Thermochemistry Quiz-11

Thermochemistry Quiz-11

Dear Readers,

Thermochemistry is the part of thermodynamics that studies the relationship between heat and chemical reactions. As we know thermodynamics is a very important topic from examination point of view. Hence as a part of it, thermochemistry also becomes important for exams. So if you want to be best in thermodynamics then you must study thermochemistry properly. So go ahead and prepare well. All the best !

Q1. For vaporization of water at 1 atmospheric pressure, the values of ΔH and ΔS are 40.63 kJ mol⁻¹ and 108.8 J K⁻¹mol⁻¹ respectively. The temperature when Gibbs energy change (ΔG) for this transformation will be zero, is:

•  273.4 K
•  393.4 K
•  373.4 K
•  293.4 K

Solution:-
ΔG = ΔH − T.ΔS
ΔG = 0, ∴ ΔH = T.ΔS
T = ΔH / ΔS = 373.4 K

Q2. Δn values in ΔH = ΔU + ΔnRT may have:

•  Integer nature
•  Fractional value
•  Positive or negative
•  All of these

Solution:-
Δn depends on stoichiometry of reaction.

Q3. It is a general principle that if a system has the less energy then it is:

•  More stable
•  Less stable
•  Unstable
•  More unstable

Solution:-
Lower is energy level of a system, more is its stability.

Q4. 1 litre-atmosphere is equal to:

•  101.3 J
•  24.206 cal
•  101.3 × 10⁷ erg
•  All of these

Solution:-
PV = 1 litre-atmosphere
= 10⁻³m³ × 0.76 × 13.6 × 9.8 × 10³ Nm⁻²
= 101.3 J

Q5. A thermally isolated gaseous system can exchange energy with the surroundings. The mode of transference of energy can be:

•  Heat
•  Work
•  Heat and radiation
•  None of these

Solution:-
Only work can be done by a thermally isolated system between it and surroundings.

Q6. Given that ΔH_{r 298 K} = −54.07 kJ/mol and ΔS^°_{r 298 K} = 10 J/mol and R = 8.314 J K⁻¹mol⁻¹. The value of log₁₀K for a reaction, A ⇌ B is:

•  5
•  10
•  95
•  100

Solution:-
ΔG° = ΔH° − TΔS°
= −54.07 − 298×10×10⁻³
= −57.05 kJ
Also, ΔG° = 2.303 RT log₁₀K

Q7. If liquids A and B form an ideal solution, then:

•  Enthalpy of mixing is zero
•  Entropy of mixing is zero
•  Free energy of mixing is zero
•  Free energy as well as the entropy of mixing are each zero

Solution:-
ΔH_mixing = 0 for ideal solutions.

Q8. The temperature at which the reaction,
Ag₂O(s) → 2Ag(s) + 1/2O₂(g)
is at equilibrium is ..... ; given, ΔH = 30.5 kJ mol⁻¹ and ΔS = 0.066 kJ mol⁻¹K⁻¹ .

•  462.12 K
•  362.12 K
•  262.12 K
•  562.12 K

Solution:-
ΔG = ΔH − TΔS
ΔG = 0, at equilibrium; ∴ ΔH = TΔS
or 30.5 = T × 0.066

Q9. When 1g atom of carbon is converted into 1g molecule of CO₂ the heat liberated is same:

•  Irrespective of whether the volume is kept constant or pressure is kept constant
•  Irrespective of the temperature at which the reaction is carried out
•  Whether the carbon is in the form of diamond or graphite
•  None of the above

Solution:-
C + O₂(g) → CO₂(g),  Δn = 0;
∴ ΔH = ΔU

Q10. For spontaneity of a cell, which is correct?

•  ΔG = 0, ΔE = 0
•  ΔG = −ve, ΔE = 0
•  ΔG = +ve, ΔE = +ve
•  ΔG = −ve, ΔE = +ve

Solution:-
ΔG = ΔH − TΔS:   ΔG = ΔE + pΔV − TΔS
For spontaneity ΔG = −ve