11th Class Chemistry Chapter 7 Thermochemistry Short Question Answers

Chemistry short QA

11th Class Chemistry Chapter 7 Thermochemistry Short Question Answers Below

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1. How the temperature of the system changes during exothermic and endothermic reaction?
In an exothermic reaction. Heat is beloved which is increases the temperature of the system . In an endothermic reaction, heat is absorbed, so the temperature of the system falls down. These statements are true when the system is isolated.
2. What is the spontaneous and a non-spontaneous process?
That process which takes place on its own is called spontaneous process, No external assistance is required. It moves from non-equilibrium state towards the equilibrium state. It is unidirectional and irreversible. A non- spontaneous process needs an external help to take place.
3. What is state and state function?
The conditions of a system is called the state of the system. The initial state is a description before a change happens. Final state is a description after a change take place. State function is the microscope property of a system, which has some definite values for initial and final states and is independent of a path which is adopted to bring out the change.
4. Define ΔHr0 (enthalpy of reaction). Can it be negative? How?
ΔHr0is the enthalpy changed under standard conditions when the reactions are converted into products. It can be negative, if the enthalpy of the products is less than the enthalpy of the reactions. This can be assessed by studying the nature of bonds which are being broken and those which are being formed.
5. Define the following terms: (a) Enthalpy (b) Endothermic reaction.
Enthalpy is the total heat content of the system. It cannot be measured. We can only measure the enthalpy change of the system. Those chemical reactions whose enthalpy change is positive and they absorb the heat during the progress of the reaction are called endothermic reactions.
6. What is the physical significance of equation, ΔH = qp?
ΔH is the enthalpy change and qp is amount of heat supplied at constant pressure. This equation shows that the expansion of the gas takes place at constant pressure and work is done by the system.    
7. What is the comparison of ΔH. Or state why ΔH= ΔE in a case of liquids and solids?
ΔH is the energy change when the pressure is constant and ΔE is the energy change when volume is constant. So ΔE can be measured for gases, liquids and solids by keeping volume constant in gases. The volume of liquids and solids are already constant. Pressure has no effect on the liquids and solids so the value of ΔH is same for liquids and solids.
8. The total energy of a system is the sum of translation, rotational and vibration motions. Justify it?
The substance is consisted of atoms, ions and molecules. If a system is in liquid or gaseous state, then its constituent particles have translational motions along three axis x, y and z in space. They need energy for this, For rotational motions along three axis they also need energy. Various types of vibration motions of the bonds make the bonds to compress and stretch. They also demand energy. So the total energy of a system is a sum of all these three types of energies.
9. State first law of thermodynamics and give its mathematical form?
According to the first law of thermodynamics, energy can neither be created nor destroyed. If a gas is enclosed in a piston cylinder arrangement and it is heated by giving some calories of energy, then the piston can be pushed to the backward direction. So some of the heat is converted into internal energy and other is used up to do the work. So
dq   =  dE + dW
Since   dw = – ve
so        dq  = dE- dw
This equation gives us quantitative relationship of heat and work.
10. How do we determine the ΔH in the laboratory for food, fuel etc?
For this purpose, bomb calorimeter is used. The substance under test is ignited electrically in the atmosphere of oxygen. The amount of heat produced is calculated from the mass of the substance, specific heat of the calorimeter and change of temperature.
11. How will you differentiate between ΔE and ΔH? Is it true that ΔH and ΔE have the same values for the reactions taking place in the solution state?
ΔH is the heat change at constant pressure, while ΔE is the heat change at constant volume. In the solution state the pressure is constant. So, there is no difference in ΔE and ΔH in such situation. There is no change in volume when the reaction happens in liquid or solid state.
PΔV    =    0
ΔH      ΔE+ PΔV
ΔH = ΔE +p *0
12. Specific heat of a substance depends upon the nature of substance. Comment?
Specific heat is the amount of a heat required to raise the temperature of 1 g of substance by 1 Kelvin. The specific heat of H2O is 4.2j g-1 K-1, while for ethanol it is 2.4 j g-1 K-1. Actually, it is the capability of the substance to process certain amount of energy. This capability depends upon the structure, bonds angles and vibration frequencies of the bonds ect.
13. How the lattice energy of the ionic compound can be measured by Born- Haber cycle?
We first of all determine heat of formation of compound in laboratory. The heat of sublimation, heat if ionization, amount of electron affinity, bond dissociation energy, are consulted from the book. The sum of all three energies and lattice energy of the substance are equalized with heat of formation.
ΔH0f  = ΔH0s + ΔH01 + ΔH0D/2 + ΔH0E + ΔH/
14. How the amount of lattice energy of an ionic compound depends upon the charge densities of the ions?
Lattice energy is the quantitative measurement of forces of attraction among the ions. Greater the amount of charge on positively change ion, greater the forces of attractions and greater is the lattice energy. Greater the size of negative ion, smaller the charge density and smaller the lattice energy with a definite positive ion.
15. Define lattice energy and give examples?
The energy required to completely separate a mole of a solid ionic compound into its gaseous ions is called lattice energy. The lattice energy of NaCl is -787 kJ mol-1.
16. What is heat of atomization?
It is the amount of heat required to break all the bonds in one mole of a compound to get the atoms.
17. Burning of a candle is a spontaneous process. Justify?
Candle needs the required temperature from outside. Then the heat evolved due to burning makes the reaction spontaneous.
18. It is true that a non- spontaneous process never happens in the universe?
ΔG is the free energy change of a reaction and ΔG = ΔH – TΔS. In a spontaneous process, ΔG is negative. To make it negative, the values and sings of ΔH and ΔS decide. ΔG is positive for it. Sometimes we have to supply energy from outside to make them happen.
19. Describe Hess’s Law of constant heat summation?
The energy change in a cyclic process is zero. The heat change of a reaction does not depend upon the path taken. The conversion of carbon and oxygen into CO2 completes through a single steps. The value of ΔH i.e. – 393.7 k.j mol-1 remains the same.
20. Enthalpy of neutralization of a strong acid and a base is always - 57.5 kcal mol-1. Why?

A strong acid and a strong base are completely dissociated in water to give H+ and OH. There ions combine to give H2O when they are mixed.

  1. Prove that qp = ΔH.
    Ans. Since q  = ΔE + W
    W  = PΔV
    qp = ΔE + PΔV
    qp  = (E2 – E1) + P (V2 – V1)
    qp = (E2 + PV2) – (E1 + PV1)
    qp = H2 – H1
    qp = ΔH

22. How do you compare law of conservation of energy and Hess’s law of heat summation?
Law of conservation of energy is first law of thermodynamics. Energy can neither be created nor destroyed in a reaction. Hess’s law is one of the best applications of first law of thermodynamic.
23. Why it is necessary to maintain the physical states of reactants and products in a chemical reaction?
When we write down a chemical reaction and Δt value is also to be written then it is necessary to maintain whether the substances are in gases, liquids or solids at the same time of reaction. The reason is that energy changes do take place when phases change.

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