Question

Calculate AU of a gas for a process in which the gas absorbs 29 J of heat and does 31 J of work by expanding A. 21 B. 60) C. -60J D. -2J Which of the following statements is not true for an exothermic reaction? A. Heat passes from the reaction system to the surroundings. B. The products have a higher heat content than the reactants. C. The temperature of the reaction system increases D. The enthalpy change for the reaction is negative Under conditions of constant pressure, for which of the following reactions is the magnitude of pressure-volume work going to be smallest? A. 2NO(g) + O2(g) → 2NO2(g) B. 2H2O2(1)→ 2H2O(l) + O2(g) C. 2KCIO3s + 2KCI(S) + 302(g) D. H2(g) + Cl2(g) 2HCl(8) Which of the following statements about enthalpy is false? A. Enthalpy is a state function. B. At constant pressure, the enthalpy change is equal to the heat absorbed or released. C. Enthalpy is an extensive property. D. The change in enthalpy of a process cannot be negative. Which of the following statements is incorrect concerning the thermochemical equation below? 2503(g) → 2802(g) + O2(g); AH° = 198 kJ The enthalpy of the reactants exceeds that of the products. 3. The reaction is endothermic. - For the reaction 2502(g) + O2(g) (g) → 2503(g); AH = -198 kl. . The external pressure is 1 atm. Based on the photoelectric effect. Einstein proposed the idea that m. the wavelength of light is inversely proportional to its frequency

Answer 1

Answer :

Q1.

According to the 1^st law of Thermodynamics -

$\Delta$U = Q + w

Where,

$\Delta$U = change in internal energy

Q = heat involved during a process. According to IUPAC convention Q is negative if heat is released by the system, and positive if heat is absorbed by the system.

w = work involved during a process. According to IUPAC convention w is negative if work is done by the system and positive if work is done on the system.

Now, it is given that, during a process the gas absorbe 28 J of heat and does 31 J of work by expansion. So heat is absorbe by the gas and work is done by the gas. So ​​​​​​, Q = 29 J ; w = - 31 J

So from 1st law of Thermodynamics,

$\Delta$U = Q + w

Or, $\Delta$ U = 29 J + (-31 J)

Or,  $\Delta$U = - 2 J

So, correct answer is option (D).

Q2.

An exothermic reaction is a type of reaction in which heat is released by the reaction system, during the course of the reaction, into the surrounding .

This is because of the fact that, in such type of reactions, the heat content of the reactant is higher than that of the product. Because of which when reactant converted into product, the extra energy of the reactant is released from the system as heat energy and the system became hot i.e. temperature of the sestem increases.

Change in enthalpy ($\Delta$H)  of such type of reaction is,

$\Delta$H = H_P- H_R

Where R stands for Reactant, P stands for Product.

As, H_R > H_p so $\Delta$ H = (-) ve

So between (A), (B), (C) amd (D), the statement which is not True for an Exothermic reaction is (B).

So, correct answer is option (B)

Q4.

Change in Enthalpy of a system is depends only on the initial and the final state of the syste, but not on the path of the process. That is why enthalpy is a state function.

We know that,

H = U + PV

Where, H = enthalpy, U = internal energy, P = pressure, V = volume.

Therefore, by differentiation,

dH = dU + PdV + VdP

Or, dH = dU + PdV

[ at constant pressed, dP = 0]

Or, dH = (dq - PdV) + PdV  

[ As dU = dq +dw

and dw = - PdV

So, dU = dq - PdV ]

Or, dH = dq

i.e dH = (dq)_p

[ (dq)_p = heat involved at constant pressure condition ]

Therefore heat involved ( release or absorb) in a process at constant Pressure condition is the measure of the enthalpy change of the process.

Enthalpy of a system depend on the amount of the system. Higher the amount of the system, higher will be the enthalpy of the system. That is why enthalpy is an extensive property.

The change in enthalpy of the system can be negative or positive, depending upon the nature of the process. As for example enthalpy change for a exothermic reaction is negative but enthalpy change for an endothermic reaction is positive.

So the false statement among (A), (B), (C) and (D) is (D)

So, correct answer is option (D)

Q5.

For the reaction,

2SO₃(g) $\rightarrow$ 2SO₂(g) + O₂(g)

It is given that ,  $\Delta$H^° = 198 kJ

As $\Delta$ H^° is positive, so the reaction is an endothermic reaction.

We know that $\Delta$ H^°= H_P^° - H_R^° ,   as  $\Delta$H^° is positive so H_P^°is greater than HR^°. That means enthalpy of the product (H_P​​​​​​) exceed that of the reactant (H_R​​​​​​) , but not the opposite.

We know sign of enthalpy of forward and reverse reaction is opposite. So for the reverse reaction,

2SO₂(g) + O₂(g) $\rightarrow$ 2SO₃(g)

$\Delta$H^° = - 198 kJ

The reactant and the products are in standard state ( as there is a ° as superscript). So the external pressure and temperature is 1 bar and 298 K respectively.

So the incorrect statement is (A)  

So, correct answer is option (A)

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