11 Calculate the energy value of a food from enthalpy of combustion data What is Biochemistry Biochemistry is the study of metabolic processes which includes a consideration of energy changes involved ID: 242770
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Slide1
Option B: Human Biochemistry (Energy)
1.1 Calculate the energy value of a food from enthalpy of combustion data.
Slide2
What is Biochemistry?
Biochemistry is the study of metabolic processes, which includes a consideration of energy changes involved Slide3
Why is Energy So Important?
Our cells contain biological molecules involved in complex reactions
Metabolism – sum of all these reactions
Cells get energy through respiration (starts
with a simple sugar molecule
like glucose)
The body needs energy-rich molecules so metabolic processes can occur These molecules come from our diet, so it is important to know energy content of food (Joules) Slide4
Bomb Calorimeter
U
sed to measure the heat of combustion of a particular reaction
Food is placed in a container (bomb)
The bomb is placed in water
T
he food is ignited electrically and burned completely. Temperature increase of water is recordedThe 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 TSlide5
Diagram Slide6
q = mc
Δ
T
q = energy evolved (Joules)
m = mass of water (grams)
c = specific heat capacity of water (
4.18 Jg-1K-1)
ΔT = temperature change in water (Kelvin) Slide7
Example 1
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
.Slide8
Solution
Temperature rise in water = 30.6 – 15.4 = 15.2°C or 15.2
K
Specific heat capacity of water = 4.18 Jg
-1
K
-1 q = mc Δ Tq = (105.10 g) (4.18 Jg
-1K-1) (15.2 K) q = 6677.63 J per 0.78 g of sample heatedThe 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
)
Slide9
Example 2
1.50 g of glucose (C
6
H
12
O
6) 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.Slide10
Solution
q
= mc
Δ
T
Temperature rise in water = 27.96 – 18.50 = 9.46°C or 9.46 KHeat evolved = (225.00 g) (4.18 Jg-1K-1) (9.46 K) = 8897.13 J
Mr (glucose) = 180 g mol-1energy value of glucose =
x 180 g mol-1 = 1,067,655.6 J mol-1 = 1070 kJ g mol-1 Slide11
Example 3
1.00 g of sucrose
,
C12
H
22
O11, 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.Slide12
Solution
M
r
for sucrose =342
H
eat
evolved = 0.631(kg)×4.18 (kJ kg-1K-1)×6.22(K) = 16.4 kJCalorific value = 5.61×103 kJ mol-1Slide13
References
Brown, C. , & Ford, M. . (2009).
Higher level chemistry developed specifically for the
ib
diploma
.Edinburgh
Gate, Harlow, Essex: Pearson Education Limited.Slide14
Any Questions?