An Overview of Class 12 Chemistry Thermochemistry Viva Questions With Answers
Thermochemistry is one of the fields of chemistry that deals with energy changes. Enthalpy change of a reaction is defined as the difference in enthalpy of product and reactant. The heat evolved in a reaction does not transfer to the surrounding. It either increases or decreases the temperature of the system. The reaction is carried out in a calorimeter to determine the enthalpy change of a reaction.
Table of Content
Aim
Apparatus Required
Theory
Procedure
Observation
Result
Precautions
Lab Manual Questions
Viva Questions
Practical Based Questions
Conclusion
Aim
To determine the heat of neutralisation of a reaction between a strong acid and a strong base.
Apparatus Required
Following is the apparatus required for conducting the above experiment
Beakers
0.2M HCL (100ml)
0.2M NaOH (100ml)
Distill water
Theory
The heat of neutralisation is defined as the change in enthalpy when 1 gram equivalent of an acid is mixed with 1 gram equivalent of a base in an aqueous solution. The enthalpy of neutralisation for strong acid and strong base is 57.1kj/mol.
H++OH- = H2O, ∆H= -57.1kj/mol.
The enthalpy of neutralisation may be positive or negative.
Procedure
The following steps must be followed while performing the procedure.
Take 100 ml of 0.2M hydrochloric acid in a beaker and keep it in a thermally insulated wooden box. Remember to find its water equivalent and note its temperature.
Now take another beaker and fill it with 100 ml of 0.2M sodium hydroxide. Note down its temperature also. Both solutions, hydrochloric acid and sodium hydroxide, should have the same temperature. If they do not have the same temperature, wait to attain the same temperature.
Now add sodium hydroxide in the beaker containing hydrochloric acid
Stir the mixture and note down the temperature of the mixture.
Observations
The volume of hydrochloric acid = volume of sodium hydroxide = 100 ml = 100gm
The temperature of hydrochloric acid = temperature of sodium hydroxide = t4°C.
The temperature of hydrochloric acid + temperature of sodium hydroxide = t5°C.
Results
The enthalpy of neutralisation for a strong acid and strong base is 57.1KJ/mol.
Precautions
Certain precautions must be followed while experimenting. These precautions are mentioned below:
The temperature of the acid and base should be the same
All the temperatures should be noted carefully
Insulate the beaker
Lab Manual Questions
Q1 Define enthalpy of neutralisation.
Ans: It is defined as the change in enthalpy on mixing 1 gram equivalent of acid with 1 gram of base.
Q2 Define enthalpy of reaction.
Ans: It is the difference in the product's and reactant's heat at constant temperature and pressure.
Q3 What is heat capacity?
Ans: Heat capacity is required to raise the temperature by 1°C.
Viva Questions
Q1 What is the significance of thermochemistry?
Ans: Thermochemistry is a critical field because it helps us to know whether a particular reaction will occur or not; it also helps us to know whether the reaction is endothermic or exothermic.
Q2 What do you mean by the enthalpy of the solution?
Ans: Enthalpy change on dissolving 1 mole of a substance above solvent at constant temperature and pressure.
Q3 Define one calorie.
Ans: One calorie is the heat required to raise the temperature of 1 gram of water by 1°C.
Q4 Define Hess law.
Ans: Hess law is defined as the energy changes in the overall reaction as the sum of energy changes of individual reactions.
Q5 What are the applications of Hess law?
Ans: It is used to calculate heat change of phase transition and also used to calculate the lattice energy of ionic compounds.
Q6 How do you calculate delta h for process ch3cooh?
Ans: delta h for the process ch3cooh is calculated by the following method. By combusting acetic acid with oxygen, we got the energy of carbon dioxide and water, after subtracting the product's energy from the reactant.
Q7 How will you define the path function?
Ans: Path functions are those of thermochemistry that depends on the path of a reaction. These are working, heat.
Q8 Is temperature a path function?
Ans: Temperature is not a path function but a state function.
Practical Based Questions
Q1 Total energy of a system is
P.E + K.E
K.E+ heat energy
Both of the above
None of the above
Ans: P.E+K.E
Q2 Thing which depends on the initial and final state of a system is
State function
Path function
Both of the above
None of the above
Ans: 1. State function
Q3 Which of the following is not a state function
Internal energy
Gibbs energy
Enthalpy
Work
Ans: 4. Work
Q4 If the internal energy of the system is increased
A chemical reaction may take place
The temperature of the system may rise
Change in the state te of the system is increased
All of the above
Ans: 4. All of the above
Q5 Which of the following is correct for the spontaneous endothermic process?
H<0
G<0
S<0
G>0
Ans: 2. G<0
Q6 A reaction has H and S both positive. The reaction will be
Spontaneous
Spontaneous and temperature dependent
Has an increasing free energy
Nonspontaneous
Ans: 4. Nonspontaneous
Q7 The environment in which a system is studied
State function
Phase
Surrounding
State
Ans: 3. Surrounding
Q8 Heat of neutralisation is minimum for which reaction?
Sodium hydroxide + sulphuric acid
Ammonium hydroxide + acetic acid
Sodium hydroxide + hydrochloric acid
Sodium hydroxide + acetic acid
Ans: 2. Ammonium hydroxide + acetic acid
Conclusion
In this article, we studied thermochemistry and learned how to determine the heat of neutralising a strong acid and a strong base.
We have also learned that the enthalpy of neutralisation for a strong acid and strong base is highest at 57.1 KJ/mol.
For weak acids and weak bases, the enthalpy of neutralisation is always less than 57.1 KJ/mol.
FAQs on Class 12 Chemistry Thermochemistry Viva Questions With Answers
1. What are the most frequently asked questions from Thermochemistry for the CBSE Class 12 board exams 2025-26?
For the Class 12 board exam, the most important questions from Thermochemistry typically revolve around numerical problems on Hess's Law of Constant Heat Summation, calculating bond enthalpy, and defining different types of enthalpies. Be prepared for questions on the enthalpy of formation, combustion, and atomization.
2. How should I structure my answer for a 3-mark question on Hess's Law to get full marks?
To secure full marks on a question about Hess's Law, you should present your answer clearly in steps.
- Begin by stating the law itself.
- Write down the target chemical equation you need to derive.
- Show how you are manipulating the given thermochemical equations (e.g., reversing or multiplying) to reach the target equation.
- Finally, calculate the total enthalpy change, making sure to include the correct positive (+) or negative (-) sign.
3. What is a common mistake students make when solving numericals on enthalpy of combustion versus enthalpy of formation?
A very common error is confusing the standard definitions. The enthalpy of formation (ΔfH°) refers to the formation of one mole of a compound from its constituent elements. In contrast, the enthalpy of combustion (ΔcH°) is the heat released when one mole of a substance is completely burned. Mixing these definitions, especially the 'per mole' basis, often leads to incorrect calculations and lost marks.
4. Which formulas from Thermochemistry are most critical for solving numerical problems in the exam?
For your board exam preparation, you must master the application of these key formulas:
- Enthalpy of reaction from enthalpies of formation: ΔrH° = ΣΔfH°(products) - ΣΔfH°(reactants).
- Enthalpy of reaction from bond enthalpies: ΔrH° = Σ(Bond enthalpies of reactants) - Σ(Bond enthalpies of products).
- Hess's Law calculations: By adding or subtracting the ΔH of given reactions.
- Calorimetry equation: q = mcΔT.
5. Why are questions related to bond enthalpy considered important for board exams?
Questions on bond enthalpy are important because they test your fundamental understanding of how energy relates to chemical bonds. They check if you know that breaking chemical bonds requires energy (an endothermic process) and forming new bonds releases energy (an exothermic process). This concept is crucial for predicting the overall energy change of a chemical reaction.
6. What kind of 1-mark or MCQ questions can be expected from basic concepts like extensive and intensive properties?
For 1-mark questions, expect direct identification or definition-based questions. For instance, you might be asked to identify the intensive property from a list like Mass, Volume, Enthalpy, and Density. You may also be asked to determine if a reaction is endothermic or exothermic based on the sign of its given ΔH value.
7. For a 5-mark question from this chapter, what is the most likely combination of topics to expect?
A 5-mark question from Thermochemistry is usually a comprehensive numerical problem. It will likely require you to use Hess's Law as the main principle. The problem might provide data for enthalpies of formation or combustion for several substances and ask you to calculate the enthalpy for a completely new reaction. These questions often have a small 1-mark definition or conceptual part attached.
