Energetics

Question 1

State the meaning of exothermic

Answer 1

release of thermal energy
temperature of surroundings goes up

Question 2

State the meaning of endothermic

Answer 2

takes in thermal energy
temperature of surroundings goes down

Question 3

What reactions are exothermic?

Answer 3

combustion
respiration
neutralisation
displacement

Question 4

What reactions are endothermic?

Answer 4

decomposition
photosynthesis

Question 5

What is the enthalpy change?

Answer 5

the thermal energy a reaction releases or absorbs per mole

Question 6

What is the symbol and units for enthalpy change?

Answer 6

Symbol: ∆H
Units: KJ/mol

Question 7

What is activation energy?

Answer 7

amount of energy required for a reaction to occur

Question 8

What level diagram is this:
enthalpy change goes down

Answer 8

exothermic
(negative as energy released)

Question 9

What level diagram is this:
enthalpy change goes up

Answer 9

endothermic
(positive as energy taken in)

Question 10

Why is a reaction exothermic?

Answer 10

more energy released making new bonds than is taken in breaking bonds

Question 11

Why is a reaction endothermic?

Answer 11

more energy is taken in breaking bonds then is released making bonds

Question 12

How do we measure if a reaction is exothermic or endothermic?

Answer 12

measure temp of surroundings

Question 13

Bond breaking =

Answer 13

endothermic

Question 14

Bond making =

Answer 14

exothermic

Question 15

How to calculate enthalpy change?

Answer 15

broken-made
- = exothermic
+ = endothermic

Question 16

What is calorimetry?

Answer 16

method of measuring energy changes
reaction carried out near or in water. temp change of water measured.

Question 17

If temp of water increases…

Answer 17

reaction has released energy
exothermic

Question 18

if temp of water decreases…

Answer 18

reaction has taken in energy
endothermic

Question 19

How can we link temp change to energy transfer?

Answer 19

Q = mc∆T

Q = energy (J)
m = mass (g)
c = S.H.C. (of water) = 4.18 J/g/ºC
∆T = temp change (ºC)

Question 20

Combustion calorimetry
What enthalpy change would you expect?

Answer 20

exothermic
temp of water should increase

Question 21

What is the method for combustion calorimetry?

Answer 21

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

Question 22

Why is it a metal calorimeter?

Answer 22

conducts thermal energy therefore more energy in water

Question 23

How do you calculate whether combustion of methanol is exothermic or endothermic?

Answer 23

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 -

Question 24

Why might results not be as accurate as expected?

Answer 24

Heat loss to surroundings
(not all energy transferred to water)
Incomplete reaction (combustion)
(less energy than expected is released)

Question 25

What is solution calorimetry?

Answer 25

reactants and water are in the same container
reaction will release into or absorb energy from the water causing a temp change

Question 26

Why is water not needed in solution calorimetry?

Answer 26

copper sulphate solution contains water

Question 27

Describe the method for solution calorimetry

Answer 27

Pipette 25cm³ of CuSO₄(aq) into a polystyrene cup
Measure initial temp of solution
Add 1g of Mg(s) and stir continuously
Measure the maximum temperature of solution.

Question 28

Why is a polystyrene cup used?

Answer 28

its an insulator

Question 29

Why might a glass beaker be in equipment list?

Answer 29

to stabilise polystyrene cup

Question 30

How do you calculate enthalpy change in following reaction:
CuSO₄(aq) + Mg(s) -> MgSO₄(aq) + Cu(s)

Answer 30

determine energy transferred to/from water:
Q = mc∆T
Convert J to KJ
Determine energy change per mole:
Q/moles of Mg
- as temp went up (exothermic)

Question 31

Why might results not be as accurate as expected?

Answer 31

• Heat transfer to atmosphere
  (improve by putting lid on/more insulating material)
• Incomplete reaction
• Assumed SHC and mass is same as water