9.2 enthalpy changes

Question 1

bond breaking is

Answer 1

endothermic
- you need energy to do it
- it has a positive enthalpy change value

Question 2

bond formation is

Answer 2

exothermic
- energy is released
- the enthalpy chabege value is negative

Question 3

define bond dissociation enthalpy

Answer 3

the amount of energy needed to break bond per mole in its gaseus state

Question 4

define and explain average bond enthalpies

Answer 4

• they’re not exact because the value is calculated average from a range of values that differ due to the shape of the compound they come from
• def: the energy needed to break one mole of bonds in the gas phase averaged over many different compounds

Question 5

To record enthalpy change you need two recordings

Answer 5

• the number of moles your reacting
• the change in temprature

Question 6

Enthalpy changes equation for value in Joules

Answer 6

q=mc^T

Question 7

units for the q=mc^T

Answer 7

q=heat lost or gained in J **
m=mass of water in a calorimeter or solution in the container in grams. the mass is measured not of the reactant but of the TOTAL mass of material that will record the heat change.
c=specific heat capacity of water 4.18Jg-1K-1**
ΔT = Tfinal – T initial (this will give the correct sign as exothermic reaction ΔH is –ve and endothermic reactions ΔH is +ve.

Question 8

is the The thermometer part of the system?

Answer 8

The thermometer is not part of the system, but contained within the
SURROUNDINGS

Question 9

system, surroundings, universe

Answer 9

system = contents of the flask
universe=suroundings and system

Question 10

Temperature is measured in kelvin but the fixed points are……

Answer 10

are equal (ice melting, water
boiling) so a 1K rise = 1oC

this means temperature CHANGE is the same in both K or oC.

Question 11

If 50cm3 of solution is added then the mass will be

Answer 11

50g

Question 12

how do you calculate enthalpy change in KJ/mol

Answer 12

ΔH = mcΔT/no moles

same as ΔH = q/

kJ mol-1

Question 13

comment on thre accuracy for the enthalpy change of combustion

Answer 13

likley to be less exothermic than expected because
* heat loss to the suroundings mainly air surounding the flame
* incomeplte combustion of methanol eg.black soot on beaker
* evaporation of methanol from the wick. try weigh imediatley after extinguishing

liley to be more exothermic because
- non standard conditions