energetics 5.2

Question 1

what is an enthalpy change?

Answer 1

the heat energy released or absorbed per mole
ΔH
kJ/mol

Question 2

if the sign is positive + what type of reaction is it and what type of energy released?

Answer 2

endothermic
absorbed energy

Question 3

if the sign is negative - what type of reaction is it and what type of energy released?

Answer 3

exothermic
released energy

Question 4

what type of reaction in endothermic?

Answer 4

absorbed energy
positive

Question 5

what type of reaction in exothermic?

Answer 5

released energy
negative

Question 6

what is the units for enthalpy changes?

Answer 6

kJ/mol

Question 7

do breaking bonds release or absorb energy?

Answer 7

absorb energy

Question 8

do making bonds release or absorb energy?

Answer 8

releases energy

Question 9

how do we represent enthalpy changes diagrammatically?

Answer 9

energy level diagrams

Question 10

how do you draw an endothermic reaction on an energy level diagram?

Answer 10

heat is absorbed and positive
products end up with more energy than the reactants

reactants low down
products high up

Question 11

how do you draw an exothermic reaction energy level diagram?

Answer 11

heat is released and negative
products ends up with less energy than reactants

reactants high up
products low down

Question 12

how can you work out enthalpy change using bond energies?

Answer 12

ΔH = bonds broken − bonds made

Question 13

what’s a reaction profile diagram?

Answer 13

like an energy level diagram
HOWEVER shows the path of the reaction it actually takes from reactants to products
activation energy - represented by an arrow from reactants to the top of the hump

Question 14

what is calorimetry?

Answer 14

experimental method used to measure an enthalpy change (ΔH)

reaction is carried out near some water
for example EXOTHERMIC
-reaction releases heat energy
-heat energy is absorbed by nearby water
-temperature of water increases

Question 15

the two formulae we use to convert measurements into ΔH:

Answer 15

Q = mcΔT
heat energy (J) = mass of water (g) x specific heat capacity x temperature change

ΔH = -Q/moles
kJ/ mol = - (heat energy in J) /moles

Question 16

what is combustion calorimetry?

Answer 16

combustion of a fuel to heat up the water

Question 17

method of combustion calorimetry:

Answer 17

1. weigh out 50g of water and place in a can above the burner
2. measure initial mass of burner and initial temperature of the water
3. ignite the burner, stir water for 30 seconds, then extinguish the burner
4. measure the finial mass of the burner and the finial temperature of water

Question 18

what are two significant sources of error in combustion calorimetry experiments?

Answer 18

heat loss
- most of the heat energy dissipating into the air instead of the water
incomplete combustion
- releases less heat energy than expected

Question 19

what is solution calorimetry?

Answer 19

the reaction you are measuring dissolves in water

reaction releases energy and water absorbs it but water and reaction are in the same place

Question 20

what is the method of solution calorimetry?

Answer 20

1. pipette 25cm3 of CuSO4 into a polystyrene cup
   1cm3 solution has a mass of 1g
2. measure the initial temperature of the solution
3. add 1g of Mg and stir continuously
4. measure the maximum temperature of the solution