9.2 Measuring Enthalpy Changes

Question 1

What is the equation for specific heat capacity?

Answer 1

Q = mcΔT

Question 2

What does “Q” stand for in the specific heat capacity equation and what are its units?

Answer 2

Q = energy

Joules

Question 3

What does “m” stand for in the specific heat capacity equation & what are its units?

Answer 3

m = mass, g

Question 4

How can you determine the standard enthalpy change of methanol experimentally?

Answer 4

1. fill beaker with known mass of water.

2. fill spirit burner with methanol & measure its mass.
3. place the spirit burner under the beaker & start burning the methanol by lighting the wick.
4. after a few minutes, put out the flame, measure the maximum temperature of the water & the mass of the spirit burner with remaining methanol.
5. calculate temperature change & mass change.
6. calculate energy change of water using Q = mcΔT, & moles of methanol burned.
7. divide energy by moles of ethanol to find standard enthalpy change of combustion of methanol.

(energy absorbed by water is approximately energy released by combustion of methanol. therefore finding the energy change of water gives us the energy released by burning said moles of methanol.)

Question 5

For what reasons may the experimental value for the standard combustion of methanol differ from the true value?

Answer 5

• ENERGY LOST TO SURROUNDINGS
  
  • energy absorbed by water < energy released by combustion, so lower temperature change
    
    • therefore experimental value is less exothermic than true value
• INCOMPLETE COMBUSTION
  
  • less energy is released during incomplete combustion compared to complete, so lower temperature change
    
    • therefore experimental value is less exothermic than true value
• METHANOL MAY EVAPORATE FROM WICK
  
  • more moles of methanol are responsible for the same temperature change (when dividing energy by moles, value is lower than true)
    
    • therefore experimental value is less exothermic than true value
• NON-STANDARD CONDITIONS