Thermochemistry

Question 1

exothermic

Answer 1

produces heat
temperature rises
delta H -ve
involves making bonds

Question 2

endothermic

Answer 2

Absorb (take in) heat
Temperature falls (feel cold)
ΔH is +ve
Involves breaking bonds

Question 3

Heat changes of Physical Changes

Answer 3

solid => gas sublimation (endo)
gas => solid (exo)
liquid => gas evaporation (endo)
gas => liquid condensation (exo)
solid => liquid melting (endo)
liquid => solid solidification (exo)

Question 4

Exothermic Reaction Profile

Answer 4

starts of high and ends low nearly touching the X-axis 
X- axis ( energy )
Y- axis ( reaction pathway )
delta Hr heat change -ve 
activation energy without catalyst 
effect of adding catalyst activation energy lowered by catalyst
reactanst 
product

Question 5

Endothermic Reaction Profile

Answer 5

X- axis energy 
Y- axis reaction pathway
reactant 
product 
activation energy 
delta H - heat taken in absorbed 
catalyst 
E act
starts of low then ends high

Question 6

bond energy

Answer 6

Average energy required to break 1 mole of bonds

into separate atoms in the gaseous state

Question 7

Heat of Combustion
Heat of Reaction
Heat of Formation

Answer 7

Heat change when one mole of a substance is burned completely in excess oxygen

Heat change when The number of moles of reactants in the balanced equation react completely

Heat change when 1 mole of compound formed from its elements in their standard states

Question 8

Hess’s Law

Law of Lavoisier and Laplace

Answer 8

The heat change for a reaction is the same whether the reaction takes place in one step or several steps

heat change for the forward reaction is equal to the heat change for the reverse reaction, but opposite in sign

Question 9

what measures heat produced when substances burn

Answer 9

Bomb Calorimeter

Question 10

how is the heat produced calculated

Answer 10

H = m c ΔT

H = heat change
m = mass (usually kg);
c = specific heat capacity:
ΔT = temperature change (oC)

Question 11

Bomb Calorimeter

Answer 11

To get accurate results we use a Bomb Calorimeter
Oxygen pumped in under pressure to ensure rapid and complete combustion
Often so rapid that it exploded
Hence the name “ bomb calorimeter”

Question 12

Bomb Calorimeter is accurate because

Answer 12

Fast
Complete
There are no side reactions
There is no heat loss

Question 13

Three things must apply if a reaction is to be suitable for study in a calorimeter

Answer 13

It must be rapid
It must go to completion
Must have no side reactions

Question 14

combustion equation

Answer 14

write down the compound to be burnt
then add
+ O2 = CO2 + H20
then balance

Question 15

calculate calorific value
example
If the heat of combustion of ethane is -1560 kJ mol-1calculate its kilogram calorific value.

Answer 15

1 mole  = -1560 kJ
      C2H6  = 12 * 2 + 1*6 = 30g
       30g  = -1560
        1 g  = -1560/30
   1000g  = (-1560/30) * 1000 
               = -52000 kJ kg-1

Question 16

How to write a heat of formation equation

Answer 16

Write the formula of the compound you want to form to the right of an = sign

Write the formulae of the elements in their standard states to the left of the equal sign in the order they appear in the formula. Put in state and check if diatomic

Balance the equation

Question 17

Points to Remember (hess law)

Answer 17

If you double an equation you double its △H value

If you half an equation you half its △H value

If you reverse an equation you change the sign of △H

Exothermic reactions have a negative △H value

Endothermic reactions have a positive △H value