The complete combustion of salicylic acid releases 21.90 kJ of energy per gram o

Question

The complete combustion of salicylic acid releases 21.90 kJ of energy per gram of salicylic acid. In a particular bomb calorimeter (initially at room temperature), the combustion of 0.1280 g of salicylic acid, in the presence of excess oxygen, causes the temperature of the calorimeter to rise by 2.46 °C. When a 0.3684-g sample of an unknown organic substance is similarly burned in the same calorimeter, the temperature rises by 3.30 °C. What is the energy of combustion per unit mass of the unknown substance? (I got -10.21 kJ/g which is incorrect).

Explanation / Answer

Ans. Step 1: Determination of calorimeter constant:

Total energy released from salicylic acid combustion =

Mass of salicylic acid / Calorific value of salicylic acid

= 0.1280 g x (21.90 kJ/ g)

= 2.8032 kJ

# Increase in temperature of calorimeter is given by-

            Q = C dT                    - equation 1

            Where, Q = energy gained by calorimeter

            C = Calorimeter constant

            dT = increase in temperature

Putting the values in equation 1-

2.8032 kJ = C x 2.460C

            Or, C = 2.8032 kJ / (2.460C)

            Hence, C = 1.1395 kJ 0C-1

# Step 2: Determine heat absorbed by calorimeter from combustion of unknown sample-

Putting the values in equation 1-

            Q1 = 1.1395 kJ 0C-1 x 3.300C = 3.76035 kJ

Therefore, combustion of 0.3684 g sample must have released 3.76035 kJ energy which was absorbed by calorimeter to increase its temperature by 3.300C.

# Step 3. Determine calorific value of sample:

We have-

            Mass of unknown sample combusted = 0.3684 g

            Energy released by unknown sample, Q1 = -3.76035 kJ

Note: The negative sign of Q1 indicates release of energy during combustion.

Now,

            Calorific value of sample = Energy released / Mass of sample

                                                = -3.76035 kJ / 0.3684 g

                                                = -10.21 kJ/ g

# Your calculated value is correct. There seems to be some technical issue with your test program.

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