HSC Science (Computer Science) 12th Standard Board Exam - Maharashtra State Board Important Questions for Chemistry

Calculate C-Cl bond enthalpy from following reaction:

CH₃Cl_(g) + Cl_2(g) → Ch₂Cl_2(g) + HCl_(g) ΔH° = -104KJ

If C-H, Cl-Cl and H-Cl bond enthalpies are 414, 243 and 431 KJ-Mol^-1 respectively.

Absolute entropies of solids, liquids and gases can be determined by

• Measuring heat capacity of substance at various temperatures
• Subtracting standard entropy of reactants from products
• Measuring vibrational motion of molecules
• Using formula ΔS° = S_T° - S_O°

Amongst the following identify the criterion for a process to be at equilibrium -

1. ΔG < 0
2. ΔG > 0
3. ΔS_total=0
4. ΔS < 0

Define the Enthalpy of fusion

Determine whether the reactions with the following ΔH and ΔS values are spontaneous or non-spontaneous. State whether the reactions are exothermic or endothermic.

(a) ΔH = -110kJ, ΔS = + 40JK^-1 at 400 K

(b) ΔH = + 40kJ, ΔS = -120JK^-1 at 250K

Write one statement of first law of thermodyamics and its mathematical expression.

For the reaction: Cl₂(g) → 2Cl(g), _______.

(A) ΔH is positive, ΔS is positive

(B) ΔH is positive, ΔS is negative

(C) ΔH is negative, ΔS is negative

(D) ΔH is negative, ΔS is positive

The equilibrium constant Kp for the reaction,

H_2(g) + I_2(g) → 2HI_(g) is 130 at 510 K. Calculate ΔG^o for the following reaction at the same temperature: 2HI_(g) → H_2(g) + I_2(g) [Given: R = 8.314 J K^-1 mol^-1 ]

If the enthalpy of vaporisation of water at 100^oC is 186.5 J.mol^-1, the entropy of vaporization will be_____________ .

(a) 4.0 J . K^-1. mol-¹                                               

(b) 3.0 J . K^-l.  mol^-1

(c) 1.5 J - K^-1. mol-¹                                             

(d) 0.5 J . K^-l. mol^-1

State and explain Hess’s law of constant heat summation.

For a chemical reaction dS=0.035 kJ/k and dH=20kJ. At what temperature does the reaction turn nonspontaneous?

Prove that ΔH=ΔU+ΔnRT. what is the condition under which ΔU=ΔH?

300 M mol of perfect gas occupies 13 L at 320 K. Calculate the work done in joules when the gas expands-

(a) isothermally against a Constant external pressure of 0.20atm.

(b) isothermal and reversible process.

(c) into vaccum until the volume of gas is increased by 3L (R=8.314J mol^-1 K^-1)

One mole of a gas expands by 3L against a constant pressure of 3 atmosphere. Calculate the work done in -

(a) L.atmosphere

(b) Joules

(c) Calories

Derive the relation between `DeltaG^@`and equilibrium constant (K) for the reaction -

aA_bB ⇌ cC+dD.​

A system absorbs 640 J heat and does work of 260 J, the change in internal energy of the system will be

(a) +380J

(b) -380J

(c) +900J

(d) -900J

Which of the following pairs is an intensive property?

(A) Density, viscosity

(B) Surface tension, mass

(C) Viscosity, internal energy

(D) Heat capacity, volume

For a chemical reaction, A→ products, the rate of reaction doubles when the concentration of
‘A’ is increased by a factor of 4, the order of reaction is

(A) 2

(B) 0.5

(C) 4

(D) 1

Define the term ‘enthalpy’.

What will happen to the internal energy if work is done by the system?