3.4 enthalpy changes

Question 1

what is the enthalpy change of a reaction?

Answer 1

when a chemical reaction occurs, there is normally a change in energy therefore:
the enthalpy change of a reaction is the heat change in a reaction at a constant pressure

Answer 2

ΔH° “delta H” (delta means ‘change in’)
(°) means substance was in the standard state under standard conditions
units: kJmol-1

Question 3

endothermic

Answer 3

reactions that absorb energy from surroundings
ΔH is positive
products higher in energy than reactants

Question 4

exothermic

Answer 4

reactions that release energy to the surroundings
ΔH is negative
products lower in energy than reactants

Question 5

ΔrH° standard enthalpy change of reaction

Answer 5

the enthalpy change of a reaction according to the molar quantities in the equation under standard conditions

Question 6

ΔneutH° standard enthalpy change of neutralisation

Answer 6

the enthalpy change when an acid and alkali react to from 1 mole of water, under standard conditions

Question 7

ΔcH° standard enthalpy change of combustion

Answer 7

the enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O, under standard conditions

Question 8

ΔfH° standard enthalpy change of formation

Answer 8

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

Question 9

bonds broken

Answer 9

energy needs to be absorbed to break a bond
bons broken in the reactants (endothermic)
ΔH is positive

Question 10

bonds made

Answer 10

energy is released when bonds are formed
bonds made when products are being produced (exothermic)
ΔH is negative

Question 11

bond dissociation enthalpy

Answer 11

the energy needed per mole to break attractive forces in bonds
(only in gaseous compounds)

Question 12

enthalpy change formula

Answer 12

enthalpy change = total energy to break bonds - total energy released forming bonds

Question 13

calorimetry

Answer 13

used to work out the enthalpy change of combustion

Question 14

how does calorimetry work

Answer 14

energy from the fuel is transferred into the water although some is lost to the surroundings too
energy transferred is calculated

Question 15

calorimetry set up

Answer 15

• weigh fuel before burning
• fuel burned to raise temperature of water by a specific amount
• lid is placed on top to prevent heat loss
• wind shields placed by flame to prevent draught moving the flame
• weigh fuel after burning and work out mass of fuel burned

Question 16

1. calc energy transferred in calorimetry
2. calc enthalpy change

Answer 16

1. Q=mcΔT
2. enthalpy = Q/n (kJmol-1)

Question 17

Q=mcΔT

Answer 17

Q - heat energy lost or gained
m = mass of water or solution (g)
c = specific heat capacity of water (4.18)
ΔT = temperature change

Question 18

Hess’s Law

Answer 18

the total enthalpy change of a reaction is independent of the route taken