Calorimetry and energy calculations

In a nutshell

Calorimetry can be used to measure the amount of energy in food. This helps us balance the amount of energy we consume with the amount of energy we use. Calorimetry can be used to measure the amount of energy in food, although there are some basic well accepted figures that can be used to estimate the amount of energy in food .

Calorimetry

Definition

Calorimetry is a way to measure the energy produced or taken in overall by a reaction. We can use this to measure the amount of energy.

Example

Calories in food.

Calorimetry experiments

To set up a calorimetry experiment to find out how much energy there is in food, you will need the following:

Test tube
$50$ $\ ml$ cold water
Food sample
Bunsen burner
Thermometer

Biology; Key concepts; KS4 Year 10; Calorimetry and energy calculations

Procedure

1.	Pour $50\ ml$ cold water into a test tube.
2.	Record the starting temperature of the water with the thermometer.
3.	Hold the test tube at $45$ degrees.
4.	Light the food sample using a Bunsen burner so that it starts burning. Make sure that it can sustain burning until all of the sample has burnt up.
5.	Immediately hold the food sample underneath the test tube until it finishes burning.
6.	Record the final temperature of the water.
7.	Use the formula below to find out how much energy was transferred from the food to the water in the form of heat energy. The formula uses the mass of water, the change in temperature caused by burning the food sample and the specific heat capacity of water which is $4.2$ . This amount of energy came from the food, and that is how the amount of calories can be measured by this experiment.

$energy\ transferred(J)= mass\ of\ water(g)\ \times\ temperature\ change(\degree C)\ \times\ 4.2(J/g/\degree C)$

This will give you the amount of energy in the food in Joules ( $J$ ). This can be converted into kilojoules ( $kJ$ ), calories ( $cal$ ) or kilocalories ( $kcal$ ).

Tip: Kilocalories are often called calories in everyday language. For example, the daily recommended intake for adults is $2000\ calories$ , although really they mean $2000\ kcal$ . If you're unsure of which form you have to convert your answer into, look at the units.

Example

A test tube containing $50\ ml$ water is heated by $12\degree C$ when a sample of olive oil is burnt. How much energy does the sample contain?

$energy\ transferred = 50\times12\times 4.2\ =\ 2520\ J$

$\underline{Therefore,\ the\ sample\ contains\ 2520\ J\ of\ energy.}$

Basic caloric values

There are some commonly known caloric values of nutrients that can be used in calculations.

Nutrient ( $1\ g$ )	Calories ( $kcal$ )
Carbohydrate	$4$
Protein	$4$
Fat	$9$

So if you know how many grams of carbohydrates, protein or fat are in the food you are looking at, you can calculate the number of calories or the amount of energy in the food.