Option B: Human Biochemistry (Energy)

1.1 Calculate the energy value of a food from enthalpy of combustion data.

What is Biochemistry? o Biochemistry is the study of metabolic processes,

which includes a consideration of energy changes involved

Why is Energy So Important? o Our cells contain biological molecules involved in complex

reactions o Metabolism – sum of all these reactions o Cells get energy through respiration (starts with a simple

sugar molecule like glucose) o The body needs energy-rich molecules so metabolic

processes can occur o These molecules come from our diet, so it is important to

know energy content of food (Joules)

Bomb Calorimeter o Used to measure the heat of combustion of a

particular reaction a. Food is placed in a container (bomb) b. The bomb is placed in water c. The food is ignited electrically and burned

completely. d. Temperature increase of water is recordede. The specific heat capacity of water (amount of

energy needed to raise the temperature of 1 gram of water by 1 Kelvin), temperature change in water, and mass of water are used to calculate the heat released by the food using the following equation: q = mc Δ T

Diagram

q = mc Δ To q = energy evolved (Joules) o m = mass of water (grams) o c = specific heat capacity of water (4.18 Jg-1K-1)o ΔT = temperature change in water (Kelvin)

Example 1o A 0.78g sample of a food substance was

combusted in a bomb calorimeter and raised the temperature of 105.10 g of water from 15.4°C to 30.6°C. Calculate the energy value of the food in kJ g-1.

Solutiono Temperature rise in water = 30.6 – 15.4 =

15.2°C or 15.2 Ko Specific heat capacity of water = 4.18 Jg-1K-1 o q = mc Δ To q = (105.10 g) (4.18 Jg-1K-1) (15.2 K) o q = 6677.63 J per 0.78 g of sample heatedo The energy value = = 8561.1 J g-1 or 8.56 kJ g-1

**If a question asks for an answer in J mol -1, multiply the answer in J g-1 by the molar mass ( g mol -1)

Example 2o 1.50 g of glucose (C6H12O6) was

completely combusted in a bomb calorimeter. The heat evolved raised the temperature of 225.00 g of water from 18.50 °C to 27.96 °C. Calculate the energy value of glucose in kJ mol-1.

Solution o q = mc Δ To Temperature rise in water = 27.96 – 18.50

= 9.46°C or 9.46 Ko Heat evolved = (225.00 g) (4.18 Jg-1K-1)

(9.46 K) = 8897.13 Jo Mr (glucose) = 180 g mol-1

o energy value of glucose = x 180 g mol-1 = 1,067,655.6 J mol-1 = 1070 kJ g mol-1

Example 3 1.00 g of sucrose, C12H22O11, was completely

combusted in a food calorimeter. The heat evolved was equivalent to increasing the temperature of 631 g of water from 18.36°C to 24.58 °C. Calculate the calorific value of sucrose (in kJ mol-1) given the specific heat capacity of water in Table 2 of the Data Booklet.

Solution Mr for sucrose =342 Heat evolved = 0.631(kg)×4.18 (kJ kg-1K-

1)×6.22(K) = 16.4 kJ Calorific value = 5.61×103 kJ mol-1

References Brown, C. , & Ford, M. . (2009). Higher level

chemistry developed specifically for the ib diploma.Edinburgh Gate, Harlow, Essex: Pearson Education Limited.

Any Questions?