Comparing the enthalpy changes of combustion of different alcohols

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Ahvashta Seeburrun

Unit 1.4b

Aim: Comparing the enthalpy changes of combustion of different alcohols

Introduction: I am going to investigate the difference in enthalpy of combustion for a number of different alcohols. My aim is to find out the best fuel out of the five alcohols. I will calculate the value for enthalpy change by burning different fuels to heat a specific amount of water using the fact that 4.2J of energy are required to raise the temperature of 1g of water by 1°C. I will also produce a wide range of results and compare them to calculate their enthalpy change of combustion.

Background information: The enthalpy change of combustion is the energy transferred to and from the surroundings at a constant pressure, when one mole of fuel burns completely.  The symbol is ΔH. The chemical reactions that release energy to their surroundings are called exothermic and the energy that transferred to them from the surroundings is called endothermic.

ΔH = ΔH products - ΔH reactants

The units are kilojoules per mole (kJmol-1)

Exothermic

 Ref:

Exothermic reactions are more common. An example of exothermic reaction is: photosynthesis in plants where the energy comes from the sunlight. Energy cannot be destroyed but it can transfer from one form to another. The total energy of a system of reacting chemicals and surroundings remains constant. The value for exothermic enthalpy change is always a negative value as the energy is lost to the surroundings

ΔH = -890.3 KJmol-1

The standard conditions to compare the enthalpy changes of various reactions are temperatures, pressures, amounts and concentrations of reactants or products. The standard conditions are:

  • A pressure of 100kilopascals (102kPa)
  • A temperature of 298K (25oC)
  • Reactants and products in physical states, normal for the above conditions.
  • A concentration of 1.0mol dm-3 for solutions.

Standard enthalpy change of combustion ΔHoc

The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound reacts completely with oxygen under standard conditions.

If energy is released on making bond is greater than that absorbed through breaking the bonds then the exothermic reaction take place.

Endothermic

 Ref:

The endothermic enthalpy change is always in positive value as the energy is gained from the surroundings.

If energy is absorbed and the bonds is greater than the energy transferred to the surroundings because bonds produced then endothermic reaction will take place.

Enthalpy changes of combustion

Ref:

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References for background information: applied science book, handouts and website:

Risk assessment:

 = highly flammable. Ref:

= irritant. Ref:

 = toxic. Ref:

= harmful. Ref:

Apparatus list:

  • Spirit burner – it is used to burn alcohols in a safe way.

 Ref:

  • Copper can – avoid burning the hand while holding it as it cools down quickly.

Ref:

  • Clamp and stand - clamp and stand is used to hold the copper can while heating up the water.

Ref:  

  • Electronic balance (to 2 d.p.) – it is used to weight the alcohols ...

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This is a good description of how to test the enthalpy of combustion for a wide range of alcohols. However, there are various mistakes throughout the report to watch out for. This can actually be quite useful, as the mistakes have been highlighted by the marker , enabling you to realise which mistakes not to make yourself. Overall, this piece of work is 3 stars out of 5