2. more on enthalpy changes

Question 1

bond breaking is

Answer 1

endothermic (delta H +)

need energy

Question 2

bond forming is

Answer 2

exothermic (delta H-)

energy is released

Question 3

the enthalpy change of reaction is the overall effect of what two changes

Answer 3

bond breaking (endothermic)
bond making (exothermic)

Question 4

what is needed to break attraction between atoms and ions

Answer 4

energy

Question 5

what is the amount of energy needed to break bonds (per mole) called

Answer 5

bond dissociation enthalpy

Question 6

bond dissociation enthalpies always involve bond breaking in what compounds
what does this allow

Answer 6

gaseous compounds

allows comparison

Question 7

average bond enthalpy is

Answer 7

the energy needed to break one mole of bonds in the gas phase averaged over many different compounds

Question 8

to measure the enthalpy change for a reaction you nee d to know what 2 things

Answer 8

1. number of moles of reactants

2. change in temperature

Question 9

equation for calculating enthalpy changes

Answer 9

q=m c deltaT

q= heat lost or gained in J
m= mass of water in the calorimeter in g 
c= specific heat capacity of water 4.18 J g-1 K-1
deltaT = change in temperature of water or solution in K

Question 10

enthalpy change units

Answer 10

kJmol-1

Question 11

how to convert from J to kJ

Answer 11

/1000

Question 12

why might the enthalpy change calculate not be the ACTUAL enthalpy change

Answer 12

heat lost and not measured e.g. may escape calorimeter, fuel might not combust completely