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CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2

Chemistry 9.5 Investigating food energy — Calorimetry and Evaluating Errors

5 min read 6 parts Lesson 5 of 12

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Lesson summary

9.5 Investigating food energy — Calorimetry and Evaluating Errors — Alhussan International Academy — Chemistry (Stages 7–9) notes: Plan and analyse an investigation that measures the energy transferred when food burns, and evaluate errors.Stored energy — foods contain stored chem…

  • Which foods
  • Scientific question
  • Hypothesis
  • Prediction
  • Reliable data
  • Relationship

Part 1 of 6Energy in food

  • Stored energyfoods contain stored chemical energy, which the body releases by respiration.
  • Calorimetry estimating the energy in food by burning a small sample under a container of water and measuring how much the water warms.
  • Which foodsfoods rich in fat or oil, such as nuts, release more energy per gram than foods rich in carbohydrate or protein.
  • Unitsenergy is measured in joules (J) or kilojoules (kJ), where 1 kJ = 1000 J.

Part 2 of 6Hypothesis and prediction

  • Scientific question — which provides more energy, cashew nuts or bread?
  • Hypothesis = a possible explanation based on evidence that can be tested further.
    • Nuts are oil rich and bread is carbohydrate rich, and oils provide more energy than carbohydrates, so nuts transfer more energy than bread.
  • Prediction — burning 1 g of nut will heat water up more than burning 1 g of bread.

Part 3 of 6Method

  1. Measure 100 g (100 cm3) of water into a metal container (or boiling tube) clamped above a heat-resistant mat.
  2. Record the starting temperature of the water.
  3. Weigh the food sample, mount it on a needle in a wooden handle and light it in a Bunsen flame.
  4. Place the burning food immediately under the container and let it burn until it goes out, relighting if needed.
  5. Stir the water and record the highest temperature reached.
  6. Repeat three times for each food and calculate the mean temperature change.
1Variables1 point
— the independent variable is the type of food and the dependent variable is the water temperature change.
  • Control the volume of water, the mass of food (about 0.5 to 1 g), the distance from the flame, the starting temperature and the container.
2Dry food
— wet foods use energy to evaporate water and give lower results.
3Safety1 point
— wear eye protection, tie back long hair, use a heat-resistant mat and handle the needle carefully.
  • People with nut allergies should not handle the nuts, and food used in a laboratory must never be tasted.

Part 4 of 6Analysing the evidence

  • Reliable data — if the three repeat values are similar, the data are reliable.
  • Relationship — the greater the water temperature change, the more energy a burning food transfers.
  • Results — the mean water temperature change for 1 g of nut is about 40 °C, compared with about 20 °C for 1 g of bread.
  • Conclusion — 1 g of burning cashew nut transfers more thermal energy than 1 g of bread, so the prediction was correct and supports the hypothesis that oil-rich cashews transfer more energy.
    • A conclusion answers the scientific question and refers to the prediction and hypothesis.

Part 5 of 6Calculating the energy transferred

  • Formula — H = m × c × ΔT.
    • H is the energy transferred, in joules.
    • m is the mass of water, in grams.
    • c is the specific heat capacity of water, 4.2 J/g °C.
    • ΔT is the temperature change of the water, in °C.
  • Per gram of food — energy per gram = energy transferred ÷ mass of food burned.
  • Nuts — 1 g of nut heating 100 g of water by 40 °C transfers 100 × 4.2 × 40 = 16 800 J, which is 1 680 000 J per 100 g.
  • Secondary data — a trusted source says 100 g of cashew nuts transfers 2 200 000 J, so the measured value is lower.

Part 6 of 6Evaluating errors

1Systematic error3 points
— it pushes every result the same way, and measured values are almost always lower than the figures on food packets.
  • Heat escapes to the air and warms the container and clamp, so some energy goes to the apparatus instead of the water.
  • Food may burn incompletely, leaving black carbon.
  • Some food may fall off or burn after being moved away from the container.
2Random error
— reading a scale slightly differently each time causes scatter in results.
3Reducing errors1 point
— repeating and taking a mean reduces random errors but not systematic ones.
  • Systematic errors are reduced by changing the method, for example a draught shield, insulation, a lid, a copper can and keeping the food close to the container.
4Accuracy and precision1 point
— results are precise if repeats are close together and accurate if close to the true value.
  • A method with large heat losses may be precise but inaccurate.

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