REQUIRED PRACTICAL 2 - enthalpy

Question 1

Level 1 for enthalpy of neutralisation

Answer 1

• Measure out set volume using a measuring cylinder and add to polystyrene cup
• Density of water = 1.00kgdm-3 so the volume of liquid = mass of acid and alkali together
• Record initial temp each minute for 3 minutes

Question 2

Level 2 for enthalpy of neutralisation

Answer 2

• On 4th minute add alkali but do not record temp until the 5th and keep recording every minute from then on until 15 minutes
• Draw graph temp vs time, extrapolate both lines back to time of mixing at 4 minutes and work out ΔT (change in temp)

Question 3

Level 3 for enthalpy of neutralisation

Answer 3

• Work out q=mcΔt
• Work out moles (mass of solid) or (mass of liquid used)
• Calculate ΔH= q/n

Question 4

In enthalpy of neutralisation, what would you use as m in q=mcΔt?

Answer 4

The volume of acid and alkali added together (they should be equal to each other e.g. 25 + 25cm^3)

Question 5

In the enthalpy graph, how many lines of best fit do you need to do?

Answer 5

2 lines, and then extrapolate from them

Question 6

What are the errors in the experiment?

Answer 6

• Heat loss

- Not stirred so incorrect measurement

Question 7

What are the errors in the experiment (neutralisation) ?

Answer 7

• Heat loss

- Not stirred so incorrect measurement

Question 8

How can you improve the error of heat loss (neutralisation) ?

Answer 8

Add a lid to the polystyrene cup (despite the cup already being a good insulator)

Question 9

How can you improve the error of not stirring (neutralisation) ?

Answer 9

Stir solutions to make sure they’re evenly heated

Question 10

Level 2 for enthalpy for a solid or liquid

Answer 10

• Add set volume of liquid using measuring cylinder to polystyrene cup
• Record the temp every minute for first 3 minutes and record in table
• As density of water is 1.00kgdm-3 the vol of liquid = mass of liquid in equation

Question 11

Level 3 for enthalpy for a solid or liquid

Answer 11

• On 4th minute add the solid and and from 5-15th minute record temp in table

Question 12

Level 3 for enthalpy for a solid or liquid

Answer 12

• On 4th minute add the solid and and from 5-15th minute record temp in table
• Draw a graph of temp vs time, extrapolate both lines back to time of mixing at 4 minutes and workout ΔT
• Work out q=mcΔt and calculate ΔH=q/n

Question 13

Enthalpy of combustion errors?

Answer 13

• Incomplete combustion of fuel
• Heat loss from flame to environment
• Evaporation of volatile liquid

Question 14

How to improve incomplete combustion of fuel?

Answer 14

Less heat given out, use a advance bomb or flame colorimeter equipment as O2 is supplied and mixture is stirred

Question 15

How to improve heat loss from flame to environment?

Answer 15

Add a draught excluder

Question 16

How to improve evaporation from a volatile liquid?

Answer 16

Use a flame or bomb colorimeter

Question 17

Level 1 of enthalpy of combustion

Answer 17

• Measure out set volume of watering measuring cylinder and add to the beaker or test tube
• Could add lid to reduce evaporation and screens to reduce draught
• As density of water is 1.00kgdm-3 the vol of liquid = mass of liquid in equation
• Record initial temp and record in table

Question 18

Level 2 of enthalpy of combustion

Answer 18

• Fix height of beaker a set distance from spirit burner
• Burn alcohol for set time
• Record highest temperature
• Calculate difference in temperature

Question 19

Level 3 of enthalpy of combustion

Answer 19

• Work out q=mcΔt
• Work out moles
• Calculate ΔH=q/n if the heat is given out then value negative