Prac 9 - Energy produced by fuels

Name the first five alchohols and their formula/structure?

1) Methane: CH₃OH

2) Ethanol: C₂H₅OH

3) Propan-1-ol: C₃H₇OH

4) Butan-1-ol: C₄H₉OH

5) Pentan-1-ol: C₅H₁₁OH

How does the number of carbon atoms affect the energy released?

More carbon atoms:

• Longer chains

• More C-C and C-H bonds to break

• More bonds will be made in CO₂ + H₂O

More energy is released, making the extra bonds in CO₂ + H₂O than is needed to break the extra C-C and C-H bonds; therefore, this energy is released as heat.

So, the higher the number of carbons, the higher the amount of energy released per gram/mole of alchol.alcohol

What is enthalpy change?

It is the change in heat energy.

It is the measure of energy released per mole of alcohol.

Looking at the mass used to heat up the water is not sufficient because they are different molecules.

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We need to work out mass used per mole of alcohol.

How do you work out enthalpy change per mole?

1) Write the formula of the molecule: e.g. CO₂ or H₂O.

2) Use the periodic table to work out the relative molecular mass (Mr):

C = 12, H = 1, O =1 6

CO₂ Mr = (1 × 12) + (2 × 16) = 44

3) Calculate heat energy released: E = mc0

4) Calculate heat energy per mole:

E = heat energy x relative molecular mass/mass of fuel burnt.

What is the relative molecular mass of Methanol?

Methanol (CH₃OH)

Mr = 32

(12 × 1) + (1 × 4) + (1 × 16)

C = 12, H = 1, O =16.

What is the relative molecular mass of Ethanol?

Ethanol (C₂H₅OH)

Mr = 46

(12 × 2) + (1 × 6) + (1 × 16)

C = 12, H = 1, O =16.

What is the relative molecular mass of Propan-1-ol?

Propan-1-ol (C₃H₇OH)

Mr = 60

(12 × 3) + (1 × 8) + (1 × 16)

C = 12, H = 1, O =16.

What is the relative molecular mass of Butan-1-ol?

Butan-1-ol (C₄H₉OH)

Mr = 74

(12 × 4) + (1 × 10) + (1 × 16)

C = 12, H = 1, O =16.

What is the relative molecular mass of Pentan-1-ol?

Pentan-1-ol (C₅H₁₁OH)

Mr = 88

(12 × 5) + (1 × 12) + (1 × 16)

C = 12, H = 1, O =16.

What is the aim of this practical?

To investigate the energy released by different alcohols (with different length Hydrocarbon chains) when heating up the same volume of water by the same temperature.

What is the hypothesis?

As the length of hydrocarbons increases, the less mass of alcohol will be needed to burn to raise the temperature of the water by 30^oc.

This happens because…

• As the alcohol increases in length, there are more C-C and C-H bonds in the reactants and more bonds in the products of CO₂ and H₂O are formed.

• Breaking the bonds in the reactants requires less energy than is released when the bonds in the products are formed and so the energy is released as heat.

What are the independent variables?

The different alcohols: Methanol, Ethanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol.

What are the dependent variables?

Quantitative: Mass (g) of alcohol used

Qualitative observations: the colour and size of flame/soothiness/carbon residue.

What are the control variables?

• Weighing with lid on

• Same temperature rise (30^oc)

• Height of calorimeter above the spirit burner

• Same volume of water (50ml)

• Same digital thermometer.

What are the Hazards, Risks, Precautions, Emergency Procedure (H&S)

Hazard - Risk - Precautions - EP

Methanol - Toxic if ingested/causes blindness - Wear goggles/lab coats/do not touch wick - First aid kit if burnt

Alcohols - Flammable - Alcohols are supplied & used at all times in glass burners with secure and intact wicks/Alcohol burners are extinguished using the cap at all times. -

Glassware - Smash/injuries - Keep away from the bench edge - Contact first aid/teacher for clean up.

Calorimeter - Burns - When the calorimeter has been heated, do not touch until cool or use heat-proof gloves to empty - Run area under cold water.

Hot water - Burns if spilt/causes injuries - contact first aid/teacher to clean up.

What equipment ia used ?

Digital thermometer - to measure the temperature of water

Clamp and stand to hold the calorimeter

Calorimeter & lid - to hold the water and keep O₂ in

Measuring cylinder (50ml) - to measure 50ml of water

Mass balance - to weigh the mass of each alcohol spirit burner with the lid on before and after being lit.

Matches - to light the fuel

Stop clock - to record when the water increased by 30 degrees

Heat-proof mat - to ensure no damage to the benches

What is the method?

1) Set up the equipment as per the diagram

2) Add 50ml of water to the calorimeter can using a 50ml measuring cylinder

3) Place the lid on the calorimeter and place the thermometer in the water

4) Record the temperature of water in the table

5) Weigh the mass of the ethanol spirit burner (with the lid on) using the mass balance and record the mass (g) of the fuel to 2dp.

6) Place the spirit burner under the calorimeter and light the fuel.

7) When the water temperature has risen by 30 degrees, place the lid back on the fuel to extinguish the flame.

8) Weigh the mass of the spirit burner and fuel, recording the final mass (g)

9) Clean the calorimeter of any soot

10) Repeat the experiment to get 3 repeats

11) Repeat the steps above with each of the 4 fuels

What was the graph like?

Bar chart

• x-axis titled alchols

• y-axis Heat energy per mole (kJ/mol)

What was the conclusion?

Same hypothesis

Just add Butan-1-ol and Pentan-1-ol would have burnt with a bigger orange flame and would have created more soot as they have more incomplete combustion.