6

Q1. The enthalpies of all elements in their standard states are

• 1) negative
• 2) zero
• 3) positive
• 4) infinite

Solution

The enthalpies of all elements in their standard states are zero.

Q2. State the effect of the following processes on the total energy content of the system (i) Work done by the system (ii) Heat transferred to the surroundings

Solution

i) Work done by the system is negative i.e. the total energy content of the system decreases. ii) When heat is transferred to the surroundings the total energy content of the system decreases.

Q3. State the condition in which the change in the internal energy of a system () is equal to the heat exchanged q.

Solution

In a system,  = q when no work is done at constant volume.

Q4. A gas is compressed by an average pressure of 0.50 atm to decrease its volume from 400 cm³ to 200cm³ . Calculate the amount of work done in this process.

Solution

 

Q5. For a reaction, Determine the temperature above which reaction will be spontaneous.

Solution

  

Q6. Calculate the internal energy of vaporization for 18 g of water at 100°C. Given,  for water at 373 K = 40.66 kJ mol^-1.

Solution

The reaction can be represented as follows   18g H₂O (l)  18 g H₂O(g) Assuming steam behaves as an ideal gas   =  40.66 kJ mol ^-1 – (1) ( 8.314 J K^-1mol^-1)( 373 K) ( 10^-3 kJ J^-1)  =  40.66 kJ mol ^-1 – 3.10 kJ mol ^-1          =  37.56 kJ mol ^-1

Q7. For a water gas reaction at 1000^°C the standard Gibb’s energy change is -8.1 kJ mol^-1. Calculate the value of equilibrium constant?

Solution

      

Q8. Give reasons for the following       (i)    The solubility of most salt in water increases with rise of temperature.       (ii)    Fluorides tend to be less soluble than corresponding chlorides.

Solution

(i) The solubility of most salt in water increases with rise of temperature because for most of the ionic compounds,  is positive and the dissociation process is endothermic. (ii) Fluorides tend to be less soluble than corresponding chloride because in former lattice enthalpy is very high.

Q9. Identify Open, closed or nearly isolated systems from the following. Earth, human beings, can of tomato soup, ice cube tray filled with water, satellite in orbit, coffee in thermos flask, helium filled balloon.

Solution

Earth Open system Human beings Open system Can of tomato soup Closed system Ice cube tray filled with water Open system Satellite in orbit Open system Coffee in thermos flask Isolated system Helium filled balloon Closed system

Q10. Give the relation between enthalpy of solution, lattice enthalpy and enthalpy of hydration.

Solution

Q11. Calculate the entropy of vaporization of water if its enthalpy of vaporization is 186.5 kJmol^-1?

Solution

 

Q12. Among the enthalpies of fusion and enthalpies of freezing, which one is positive and which one is negative and why?

Solution

Melting is endothermic process, so all enthalpies of fusion are positive because ice requires heat for melting. While in freezing of water equal amount of heat is given to surrounding i.e. freezing is exothermic process. Thus all enthalpies of freezing are negative.

Q13. How does the intermolecular interaction affect the magnitude of enthalpy change during phase transformation?

Solution

The strength of intermolecular interaction determines the magnitude of enthalpy change. For example in water there is strong intermolecular interaction in the form of hydrogen bond and in acetone the intermolecular interaction is weak dipole-dipole interaction. Hence enthalpy of vaporization is greater in case of water.

Q14. For the reaction: Calculate the temperature at which Gibbs energy change ΔG is equal to zero. Predict the nature of the reaction at this temperature and above it.

Solution

   

Q15. Calculate the difference between heats of reaction at constant pressure and at constant volume for the reaction at 25^oC in kJ.

Solution

Q16. The enthalpies of formation of C₂H_{4 (g),} CO_{2 (g)}, H₂O _(l) at 25^oC atmospheric pressure are 52, -394 and -286 kJ mol^-1 respectively. Calculate the enthalpy of combustion of C₂H₄.

Solution

The chemical equation for the above reaction is:  

Q17. State the Zeroth law of thermodynamics.

Solution

The Zeroth law states that if two bodies A and B are in thermal equilibrium with another body C, then the bodies A and B will also be in thermal equilibrium with each other.

Q18. One mole of ethyl alcohol is completely burnt in oxygen to form water and carbon dioxide at 25^oC to give 327 k cals. Calculate the heat of reaction at constant volume.

Solution

The chemical equation for this reaction is: