enthalpy changes: content

Question 1

exothermic reactions

Answer 1

the products are at a lower energy than the reactants: excess heat is transferred from the system to the surroundings
ΔH is negative

Question 2

endothermic reactions

Answer 2

products are at a higher energy than the reactants: heat is taken in from the surroundings to the system
ΔH is positive

Question 3

enthalpy profile diagrams

Answer 3

• gaseous atoms
• reactants
• products

Question 4

standard conditions [ ]ᶿ

Answer 4

298K
100kPa
1.00moldm⁻³

Question 5

standard enthalpy change of formation for an element

Answer 5

zero (by definition)

Question 6

examples of exothermic reactions

Answer 6

combustion
respiration
neutralisation

Question 7

examples of endothermic reactions

Answer 7

thermal decomposition
evaporation
photosynthesis

Question 8

what are the two calorimetry equations

Answer 8

q=mcΔT
q - energy change (J)
m - mass of solution/surroundings (g)
c - specific heat capacity of solution/surroundings (J kg⁻¹ K⁻¹)
ΔT - temperature of solution/surroundings (K)

ΔH=±q/n
ΔH - enthalpy change (kJ mol⁻¹)
q - enthalpy change (kJ)
n - moles of reactant/system (mol)

Question 9

methods to determine enthalpy changes

Answer 9

combustion calorimetry
calorimetry with a reaction in solution

Question 10

combustion calorimetry overview

Answer 10

a known mass of substance/fuel is burned to heat a known mass of water

system- fuel burning
surroundings- water

ALWAYS exothermic - ΔH is negative

Question 11

sources of error in combustion calorimetry

Answer 11

• heat lost to the air
• incomplete fuel combustion
  [evaporation of fuel/water from wick/beaker]

Question 12

how to minimise error in combustion calorimetry

Answer 12

• ensure a plentiful oxygen supply (in excess)
• add a lid to the beaker
• use a copper can instead of a beaker (better conductor)
• cover wick when not burning
• use an insulated/bomb calorimeter

Question 13

calorimetry with a reaction in solution overview

Answer 13

a known quality of reactant (limiting!!) reacts in a known volume of water

system- the chemicals reacting or dissolving
surroundings- the solution

Question 14

sources of error in reaction in solution calorimetry

Answer 14

• heat loss/gain to/from the air
• water evaporates from the beaker
• incomplete reaction
• c of solution ≠ c of water

Question 15

how to minimise error in reaction in solution calorimetry

Answer 15

• use a polystyreine cup
• use a lid to prevent heat loss/gain
• ensure standard conditions

Question 16

bond enthalpies are always _____

Answer 16

endothermic (positive ΔH values)

Question 17

bond breaking is

Answer 17

endothermic
(=activation energy)

Question 18

bond making is

Answer 18

exothermic

Question 19

how to calculate ΔH

Answer 19

• draw out the equation
• calculate totals for bonds made/broken
• use the equation to calculate overall enthalpy changes
• [sketch reaction profile]

Question 20

why can reactions be exothermic

Answer 20

more energy is released (forming bonds in the products) than is required (breaking bonds in the reactants)

Question 21

why might it not be possible to directly measure enthalpy changes from experiments

Answer 21

• very high activation energy Eₐ
• very slow rate of reaction
• occurrence of side reactions

Question 22

how to indirectly calculate enthalpy changes

Answer 22

hess’ law: the enthalpy change for a given reaction is independant of the route taken
draw an enthalpy cycle (kinda like vectors)
BE CAREFUL of the enthalpy change direction

Question 23

required practical: enthalpy determination

Answer 23

• pour the aqueous reactant into a polystyreine cup and cover
• use a thermometer to record the temperature every minute for three minutes to get an accurate start temperature
• add the other reactant and stir to ensure a homogenous mixture
• record the temperature after one minute
• continue to record the temperature at one minute intervals
• calculate ΔT
• plot ΔT against time
• extrapolate two lines of best fit
• read ΔT at the point of mixing