All living things require the input of energy to exist – this energy is used to drive the thousands of biochemical reactions that occur to allow the organism to grow, reproduce and sustain life. This energy comes almost always from the Sun, in the rst instance – energy from sunlight is captured by photosynthetic organisms (e.g. plants, algae, certain bacteria) and converted into carbohydrates. These are then broken down by a process called cellular respiration, to produce energy-rich molecules (e.g. adenosine triphosphate, or ATP) that release energy to drive biochemical reactions. Photosynthetic organisms can by ingested by nonphotosynthetic animals, and the carbohydrates (and other biomolecules) can be broken down and used for cellular respiration. As we ourselves are non-photosynthetic organisms, we must obtain our energy through what we ingest, i.e. via our diet, so that our cells are able to carry out all the necessary biochemical reactions. The amount of energy required by an individual will depend on the amount of physical activity they perform, but in general an average man requires about 10 500 kilojoules (kJ), equating to 2500 kilocalories (kcal) per day, while an average woman needs approximately 8400 kJ (2000 kcal) per day. The amount of energy found within di erent foods we buy is often displayed on the food packaging. This energy value is worked out through a process known as food calorimetry. A food (or bomb) calorimeter can be used, which measures the heat of combustion. Here, a known mass of a particular food is ignited and completely burnt in the presence of oxygen. The energy released is transferred to water and the rise in temperature of the water is measured. The energy contained in the food can then be calculated using the following equation: q = mc∆T where: q = heat evolved (J) m = mass of water (g) c = speci c heat capacity of water (4.18 J g−1 K−1 or 4.18 J g−1 °C) (This is included in the IBO Chemistry Data booklet.) ∆T = temperature change of the water (in °C or K)
Calculate the energy value of food using enthalpy of combustion data
1 kJ = 0.24 kcal
When 1.00 g of tomato soup was burnt in a food calorimeter containing 100 g water, it raised the temperature of the water from 20.4 °C to 28.0 °C. Calculate the energy content of 100 g of tomato soup. From the equation q = mc∆T, for 1.00 g of tomato soup, we know that: m = 100 g c = 4.18 J g−1 K−1 ∆T = 28.0 − 20.4 = 7.6 °C (which is 7.6 K, as it is the change in temperature that we are looking at, not the actual temperature).
Therefore: q = 100 × 4.18 × 7.6 = 3176.8 J, i.e. 3.18 kJ per 1 g So, in 100 g of tomato soup, there will be 3.18 × 100 = 318 kJ.
Examiner’s tip You could also be told the heat capacity of the whole system (water and calorimeter together) and asked to work out the enthalpy change. In that case, in the rst step you just multiply the heat capacity by the temperature change.
This method does not account for the heat lost through the container. The heat capacity of the thermometer and container of the food calorimeter must also be taken into account to increase accuracy of the results.
1 Complete combustion of 2.50 g of a snack food raised the temperature of 200.0 g of water by 17.9 °C. Calculate the energy value per 100 g of the food. 2 10.0 g of a biscuit was completely combusted in a food calorimeter. The heat capacity of the whole system was 8.50 kJ °C−1, and the temperature of the system increased 12.1 °C. Calculate the energy value of 100 g of the biscuit. 3 If 100 g of cooked rice contains 530 kJ energy, by how many degrees Celsius does the water temperature rise when 1 g of cooked rice is completely burnt in a food calorimeter containing 100 g water?
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