THERMOCHEMISTRY
Q.01: Choose the correct answer:
(i) If an endothermic reaction is allowed to take place very rapidly in the air, the temperature of the surrounding air:
(a) Remains constant
(b) Increases
(c) Decreases
(d) Remains unchanged
ANSWER: (c)Decreases
EXPLANATION: The endothermic reactions proceed by absorbing heat into the system from the surrounding. So, the temperature of surrounding air decreases.
(ii) In endothermic reaction, the heat content of the:
(a) Products is more than that of reactants
(b) Reactants is more than that of products
(c) Both ‘a’ & ‘b’
(d) Reactants and products are equal
ANSWER: (a) Products is more than that of reactants
EXPLANATION: The endothermic reactions proceed by absorbing heat into the system from surrounding. So, the heat content of the products increases as compared to reactants. It means in endothermic reactions, the products are at high energy state than reactants.
(iii) Calorie is equivalent to:
(a) 0.4184 J
(b) 41.84 J
(c) 4.184 J
(d) 418.4 J
ANSWER: (c) 4.184 J
EXPLANATION: Calories and Joules are two different units used for measuring heat energy changes. By comparing each other, 1 calorie is 4.184 times bigger than 1 Joule. Thus, 1 cal. = 4.184 J
(iv) The change in heat energy of a chemical reaction at constant temperature and pressure is called:
(a) Enthalpy change
(b) Heat of sublimation
(c) Bond energy
(d) Internal energy change
ANSWER: (a) Enthalpy change
EXPLANATION: For a reaction occurring at constant temperature and pressure, the heat energy change is more suitably referred as enthalpy change. This is because enthalpy is more comprehensive term as compared to energy change because the reactions occurring at constant pressure involve volume changes which is a form of pressure volume work, and the enthalpy represents both energy changes and the pressure volume work changes. i.e., DH=DE+PDV
(v) Which of the following statements is contrary to the first law of thermodynamics?
(a) Energy can neither be created nor destroyed.
(b) One form of energy can be transferred into an equivalent amount of other kinds of energy.
(c) In an adiabatic process, the work done is independent of its path.
(d) Continuous production of mechanical work without supplying an equivalent amount of heat is possible.
ANSWER: (d) Continuous production of mechanical work without supplying an equivalent amount of heat is possible.
EXPLANATION: The production of any form of work or energy without providing any input or in other words constructing a perpetual motion machine, is impossible because it is against the law of conservation of energy which says that energy can neither be created nor destroyed, though it may be changed from one form into another form.
(vi) For a given process, the heat changes at constant pressure (qp) and at constant volume (qv) are related to each other as:
(a) qp=qv
(b) qp<qv
(c) qp >qv
(d) qp=qv/2
ANSWER: (c) qp >qv
EXPLANATION: It means heat supplied at constant pressure is greater than the heat supplied at constant volume. This is because at constant pressure, a part of the heat provided (q) is used in volume expansion or doing pressure volume work. Thus, the total heat change will be the sum of energy change and pressure volume work.
(vii) For the reaction: NaOH + HCl → NaCl + H2O the change in enthalpy is called:
(a) Heat of reaction
(b) Heat of formation
(c) Heat of neutralization
(d) Heat of combustion
ANSWER: (c) Heat of neutralization
EXPLANATION: The reaction between an acid and a base to give salt and water is called ‘neutralization’, and the heat released when certain number of moles of an acid react with certain number of moles of a base to give one mole of H2O is termed as ‘enthalpy of neutralization’.
(viii) The net heat change in a chemical reaction is same, whether it is brought about in two or more different ways in one or several steps. It is known as:
(a) Henry’s law
(b) Joule’s principle
(c) Hess’s law
(d) Law of conversation of energy
ANSWER: (c) Hess’s law
EXPLANATION: The enthalpy change in a reaction depends only upon the initial and final states of the system. It is independent of the path followed by the system to bring about that change. This is called Hess’s law of heat summation or second law of thermochemistry.
(ix) Enthalpy of neutralization of all the strong acids and strong bases has the same value because:
(a) Neutralization leads to the formation of salt and water.
(b) Strong acids and bases are ionic substances.
(c) Acids always give rise to H+ ions and bases always furnish OH– ions.
(d) The net chemical change involves the combination of H+ and OH– to form water.
ANSWER: (d) The net chemical change involves the combination of H+ and OH– to form water.
EXPLANATION: This is because the anions and cations coming from strong acids and strong bases do not react but remains ionized in the solution. The only reaction taking place is between H+ ions and OH– ions which combine to form H2O. So, the heat released is due to water formation only which is the same in case of all strong acids and strong bases. That’s why, the enthalpy of neutralization of all strong acids and bases is the same i.e., 57.4 kJmole1-.
Q.02: Fill in the blanks with suitable words.
(i) The change in internal energy __________ be measured. (cannot)
(ii) Solids which have more than one crystalline forms possess _________ values of heats of formation. (more than one)
(iii) A process is called __________ if it takes place on its own without any external assistance. (spontaneous)
(iv) A __________ is a macroscopic property of a system which is __________ of the path adopted to bring about that change. (sate function, independent)
Q.03: Indicate the true or false as the case may be.
(i) It is necessary that a spontaneous reaction should be exothermic. (FALSE)
EXPLANATION: No, it is not necessary that a spontaneous reaction must always be exothermic. There are certain reactions which are endothermic as well as spontaneous. For example, the reaction between atmospheric nitrogen and oxygen during thunder bolts and lightning to form nitric oxide (NO) is an endothermic reaction that takes place spontaneously.
(ii) Amount of heat absorbed at constant volume is internal energy change. (TRUE)
(iii) The work done by the system is given the positive sign. (FALSE)
EXPLANATION: The work done by the system on the surrounding is given negative sign because it causes to decrease the internal energy of the system.
(iv) Enthalpy is a state function but internal energy is not. (FALSE)
EXPLANATION: Both enthalpy and internal energy are state functions because both are macroscopic properties of the system that depend upon the initial and final states of the system and not the path followed by the system during that change.
(v) Total heat content of a system is called enthalpy of the system. (FALSE)
EXPLANATION: Total heat content of a system measured at constant pressure will be called enthalpy.