Tyler Brooks
Calculating the heat of combustion of a candle is a valuable learning experience for chemistry students. It involves lighting a candle beneath a container of water for a set period and using the change in mass of the candle, the mass of the water, and the change in temperature of the water to calculate the heat of combustion. The specific heat of water, usually expressed in joules per gram per degree Celsius, is multiplied by the mass of water in grams and the change in temperature in degrees Celsius to calculate the heat taken in by the water. This value is assumed to be equal to the heat released by the candle. The number of moles of the candle burned can then be used to calculate the heat of combustion of the candle wax.
| Characteristics | Values |
|---|---|
| Formula for wax in the candle | C32H66 or C25H52 |
| Molar mass of candle wax | 353 grams per mole |
| Specific heat of water | 4.184 joules per gram per degree Celsius |
| Mass of water | 100ml or 1000 grams |
| Change in temperature of water | 5 degrees Celsius |
| Mass of the candle | Variable |
| Mass of empty can | Variable |
| Mass of small can and water | Variable |
| Initial temperature of water | Variable |
| Final temperature of water | Variable |
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What You'll Learn
Define heat of combustion and specific heat
The heat of combustion of a substance, also known as its calorific value or energy value, is the amount of heat released when a given amount of the substance undergoes combustion. It is usually considered a synonym of calorific value, which is the total amount of energy released when a given mass of a substance undergoes complete combustion in the presence of oxygen under standard conditions of temperature and pressure. The potential of hydrocarbons used in fuels is expressed in terms of calorific value, based on the combustion reaction they undergo with oxygen to form water and carbon dioxide. The heat of combustion of a substance can be expressed in terms of energy (in joules or kilojoules) liberated when one mole of the fuel undergoes complete combustion with oxygen.
The heat of combustion is normally measured calorimetrically but can also be calculated accurately from the elemental composition of the substance. For example, in the case of coal, the heat of combustion is the heat given off when a given amount of coal is completely burned. The specific heat records the heat necessary to cause a given temperature rise in a substance. It is dependent on the substance's composition and structure. For instance, in the case of coal, the specific heat of coal decreases with an increase in carbon content.
The higher heating value (HHV) takes into account the latent heat of vaporization of water in the combustion products. It is useful in calculating heating values for fuels where condensation of the reaction products is practical. The HHV assumes that all the water components are in a liquid state at the end of combustion and that heat delivered at temperatures below 150 °C can be utilised. The lower heating value (LHV) is another measure of available thermal energy produced by combustion, measured as a unit of energy per unit mass or volume of substance. It considers energy losses such as the energy used to vaporize water.
To calculate the heat of combustion of a candle, an experiment can be set up where a candle is lit underneath a pail of water for a set period of time. The change in molar heat can be calculated using the candle's change in mass, the water's change in temperature, and the water's mass. The calculation is based on the assumption that the heat released by the candle is equivalent to the heat absorbed by the water. The heat absorbed by the water can be calculated by multiplying the specific heat of water by the mass of the water in grams and the change in temperature in degrees Celsius. The specific heat of water is 4.184 joules per gram per degree Celsius.
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Calculate the number of moles of the candle burned
To calculate the number of moles of a candle that have burned, you need to first calculate the mass of the candle burned. This is done by subtracting the post-experiment weight of the candle from its pre-experiment weight.
For example, if the candle weighed 50g before burning and 45g after, then the mass of the candle burned is 5g.
The next step is to calculate the molar mass of the candle wax. This is done by adding up the atomic masses of all atoms present. For candle wax with the chemical formula C24H50, the molar mass would be (24 x atomic mass of carbon) + (50 x atomic mass of hydrogen). Using 12.01g/mol for carbon and 1.008g/mol for hydrogen, the molar mass of candle wax is approximately 338.33g/mol.
Finally, to calculate the number of moles of the candle burned, divide the mass of the candle burned by the molar mass of the wax. In the example above, the calculation would be 5/338.33, which is approximately 0.0148 moles.
Another example uses a candle with the chemical formula C25H52, which has a molar mass of 353 grams per mole. If the change in mass of the candle is 2 grams, you would divide 2 by 353 to obtain 0.0056 moles.
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Find the heat taken in by the water
To calculate the heat of combustion of a candle, a common experiment is to light a candle under a pail of water for a set period of time. The heat released by the candle is assumed to be equivalent to the heat taken in by the water. This allows students to calculate the change in molar heat.
To find the heat taken in by the water, you need to know the specific heat of water, the mass of the water, and the change in temperature. The specific heat of water is 4.184 joules per gram per degree Celsius. Multiply this by the mass of the water in grams and the change in temperature in degrees Celsius. For example, if you have 1,000 grams of water and a 5-degree change in temperature, you would multiply 4.184 by 1,000 by 5, which equals 20,920 joules. This is the amount of heat taken in by the water.
It's important to note that this calculation assumes that the heat released by the candle is equal to the heat absorbed by the water. In reality, a burning candle also releases light and creates water vapour and carbon dioxide. The heat of the flame also melts the wax near the wick, and the flame's temperature can vary, with the hottest part typically reaching around 1400°C. Additionally, the oxygen depletion in the container is not solely responsible for the water rising, as the water level rises rapidly at the end when the candle goes out.
To improve the accuracy of the experiment, one suggestion is to light the candle inside the container to exclude excessive preheating. The size of the container can also affect the results. Furthermore, the chemical equation used in the experiment should be balanced correctly. For example, during the burning process, two oxygen molecules result in one carbon dioxide molecule and two water molecules, rather than one carbon dioxide molecule replacing one oxygen molecule.
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Measure the change in temperature of a mass of water
To measure the change in temperature of a mass of water, you can follow these steps as part of an experiment to calculate the heat of combustion of a candle.
Firstly, you will need to record the initial temperature of the water. Light a candle and place it underneath a pail of water for a set period of time. It is important to ensure that the candle is lit and stable before placing it under the pail of water, as you want to measure the change in temperature of the water caused by the burning candle. While the candle is burning, gently stir the water with a glass rod to ensure even heat distribution.
After a set period of time, usually around 10 minutes, blow out the candle and immediately record the final temperature of the water. It is important to act quickly to get an accurate measurement of the water temperature directly after the combustion. The change in temperature of the water can then be calculated by subtracting the initial temperature from the final temperature. This value will be used to determine the heat of combustion of the candle.
It is also important to note that the accuracy of the temperature change measurement depends on factors such as the mass of water, the specific heat of water, and the duration of combustion. These factors should be considered when designing the experiment to ensure meaningful data collection.
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Calculate the heat of combustion for wax
To calculate the heat of combustion for candle wax, you can perform the following steps:
Firstly, you need to gather your equipment. You will require a candle, a thermometer, a small metal can, a large metal can, water, and a way to stir the water (such as a glass rod).
Next, light the candle and drip some molten wax onto the lid of the small can. Attach the candle to the lid while the wax is still liquid, and then blow out the candle. Measure 100ml of water and pour it into the small can, ensuring that the thermometer is suspended in the water without touching the sides or bottom of the can.
Replace the small can and water inside the large can and ensure the candle is lit underneath. Time this part of the experiment for 10 minutes, gently stirring the water throughout. After the 10 minutes are up, blow out the candle and record the final temperature of the water.
Now you can begin your calculations. The formula for calculating the heat of combustion is:
> Quantity of heat in joules = (mass of water) x (change in temperature) x (specific heat of water)
The specific heat of water is 4.184 joules per gram Celsius. For example, if you had 1000 grams of water with a temperature change of 5 degrees Celsius, the calculation would be:
184 x 1000 x 5 = 20,920 joules
To find the heat of combustion for wax, you need to calculate the moles of candle wax burned. This is done by dividing the change in mass by the candle wax's molar mass. Candle wax has the chemical formula C25H52 and a molar mass of 353 grams per mole. For example, with a change in mass of 2 grams:
2 / 353 = 0.0056 moles
Finally, to find the heat of combustion for wax, multiply the change in heat by the moles of candle wax burned. Given that the change in water temperature is equivalent to the change in candle heat, we can use the previous value of 20,920 joules:
20,920 x 0.0056 = 117.52 joules/mole
So, the heat of combustion for wax in this example would be 117.52 joules/mole.
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Frequently asked questions
The formula for calculating the heat of combustion of a candle is: Quantity of heat in joules = (mass of water)(change of temperature)(specific heat of water). The specific heat of water is J/(g°C).
You will need a candle, a small metal can, a large metal can, water, a thermometer, and a glass stirring rod.
First, measure 100 mL of water and pour it into the small metal can. Suspend the thermometer in the water without letting it touch the sides or bottom of the can. This device is called a calorimeter. Cut the large soda can 1 or 2 inches from the bottom and throw away the top. Measure the mass of the bottom of the large soda can and the candle. Place the candle in the bottom of the large soda can.
Light the candle and immediately place the small can of water on top of it. Time the experiment for 10 minutes. Gently stir the water with the glass stirring rod. After 10 minutes, blow out the candle and record the final temperature of the water.
