AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The chemical energy store in dry biomass can be estimated using calorimetry.
Full specification wording
Energy and ecosystems (A-level only): The chemical energy store in dry biomass can be estimated using calorimetry.
Calorimetry measures the chemical energy in dry biomass by burning it completely and capturing the heat released.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 6 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Calorimetry measures the chemical energy in dry biomass by burning it completely and capturing the heat released. A weighed, dried sample is ignited in a sealed vessel, a bomb calorimeter, in pure oxygen so that combustion is complete. The vessel sits in a known mass of water and the temperature rise of that water is recorded. Energy released is calculated from the mass of water multiplied by its specific heat capacity multiplied by the temperature change, then divided by the dry mass of the sample to give an energy value per gram, usually kJ g-1, which can be scaled to kJ m-2 for an area. The sample must be dried first, because energy would otherwise be absorbed evaporating water and the value would be too low.
What earns the marks
- Naming calorimetry, or a calorimeter, as the technique used.
- Burning a known dry mass of the sample completely, in oxygen.
- Measuring the temperature rise of a known mass or volume of water.
- Calculating energy as mass of water multiplied by specific heat capacity multiplied by temperature change, divided by the dry mass.
- A named source of error, such as heat lost to the surroundings or incomplete combustion.
Common mistakes and how to put them right
Writing colorimeter instead of calorimeter.
Put it right: Phonetic spellings of calorimeter are accepted but colorimeter is rejected outright.
Burning fresh rather than dried material, so energy is absorbed evaporating water.
Giving an answer in joules without dividing by the sample mass, so values cannot be compared.
Ignoring heat lost to the apparatus and surroundings when asked why the value is an underestimate.
Assuming the temperature rise alone is the energy value, without using the specific heat capacity of water.
Exam tips
- Write 'calorimeter' clearly and distinguish it from 'colorimeter', which is a different instrument.
- If asked why the measured energy value is lower than the true value, answer heat lost to the surroundings or incomplete combustion.
- State that the sample is dried to constant mass before burning whenever you write the method.
Try the lesson
The start of the Energy and ecosystems (A-level only): The chemical energy store in dry biomass can be estimated using calorimetry. lesson, made from checked teaching notes for this specification point.
Have a go first
Which piece of apparatus is used to estimate the chemical energy store in a sample of dry biomass?
Listen
Calorimetry measures the chemical energy in dry biomass by burning it completely and capturing the heat released. A weighed, dried sample is ignited in a sealed vessel, a bomb calorimeter, in pure oxygen so that combustion is complete. The vessel sits in a known mass of water and the temperature rise of that water is recorded. Energy released is calculated from the mass of water multiplied by its specific heat capacity multiplied by the temperature change, then divided by the dry mass of the sample to give an energy value per gram, usually kJ g-1, which can be scaled to kJ m-2 for an area. The sample must be dried first, because energy would otherwise be absorbed evaporating water and the value would be too low. A strong answer earns marks for these points. naming calorimetry, or a calorimeter, as the technique used. burning a known dry mass of the sample completely, in oxygen. measuring the temperature rise of a known mass or volume of water. calculating energy as mass of water multiplied by specific heat capacity multiplied by temperature change, divided by the dry mass. Exam tip: Write 'calorimeter' clearly and distinguish it from 'colorimeter', which is a different instrument.
Key words
Say what each word means before you turn the card.
calorimetry
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bomb calorimeter
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temperature rise of water
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energy per gram of dry mass
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See every definition
- calorimetry
- A technique used to estimate the chemical energy store in dry biomass by burning it completely and capturing the heat released.
- bomb calorimeter
- A sealed vessel in which a weighed, dried sample is ignited in pure oxygen so that combustion is complete.
- temperature rise of water
- Recorded for a known mass of water in which the vessel sits, and used with the specific heat capacity to calculate energy released.
- energy per gram of dry mass
- Calculated by dividing the energy released by the dry mass of the sample, usually given in kJ g-1 and scalable to kJ m-2 for an area.
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