State the meaning of exothermic
release of thermal energy
temperature of surroundings goes up
State the meaning of endothermic
takes in thermal energy
temperature of surroundings goes down
What reactions are exothermic?
combustion
respiration
neutralisation
displacement
What reactions are endothermic?
decomposition
photosynthesis
What is the enthalpy change?
the thermal energy a reaction releases or absorbs per mole
What is the symbol and units for enthalpy change?
Symbol: ∆H
Units: KJ/mol
What is activation energy?
amount of energy required for a reaction to occur
What level diagram is this:
enthalpy change goes down
exothermic
(negative as energy released)
What level diagram is this:
enthalpy change goes up
endothermic
(positive as energy taken in)
Why is a reaction exothermic?
more energy released making new bonds than is taken in breaking bonds
Why is a reaction endothermic?
more energy is taken in breaking bonds then is released making bonds
How do we measure if a reaction is exothermic or endothermic?
measure temp of surroundings
Bond breaking =
endothermic
Bond making =
exothermic
How to calculate enthalpy change?
broken-made
- = exothermic
+ = endothermic
What is calorimetry?
method of measuring energy changes
reaction carried out near or in water. temp change of water measured.
If temp of water increases…
reaction has released energy
exothermic
if temp of water decreases…
reaction has taken in energy
endothermic
How can we link temp change to energy transfer?
Q = mc∆T
Q = energy (J)
m = mass (g)
c = S.H.C. (of water) = 4.18 J/g/ºC
∆T = temp change (ºC)
Combustion calorimetry
What enthalpy change would you expect?
exothermic
temp of water should increase
What is the method for combustion calorimetry?
Weigh out 50g of water and place in a can above burner
Measure initial mass of burner and initial temp of water
Ignite burner, stir water for 30 seconds, extinguish burner
Measure final mass of burner and final (max) temp of water
Why is it a metal calorimeter?
conducts thermal energy therefore more energy in water
How do you calculate whether combustion of methanol is exothermic or endothermic?
Determine energy transferred to/from water:
Q = mc∆T
Convert J to KJ
Determine energy change per mole:
Q/moles of methanol
Temp of water went up so put -
Why might results not be as accurate as expected?
Heat loss to surroundings
(not all energy transferred to water)
Incomplete reaction (combustion)
(less energy than expected is released)
