Comparing the Enthalpy Changes of Combustion of Different Alcohols

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Comparing the Enthalpy Changes of Combustion of Different Alcohols

Introduction

In this investigation I will set out to find the differences in the enthalpies of combustion of 6 different alcohols. These are methanol, ethanol, butan-2-ol, propan-1-ol, propan-2-ol and 2Methylpropan-1-ol. I can then find the variation between straight chain and branched molecules and isomers.

Planning

Method

Carrying Out The Experiment

To carry out the experiment, I will fill my calorimeter with 75g of water. It is important that this is accurate, as it is needed to find the enthalpy change. The calorimeter will be held up by a clamp, connected by a boss to a clamp stand so that heat can be applied without having to hold the calorimeter up myself. The calorimeter always needs to be the same distance away from the heatproof mat at the bottom. The calorimeter is made of copper and is a good conductor which means that most of the heat the copper absorbs will be passed onto the water inside it.I will then need another clamp stand, clamp and boss to set up a draft-extrusion system. This will be 3 heatproof mats which will surround the calorimeter, ensuring that as much of the heat as possible goes to the calorimeter and not its surroundings. I will then need to record the starting temperature, as this is needed to find the change in temperature. Now, I take the mass of the spirit burner I have so that I can calculate the amount of alcohol that was used during the combustion. I can now light it and let it burn. I will aim for a temperature change of 20°C. Once it reaches that temperature change, I will need to pull away the spirit burner and then record the final temperature of the water and also weigh the final mass of the fuel. I can then find out the change in temperature, which is the final temperature - the initial temperature. The mass of alcohol used can be calculated by the initial mass of alcohol used - the final mass of alcohol. This will need to be repeated 3 times for each alcohol so there will be 18 tests overall. This enables me to obtain the most accurate results.

Specified Instructions for Experiment

Instruction

Amount

Change in temperature

20°C

Distance of calorimeter from base of experiment

2cm

Mass of water

75g

What to do in steps

o Put apparatus together, ensuring the calorimeter is 12cm from the base of the experiment

o Measure out water ensuring that it is 75ml or 75g. Record the initial mass of the alcohol

o Take the reading for the initial temperature

o Apply lit spirit burner to the base of the calorimeter. Aim to finish on a temperature 20°C away from the initial temperature

o Once finished, record the final temperature of the water and the final mass of the alcohol, calculating the differences

Diagram

Apparatus to be used

Apparatus

Reason For Use

Spirit burner

Will contain the fuel and be used to heat up the water

Calorimeter

Used to hold the water when being heated

Balance

Needed to measure the amount of water being used and to measure the change in weight of the alcohols before and after burning

Clamp Stand x2

Will be used to hold the calorimeter and heatproof mats in place

Heatproof mats x4

The mats will be needed to create a draft extrusion system so that as much heat as possible gets to the calorimeter

Clamp x2

Needed to hold heatproof mats in place as well as the calorimeter for heating

Boss x2

Used to join the clamp to the clamp stand

Thermometer

Will measure the temperature change of the water before and after heating

Substances to be used

Substance

Formula

State

(Alcohols(

Methanol

CH3OH

l

Ethanol

C2H5OH

l

Propan-1-ol

Isomers

C3H7OH

l

Propan-2-ol

C3H7OH

l

Butan-2-ol

Isomers

C4H9OH

l

2Methylpropan-1-ol

C4H9OH

l

Water

H2O

l

Obtaining Accurate Results

The formula to find out the enthalpy change of combustion of an alcohol is:

(Hc = cm(T

As I am finding (H, it is important that c, m and (T are accurate so that I can produce an accurate value for the enthalpy change of combustion. I need to find out the energy change per mole of an alcohol and I need to find the mass of alcohol used to calculate this so I need to be accurate when finding the amount of alcohol used, also.

Specific Heat Capacity (c)

The specific heat capacity of water is already known, however to gain the most accurate results possible, the number needs to be as precise as possible. To do this, the value should be kept as 4.17 and not rounded up to 4.2, as 4.17 is a more accurate value and this will result in a more accurate enthalpy change of combustion.

Mass of Water (m)

The mass of water is a fixed amount which I have decided upon as 75ml (75g). However, when carrying out the experiment it is important that the mass of water used is accurate. For instance, if the mass of water in the calorimeter is less than 75g, the change in temperature will be bigger as there is less water to heat. If the mass is bigger than 75g, then there will be a smaller change in temperature because there is more water to heat. Either way, this is inaccurate so it is important to measure out the water with care so that it is as accurate as possible.

Change in Temperature ((T)

To calculate this, I need to take two different readings - one at the start and one at the end of the experiment. These need to be as accurate as possible as they affect the final value for the enthalpy change.
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The Calorimeter

When being heated, the water will not take in all of the heat applied because the calorimeter will absorb it first. Because of this and to gain more accurate results, I will need to find out how much heat the calorimeter has absorbed and add it on to how much heat the water will have absorbed. The calorimeter is made of copper. This is because copper has a low specific heat capacity so most of the heat it absorbs will be passed onto the water and it also has high thermal conductivity which again means ...

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