Test Session Chapter 6 Chemical Energetics (1st Year Chemistry)

Q1. Multiple Choice Questions

10 Marks

Choose the correct option for each of the following questions. Each question carries 1 mark.

1. Which of the following quantities is NOT typically determined using Hess’s Law?

A Enthalpy change of formation

B Enthalpy change of combustion

C Activation energy

D Enthalpy change of reaction

2. Which of the following is an exothermic reaction?

A Photosynthesis

B Formation of water

C Electrolysis

D Bond breaking

3. Bond energy is useful in predicting:

A Atomic radius

B Ionization energy

C Lattice energy

D Enthalpy change

4. The specific heat capacity of water is:

A 4.18 J g^-1 K^-1

B 1.00 J g^-1 K^-1

C 2.18 J g^-1 K^-1

D 5.00 J g^-1 K^-1

5. The final enthalpy change ΔH is calculated using:

A -q × ΔT

B -q/n

C q × m × T

D c/mΔT

6. The energy released from food digestion is equal to:

A Energy used in cooking

B Energy required for freezing

C Energy released during combustion

D None of the above

7. Hess’s law is especially useful when:

A A reaction is fast

B Enthalpy change can’t be measured directly

C Energy is negative

D Heat is not absorbed

8. In Hess’s cycle, if ΔH₁ = -100 and ΔH₂ = -200, ΔH (overall) = ?

A -300

B +300

C -100

D -200

9. The reaction: C (graphite) + ½O₂ → CO is calculated using Hess’s law from:

A Combustion of CO

B Formation of CO₂

C Both A and B

D Ionization of C

10. Hess’s law can be used to calculate:

A Boiling points

B Lattice energy

C Molar mass

D Density

Q2. Short Answer Questions

15 Marks

Write down short answers to the following questions.

Differentiate between exothermic and endothermic reactions.
Write equations, including state symbols, that represent the enthalpy change of atomization of:
(i) Oxygen
(ii) Barium
(iii) Bromine (Quick Check 6.2)
Draw enthalpy cycle of sample problem (Propane (C₃H_8(g)) burns in oxygen according to the equation:
C₃H_8(g) + 5O_2(g) → 3CO_2(g) + 4H₂O_(l)
When 14.64 g of propane is burned in an excess of oxygen in a calorimeter at 25 °C and 1 atm pressure, 678.6 kJ of heat is evolved. Calculate the standard enthalpy of formation of propane. The standard enthalpies of formation of CO_2(g) and H₂O_(l) are -393.51 kJ mol^-1 and -241.82 kJ mol^-1 respectively.) according to Hess’s law to validate the calculation ΔH_f (C₃H₈) = -104 kJ mol^-1. (Quick Check 6.5)

Calculate the enthalpy change for the following reaction:

C₂H₅OH_(l) + 3O_2(g) → 2CO_2(g) + 3H₂O_(l)

The bond energies of various bonds (in kJ mol^-1) are given below:

Bond	Energy (kJ mol^-1)
C-C	+347
C-H	+410
C-O	+336
O=O	+496
C=O	+805
O-H	+465

(Quick Check 6.6)

Give two examples of exothermic reactions.
Write the equation for NH₄Cl dissolution and its ΔH value.
What is the role of NH₄Cl in cold packs?
Define enthalpy of formation with an example.
What is the standard enthalpy of combustion of ethanol?
Give bond energy of H-H and O=O bonds.
What causes ΔH_r in chemical reactions?
How does ΔH_sol affect temperature of solution?
Why is ΔH_sol of NaOH negative?
What is the Born-Haber cycle?
Name the steps involved in the Born-Haber cycle of NaCl.

Q3. Detailed Answer Questions

10 Marks

Write detailed answers to the following questions. (Answer any 2)

1. Propane (C₃H_8(g)) burns in oxygen according to the equation:

C₃H_8(g) + 5O_2(g) → 3CO_2(g) + 4H₂O_(l)

When 14.64 g of propane is burned in an excess of oxygen in a calorimeter at 25 °C and 1 atm pressure, 678.6 kJ of heat is evolved. Calculate the standard enthalpy of formation of propane. The standard enthalpies of formation of CO_2(g) and H₂O_(l) are –393.51 kJ mol^-1 and –241.82 kJ mol^-1 respectively. (Sample problem 6.6)

2. Explain in detail the concept of enthalpy change. How do exothermic and endothermic reactions differ in terms of ΔH?

3. With the help of a labeled diagram, explain enthalpy change in exothermic and endothermic reactions.

Chemistry Examination

Chapter 6: Chemical Energetics

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