Chemical Thermodynamics Quiz-4

CHEMICAL THERMODYNAMICS QUIZ-4

Dear Readers,

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Q1. Calorific value of carbohydrates is approximately:

•  4.0 kcal/g
•  16.0 kcal/g
•  20 kcal/g
•  9.0 kcal/g

Solution
a) Fuel value per g of substance produced on burning

Q2.Heat given to a system under isochoric process is equal to:

•  W
•  q_p
•  ∆U
•  ∆H

Solution
c) For an isochoric process V= constant ; thus ∆V=0 from 1st law q=∆U-W or q_v=∆U

Q3.  An engine operating between 150℃ and 25℃ takes 500 J heat from a higher temperature reservoir
if there are no frictional losses, then work done by engine is

•   165.85 J
•  169.95 J
•  157.75 J
•  147.7 J

Solution
d) T_2=150+273=423 K, T_1=25+273=298 K Q=500 K W/Q=(T₂-T₁)/T₂ ∴W=500 ((423-298)/423)=147.7 J

Q4. The work done during the expansion of a gas from a volume of 4 dm³ to 6 dm³ against
a constant external pressure of 3 atm is

•  -6 J
•  -608 J
•  +304 J
•  -304 J

Solution
b) Work done(W)=-p_ext (V₂-V₁) =-3×(6-4)=-6 L atm = -6×101.32 J(∴ 1 L atm=101.32 J) = -607.92≈-608 J

Q5.If the enthalpy change for the transition of liquid water to steam is 30 kJ mol^-1 at 27°C,
the entropy change for the process would be:

•  100 J mol^-1K^-1
•  10J mol^-1K^-1
•  1.0J mol^-1K^-1
•  0.1J mol^-1K^-1

Solution
 a) ∆S=∆H/T=(30×10³)/300=100 J mol^-1 K^-1

Q6. The ∆G in the process of melting of ice at -15℃ is:

•  ∆G=-ve
•  ∆G=+ve
•   ∆G=0
•  All of these

Solution
b) The melting of ice at -15℃ is not an spontaneous process

Q7. The gas in a refrigerator causes cooling on expansion because:

•  Work done by the gas is converted into heat
•  Heat of the gas is lost as work is done by the gas
•  The heat is spread over a large space
•  None of the above

Solution
b) The gives rise to cooling.

Q8.Heat of reaction at constant volume is equal to :

•  ƩU_P-ƩU_R
•  ƩU_R-ƩU_P
•  ƩH_P-ƩH_R
•  ƩH_R-ƩH_P

Solution
a) At constant volume, heat of reaction is ∆U.

Q9. Which phenomena cannot be described as oxidation?

•  Oxidation of coal in air
•  Burning of magnesium in nitrogen
•  Reaction of antimony with chlorine
•  Lighting of an electric lamp

Solution
d) It is a physical change.

Q10. Combustion of glucose takes place according to the equation
C₆H₁₂O₆+CO₂→6CO₂+6H₂O;∆H=-72kcal. How much energy will be required
for the production of 1.6 g of glucose (Molecular mass of glucose = 180)?

•  0.064 kcal
•  0.64 kcal
•  6.4 kcal
•  64 kcal

Solution
b) ∆H per 1.6 g =(72×1.6)/180=0.64 kcal