Formulae of Alcohols

Specification Point 9.26C:
  • Recall the formulae of molecules of the alcohols, methanol, ethanol, propanol (propan-1-ol only) and butanol (butan-1-ol only), and draw the structures of these molecules, showing all covalent bonds
  • Alcohols are a homologous series of compounds that have the general formula  CnH2n+OH.
  • They differ by one -CH2 in the molecular formulae from one member to the next.
  • They show a gradation in their physical properties:
    • Boiling points increase with increased carbon chain length.
    • Viscosity increases with increased carbon chain length.
  • They have similar chemical properties.
  • The first four members of the series are methanol, ethanol, propanol and butanol.

Alcohol formulae table, Edexcel GCSE Chemistry

Alcohol Functional Group

Specification Point 9.27C:
  • Recall that the functional group in alcohols is –OH
  • All alcohols contain the hydroxyl (-OH) functional group, which is the part of alcohol molecules that is responsible for their characteristic reactions.
  • As with other organic molecules, a large R represents the side chain which doesn´t  affect the chemistry of the molecule.

Alcohol Functional Group, Edexcel GCSE Chemistry

Diagram illustrating the side chain and the  -OH group in ethanol which characterizes its chemistry

  • Alcohols are colourless liquids that dissolve in water to form neutral solutions.
  • They undergo combustion to form carbon dioxide and water, giving off heat energy and are used as fuels.
  • There are alcohols that have more than one -OH group on the chain, which are called di-ols.
  • We have already seen their use in condensation polymerisation reactions.
  • Alcohols undergo dehydration reactions in the presence of concentrated acid catalyst and heat, producing an alkene and water.

A water molecule can be removed from an alcohol to produce an alkene

Core Practical: Heat of Combustion of Alcohols

Specification Point 9.28C:
  • Investigate the temperature rise produced in a known mass of water by the combustion of the alcohols ethanol, propanol, butanol and pentanol
  • Alcohols can be used as fuels as they readily undergo combustion and release heat energy.
  • Ethanol for example combusts in excess oxygen:

CH3CH2OH + 3O2 → 2CO2 + 3H2O

  • Some alcohols are better fuels than others i.e. they release more heat energy per mole than other alcohols.
  • Calorimetry studies can be performed to investigate the efficiency of alcohol fuels by measuring how much of each alcohol is needed to raise the temperature of a fixed amount of water by a set number of degrees.

Calorimetry Investigation

Aim:

  • To investigate the efficiency of different alcohol fuels by calorimetry studies

Materials:

  • Copper calorimeter can with lid, thermometer, water, spirit burner, balance.
  • Solutions of ethanol, propanol, butanol and pentanol.

Copper Calorimeter, Edexcel GCSE Chemistry

Diagram showing the calorimetry experiment for combustion

Method:

  • Using a measuring cylinder, place 100 cm3 of water into a copper can.
  • Record the initial temperature of the water and the mass of the empty burner.
  • Fill the burner with the test alcohol and record its new mass.
  • Place the burner under the copper can, light the wick and place the lid on.
  • Stir the water constantly with the thermometer (calorimeter lids allow for this) and continue heating until the temperature rises by 25ºC.
  • Immediately measure and record the mass of the spirit burner.
  • Repeat procedure for other alcohols, making sure the variables are kept the same:
    • Volume of water (water should be changed each time)
    • Mass of alcohol
    • Distance between wick and bottom of stand
  • Record your results neatly in tabular format.

Calculation:

  • Calculate the change in mass for each alcohol.
  • This is equal to the amount of each alcohol used to increase the set amount of water by the set amount of degrees, allowing for direct comparison of the alcohols in terms of their fuel efficiency.
  • Their efficiencies could also be compared using the equation for enthalpy of reaction.
  • Rise in temperature of water    =    final temperature – initial temperature.
  • Mass of alcohol burnt     =    initial mass – final mass.
  • Enthalpy change equation:  Q = m x c x ΔT, where:
    • Q – energy transferred to water
    • m – mass of water heated
    • c –  the specific heat capacity – is the amount of heat needed to raise the temperature of 1 gram of a substance by 1 oC.

For water, the value is 4.18 J g-1C-1 (Joules per gram per degree Celsius).

    • ∆T – change in temperature
  • When found Q you have calculated amount of heat released when you have burnt the mass of alcohol in the experiment.
  • You can work out:
    • Amount of heat released from 1g of substance = Q/mass of substance burnt.
    • Amount of heat released from 1 mole of substance = Q/mass of substance burnt x molecular weight of substance.

Analysis of Results:

  • The results will show that less amounts of the longer chain alcohols are needed to raise the temperature of the water.

Conclusion:

  • Therefore longer chain alcohols are more efficient fuels than shorter chain alcohols.

Example:

The energy from burning 1.0 g of propane was transferred to 150 g of water to raise its temperature by 30°C. Calculate the heat energy change (in kJ).

Q = m x c x ΔT

Mass of Water = 150 g

Heat capacity of Water = 4.2 J g-1 C-1

Temperature rise = 30°C

Energy transferred = 150 x 4.2 x 30 = 18900 J

*1000 J = 1 kJ

So 18900 J   =  18.9 kJ

        Energy transferred = 18.9 kJ

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Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.