations

Answer 16

step 1: energy put into break bonds forming separate gaseous atoms
step 2: gaseous atoms combine to form bonds and the energy released is equal and opposite to that of the breaking bonds

step 1- step 2

Question 17

standard enthalpy change of neutralisation

Answer 17

acid and base in standard states react to form 1 mole of H20 under standard conditions

Question 18

sources of error when determining enthalpy change of reaction

Answer 18

heat loss to surroundings

Question 19

methods for reducing heat loss

Answer 19

lid
insulation
draught shields

Question 20

sources of error when determining enthalpy change of combustion

Answer 20

heat loss
incomplete combustion
evaporation from wick
non-standard conditions

Question 21

Ea activation energy

Answer 21

energy barrier
minimum energy required for reaction to take place

Question 22

small Ea

Answer 22

rapid reaction, energy readily available from surroundings

Question 23

limitations of average bond enthalpy

Answer 23

average value obtained from many molecules, actual value may be slightly different

Question 24

weak acids

Answer 24

some energy used to dissociate
less exothermic

Question 25

is zero enthalpy possible?

Answer 25

no!

Question 26

hess cycle with formation

Answer 26

intermediary step = elements

Question 27

hess cycle of combustion

Answer 27

intermediary step = H2O + CO2

providing O2 isn’t reactant

Question 28

calorimetry

Answer 28

method of measuring the change in energy of a system
analysing heat exchange between the system and its surroundings

Question 29

bond enthalpies are always

Answer 29

positive

Question 30

Hess’ cycle calculations

Answer 30

pick species that only appear once
you can reverse reaction and change sign
remember to multiply through