Important Questions and Answers: Energetics of Chemical Reactions

1. State whether the following are extensive or intensive property with reasons:

Ans: Entropy: Since, entropy depends upon mass of the substance, it is extensive property.

Temperature: Since, temperature do not depend upon mass of the substance, it is intensive property.

2. Define exothermic and endothermic reaction.

Ans: The reaction in which heat is evolved is called exothermic reaction.

Example: C(s) + O2 (g) →  CO2(g); ΔH = -393.5KJ

The reaction in which heat is absorbed is called endothermic reaction.

Example: C(s) + 2S(g) →  CS2(g); ΔH = +92.0KJ

3. State and explain Hess law of constant heat summation.

Ans:  It states that, “The amount of heat evolved or absorbed in a given chemical reaction is always same regardless the step in which reaction completes.”

Let Q be the amount of heat evolved during a chemical change, where A changes to D.

i.e., A → E + Q……(i)

Again, the reactant A is converted into the same product D through several steps.

A → B → C → D……(ii)

Now, according to Hess law of constant heat summation, the amount of heat evolved or absorbed in this reaction is same.

4. Draw the energy profile diagram for exothermic and endothermic reaction.

Ans: The energy profile diagram for exothermic and endothermic reaction is given below.

5. State the first law of thermodynamics.

Ans: First law of thermodynamics states that, “When dQ amount of heat is given to a system, the partial heat is applied to increase internal energy and partly as the external work done by the gas. ”

So, dQ = dU + dW

Here, dU is the internal energy of the gas and dW is the work done by the gas.

6. What is enthalpy of a reaction?

Ans: The amount of heat content in the reaction is called enthalpy of a reaction. It is represented by ΔH. Enthalpy might be negative or positive depending upon the situation.

7. What are the limitations of first law of thermodynamics?

Ans: First law of thermodynamics does not explain:

  • The extent and direction of the convertibility of one form of energy to another.
  • The nature of changes (reversible or irreversible).
  • The feasibility and spontaneity of a process.
  • Reason for why heat cannot be converted into work.

8. Differentiate between internal energy and enthalpy.

      Internal Energy         Enthalpy
It is the sum of different forms of energies associated with the atoms, ions or molecules in a system. Enthalpy is the heat content of a system.
Internal energy of a substance depends upon its chemical nature and its state of existence. Enthalpy of a substance depends on its temperature and pressure.
The change in internal energy of a system is equal to the heat supplied to or absorbed by the system at constant volume. Enthalpy of a substance depends upon its temperature and pressure.
It is represented by U. It is represented  by H.

9. Define state function and give any two examples of it.

Ans: The measurable property of a system which depends only upon the state of the system is called state function of a system. Pressure, volume, temperature, entropy are some of the example of state function.

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