An Overview of Class 12 Chemistry Thermochemistry Experiment
Introduction
Is the heat gain by a cold body always equal to heat loss by a hot body? Yes, it is true. Heat gain by a cold body always equals heat loss by a hot body. It is the principle of the calorimeter. There is no energy loss. Energy is always conserved; it can only be converted from one form to another. Do you know what heat capacity is and how it can be calculated?
To know more about calorimeters and water equivalents, continue reading this article!
Table of Content
Aim
Apparatus Required
Theory
Procedure
Observations
Results
Precautions
Lab Manual
Viva Questions
Practical Based Questions
Aim
To determine the water equivalent of a calorimeter.
Apparatus Required
Calorimeter
Thermometer
Distilled water
250ml Beaker
Cotton
Hot plate
Measuring jar
Wooden box with lid
Stirrer
Theory
The water equivalent of a calorimeter is defined as the amount (mass) of water that will absorb or lose the same quantity of heat as any other given substance for the exact change in temperature. It is necessary to measure the water equivalent for correct calculation. Because during the heat exchange calorimeter also takes or loses some heat or energy. There are different types of calorimeters - adiabatic calorimeter, reaction calorimeter, constant pressure calorimeter, and differential scanning calorimeter.
Procedure
Take 50 ml of water in a calorimeter using a measuring jar
Then measure the temperature of the water using a thermometer in a calorimeter. Let's assume the temperature of the water is t1°C.
The calorimeter is placed in a wooden box, and cotton is filled in the space between the calorimeter to avoid heat loss.
Now a stirrer is inserted into the calorimeter.
Pour 50 ml of distilled water is in the beaker, then heat the water
Stop heating, and note the temperature of the water. Lets the temperature of the water be t2
Add warm water to the calorimeter
Now stir the water and note the reading
Continue stirring water using a stirrer until its temperature becomes constant. Let the temperature be t3.
Observations
The volume of water in the calorimeter is 50 ml
The temperature of the water is t1
The volume of water added is 50ml
The mass of water is 50g
The temperature of warm water is t2
Temperature after mixing is t3
By using the formula
Heat given by hot water = heat taken by cold water
We can calculate the water equivalent of a calorimeter by the above formula.
Results
The water equivalent of the calorimeter can be calculated by the above formula.
Precautions
Several precautions need to be followed while experimenting. These precautions are mentioned below.
The final temperature should be noted only after stirring the solution well.
All the temperatures should be noted carefully.
Insulate the beaker thermally as perfectly as possible.
Lab Manual Questions
Q1 What do you mean by water equivalent?
Ans: Water equivalent is the amount of heat required to increase the temperature of any substance by one degree.
Q2 What is the principle of finding the water equivalent?
Ans: Heat gain is equal to the heat loss principle used to find the water equivalent of the calorimeter.
Q3 Define the enthalpy of a reaction.
Ans: The enthalpy of a reaction is defined as the difference in the heat content of the reactant and product of a chemical reaction at constant pressure.
Viva Questions
Q1 What is thermochemistry?
Ans: Thermochemistry is the study of energy changes in a chemical reaction.
Q2 What is an exothermic reaction
Ans: Exothermic reactions are those reactions that release heat or energy.
Q3 What do you mean by an enthalpy of the solution?
Ans: Enthalpy of a solution is the change in energy when a one-mole substance is dissolved in a solvent at constant temperature and pressure.
Q4 Define heat capacity?
Ans: Heat capacity is the heat required to raise the temperature by one-degree celsius.
Q5 What is the first law of thermodynamics?
Ans: The first law of thermodynamics defines energy that cannot be created or destroyed. It can be converted only from one form to another form.
Q6 What is the principle used for determining the heat of neutralisation?
Ans: Hea gain = Heat loss is the principle used for determining the heat of neutralization.
Q7 What is the principle of the calorimeter?
Ans: Heat gain by a cold body always equals heat loss by a hot body.
Q8 What are thermochemical equations?
Ans: Thermochemical equations are balanced chemical equations with their physical states.
Practical Based Questions
Q1 What is the most expected error in determining the heat of neutralisation?
Temperature error
Systematic error
Both of the above
None of the above
Ans: 1. Temperature error
Q2 What is the value of enthalpy of neutralisation of strong acid and a strong base?
62.4 kj/mol
50 kj/mol
57.1 kj/mol
None of the above
Ans: 57.1 kj/mol
Q3 In endothermic reactions, heat is
Absorbed
Released
Both of the above
None of the above
Ans: 1. Absorbed
Q4 What type do deviation is observed in the acetone and chloroform mixture?
Positive deviation
Negative deviation
Both of the above
None of the above
Ans: 2. Negative deviation.
Q5 Heat evolved or absorbed on mixing chloroform and acetone
Evolve
Absorbe
Remain same
None of the above
Ans: 1. Evolve
Q6 In which of the following reactions will the product be stable?
Endothermic reactions
Exothermic reaction
Both of the above
None of the above
Ans: 1. Exothermic reaction
Q7 For which of the following substances is the heat of formation in the standard state 0?
Carbon dioxide
C (diamond)
Ozone
C (graphite)
Ans: 4. C (graphite)
Q8 If the change of enthalpy is negative, what will be the reaction?
Endothermic reactions
Exothermic reaction
Maybe exothermic or endothermic reactions
None of the above
Ans: 2. Exothermic reaction.
Conclusion
In this article, we have discussed calorimeter and its principle
We have also discussed how to measure the water equivalent of a calorimeter.
Calorimeter, thermometer, beaker, and distilled water are some of the articles required for this experiment.
Certain precautions, such as proper temperature measurement, must be followed while experimenting.
FAQs on Class 12 Chemistry Thermochemistry Experiment
1. What are the most frequently asked types of questions from Thermochemistry for the Class 12 board exams?
For the CBSE Class 12 Chemistry exam, important questions from Thermochemistry typically focus on a few key areas. You can expect:
- Numerical problems based on Hess's Law of Constant Heat Summation.
- Calculations involving bond enthalpies, enthalpy of formation, and enthalpy of combustion.
- Definitions of key terms like standard enthalpy of formation, lattice enthalpy, and atomization for 1-mark questions.
- Questions asking you to differentiate between exothermic and endothermic reactions with examples.
2. Why is it crucial to mention the physical states (s, l, g) of substances in a thermochemical equation?
Mentioning physical states is essential because the enthalpy change (ΔH) of a reaction directly depends on the state of the reactants and products. For example, the energy required to convert liquid water to gaseous water is significant. Not specifying the state can lead to an incorrect value for the reaction's enthalpy, making the entire calculation wrong in an exam.
3. How are questions on Hess's Law typically framed in the Class 12 Chemistry board exam?
In the board exam, questions on Hess's Law are almost always 3-mark numerical problems. You will typically be given two or three thermochemical equations with their ΔH values and asked to calculate the enthalpy for a different, target reaction. To solve this, you must manipulate the given equations (by reversing or multiplying them) to derive the target equation.
4. What is a common mistake students make when solving problems related to bond enthalpy?
A very common error is confusing the formula for reaction enthalpy calculation. Remember, for bond enthalpies, the formula is ΔrH° = Σ (Bond enthalpies of reactants) - Σ (Bond enthalpies of products). Students often incorrectly subtract reactants from products, which is the method used for enthalpy of formation. Always break down all bonds in reactants and form all bonds in products.
5. How much weightage does the Thermodynamics unit carry in the CBSE Class 12 Chemistry exam for 2025-26?
As per the CBSE 2025-26 syllabus, the unit on Thermodynamics, which includes all the concepts of Thermochemistry, is quite important. While the exact marks can vary, you can typically expect questions worth around 4 to 6 marks from this unit, which often includes a mandatory numerical problem.
6. Why is the standard enthalpy of formation for an element like O₂(g) or C(graphite) considered zero?
The standard enthalpy of formation (ΔfH°) is the heat change when one mole of a substance is formed from its constituent elements in their most stable states (also known as their reference states). Since an element like O₂ gas is already in its most stable natural form, no energy change is required to 'form' it from itself. By convention, its formation enthalpy is set to zero to provide a universal baseline for calculating the enthalpies of compounds.
7. Besides Hess's Law, what other numerical problems are important from this chapter?
Besides Hess's Law, you should definitely practice numericals based on calculating the enthalpy of a reaction (ΔrH°) using standard enthalpies of formation (ΔfH°). The key formula to use here is ΔrH° = Σ (ΔfH° of products) - Σ (ΔfH° of reactants). These are also frequently asked as 2 or 3-mark questions in board exams.
