Determining enthalpy change of combustion

Question 1

Equipment used

Answer 1

A calorimeter filled with water above the organic compound being combusted
A thermometer

Question 2

Overall idea of this method?

Answer 2

Measure the temperature change of the water in calorimeter to find temp change so we can calculate the enthalpy change of combustion

Question 3

Why do we use this method?

Answer 3

Because it’s too difficult to measure temperature change of combusting organic material so when the heat is released (exothermic) it heats up water so measure temp rise of water

Question 4

Step 1

Answer 4

Use a balance to measure the starting mass of the organic fuel AND spirit burner

Question 5

Step 2

Answer 5

Pour a known mass of water into calorimeter using volumetric pipette and measure its initial temp

Question 6

Step 3

Answer 6

Remove cap of spirit burner and light the wick

Question 7

When should you remove cap of spirit burner lid?

Answer 7

Only when lighting wick: or else it evaporates
And thus not be an accurate measure or the mass change (larger mass change so larger enthalpy change as a result)

Question 8

Step 4

Answer 8

Energy released of this combustion will heat up water causing water temperature to rise (exothermic)

Question 9

Step 5

Answer 9

Still water with thermometer in calorimeter filled with water: distribute absorbed thermal energy of water

Question 10

Step 6

Answer 10

Extinguish flame on wick by placing cap on wick after 3 mins

Question 11

Step 7

Answer 11

Use thermometer to read the final temp of water

Question 12

Step 8

Answer 12

Reweigh mass of spirit burner using balance, subtract this value from the fuel + spirit burner weighed to find mass of fuel burnt

Question 13

What equation can we use to identify the enthalpy change of this reaction?

Answer 13

Q = mc△T
Energy = mass x specific heat capacity x change in temperature

Question 14

What values from this experiment have we collected which we can use to find enthalpy change?

Answer 14

Temperature change of water
Mass of water (same as volume)
The mass of fuel burnt

Question 15

What does the enthalpy of this experiment tell us that we calculated?

Answer 15

The heat energy absorbed by the water thus the heat energy released by the fuel combusting

Question 16

How to find enthalpy change?

Answer 16

= mass of water x shc of water x temp change of water

Question 17

How to find enthalpy change PER MOL?

Answer 17

Divide the enthalpy released we calculated (in kJ) by the number of moles of fuel burnt, calculated from the mass of fuel burnt we recorded

Question 18

What is the enthalpy change symbol of the water?

Answer 18

None: just calculating the change in enthalpy

Question 19

What is the enthalpy change per mol symbol?

Answer 19

Negative: it is an exothermic reaction which releases energy

Question 20

Why the published value of enthalpy change or combustion is larger than the one we calculated

Answer 20

Leaving the unlit spirit burner uncapped
Some of the heat transferred not to water but metal calorimeter
Not all fuel underwent complete combustion
Not completed under standard conditions

Question 21

How does leaving spirit burner uncapped decrease the enthalpy change per mol of combustion below the published value?

Answer 21

Because it causes fuel to evaporate so thus we find a larger mass of fuel combusted (thus more moles)
So when dividing enthalpy change by a larger value, the enthalpy change per mol is smaller

Question 23

If a substance burns in air with a smoky flame, what happens?

Answer 23

This means incomplete combustion occurred
So then the value of enthalpy change of combustion decreases: because less energy is released in incomplete combustion so the temp raise is smaller = incomplete combustion