9.5.3 Enthalpy
This is the third lecture from Chapter 5: “Energetics” in the new Class 9 Chemistry book (Punjab Board – PCTB). It discusses the concepts of enthalpy and difference between enthalpy and heat. The lecture also includes a multiple-choice quiz, short question and long question notes.
MCQs Based Quiz
Short Questions
Q1. What is meant by heat content of a molecule?
The total amount of heat energy present in a molecule under standard conditions (0°C temperature and 760mmHg pressure) is called the heat content of a molecule. Its unit is $\mathrm{kJmol^{-1}}$.
Q2. How can you define the enthalpy of a system?
The total heat content of a system is called its enthalpy. It is represented by (H) and the change in the enthalpy of the system is represented by ΔH. Its unit is $\mathrm{kJmol^{-1}}$.
Q3. How can you measure the enthalpy (H) of a system?
The enthalpy (H) of a system cannot be measured directly. Instead, we can only study the change in enthalpy (ΔH) of a system.
Q4. What is standard enthalpy of a reaction (ΔH°)?
The enthalpy change when reactants react to produce products under standard conditions (0°C temperature and 760mmHg pressure) is called standard enthalpy of a reaction (ΔH°).
$$\mathrm{2\,CO_{(g)} + O_{2(g)} \longrightarrow 2\,CO_{2(g)}} \quad \boxed{\Delta H^\circ = -566.0\,\text{kJ}}$$ kjmol-1
Q5. What is meant by heat of reaction at constant pressure?
The enthalpy change when reactants react to produce products under standard conditions (0°C temperature and 760mmHg pressure) is called standard enthalpy of a reaction (ΔH°).
$$\mathrm{2\,CO_{(g)} + O_{2(g)} \longrightarrow 2\,CO_{2(g)}} \quad \boxed{\Delta H^\circ = -566.0\,\text{kJ}}$$
Q6. Give an example of exothermic reaction.
$$\mathrm{2\,CO_{(g)} + O_{2(g)} \longrightarrow 2\,CO_{2(g)}} \quad \boxed{\Delta H^\circ = -566.0\,\text{kJ}}$$
Q7. How would you define heat?
Heat is a form of energy that flows from a hot system to a cold system due to difference in temperature. It is measured in joules (J). It is also called transfer of thermal energy.
Q8. On what does the enthalpy depend on?
Enthalpy depends on:
- Number of particles (atoms, ions, molecules etc.) in a system
- Chemical composition of the particles
- Structure of the particles
Q9. What is the relationship between heat and enthalpy at a constant pressure?
When heat leaves or enters a system, it causes the enthalpy change. At constant pressure, the enthalpy change is equal to heat evolved or absorbed by the system.
Q10. Can energy be transferred in a form other than heat during a chemical reaction?
Yes, during a chemical reaction, energy can be transferred in forms other than heat. For example, a firework can release energy in the form of light and sound, in addition to heat.
Q11. Why is it not possible to calculate the enthalpy of a system?
This is because, when measuring physical quantities like temperature, we need a starting point of zero. But some quantities, like enthalpy, do not have a zero point. This is why we cannot directly measure the enthalpy of a system. We can only measure the change in it.
Descriptive Questions
Q1. Write a detailed note on enthalpy.
Enthalpy:
The total amount of thermal energy stored in a system is called the enthalpy of that system. Enthalpy is represented by letter ‘H’ and its unit is kJ mol-1.
It is the measurement of energy in a thermodynamic system and the quantity of enthalpy is equal to the heat content of a system.
The total amount of heat energy present in a molecule or system under standard conditions (0°C and 760 mmHg) is called its heat content.
Change in Enthalpy (ΔH):
Enthalpy (H) is an essential part of the system, but we cannot directly measure it. However, change in the enthalpy of the system (ΔH) can be measured relatively easily.
Relationship Between Energy and Enthalpy:
- Positive Enthalpy (+): In an endothermic change, the system absorbs (gains) energy or heat from the surroundings. This results in an increase in the total enthalpy of the system.
- Negative Enthalpy (-): In an exothermic change, the system releases (loses) energy or heat to the surroundings. This results in a decrease in the total enthalpy of the system.
Standard Enthalpy of a Reaction (ΔH°)
The enthalpy change when reactants react to produce products under standard conditions (0°C temperature and 760mmHg pressure) is called standard enthalpy of a reaction (ΔH°).
$$\mathrm{2\,CO + O_2 \rightarrow 2\,CO_2} \quad \boxed{\Delta H^\circ = -566.0\,\text{kJ}}$$
Standard enthalpy of a reaction (ΔH°) can also be called heat of reaction at constant pressure.
Importance of Standard Enthalpy of a Reaction (ΔH°):
The standard enthalpy of a reaction can be used to determine whether a reaction is endothermic or exothermic.
- Exothermic reactions have negative (-) value of ΔH°.
- Endothermic reactions have positive (+) value of ΔH°.
Q2. Explain the differences between heat and enthalpy.
Heat
Enthalpy
Heat it a form of energy that flows from a hot system to a cold system due to difference in temperature.
The total amount of thermal energy stored in a system is called the enthalpy of that system.
Heat is represented by ‘q’.
Enthalpy is represented by ‘H’.
It is not an essential part of the system.
It is an essential part of the system.
A system can gain or lose heat.
Enthalpy change is equal to change in heat at constant pressure.
It is measured in joules (J).
It is measured in kJ mol-1.