2) Which of the following processes are spontaneous? 3) The normal freezing poin

Question

2) Which of the following processes are spontaneous? 3) The normal freezing point of chloroform (CHCl3) is -63.5 degree C. 4) The element aluminum (Al) freezes at 660. Degree C, and its molar enthalpy of fusion is deltaHfus = 10.7 kJ/mol. 5) Express the second law of thermodynamics as a mathematical equation. (select the appropriate symbol) 6) In a particular spontaneous process, the entropy of the system decreases. What can you conclude about the sign and magnitude of deltaSsurr? 7) During a certain reversible process, the surroundings undergo an entropy change, deltaSsurr = -64 J/K. What is the entropy change of the system for this process? 8) In a chemical reaction two gases combine to form a liquid. What do you expect for the sign of deltaS?

Explanation / Answer

2. Which of the following processes are spontaneous?
Answer: a, c and d

a) the reaction of hydrogen gas with oxygen gas to form water vapor
Reaction is spontaneous and the mixture makes a pop sound releasing heat and the total entropy of system and surroundings increase.
b) separating a mixture of N2 and O2 into two separate samples, one is pure N2 and one is pure O2
Not spontaneous as separation will cause decrease in total positional entropy.

c) the melting of ice cubes at 10C and 1 atm pressure
Spontaneous as entropy will increase

d) alignment of iron fillings in a magnetic field
spontaneous as this will decrease the energy and hence total entropy will increase, though the system entropy decreases.

3. The normal freezing point of chloroform (CHCl3) is -63.5C
Answer:
a) Is the freezing of chloroform an endothermic or exothermic process?
Freezing is an exothermic process, DH is negative.

b) In what temperature range is the freezing of chloroform a spontaneous process?
For temperature range from -273 to -63.5, freezing will be spontaneous process

c) In what temperature range is it a nonspontaneous process?
Temperature above -63.5C, Surround entropy will increase less than the decrease of entropy by melting.

d) At what temperature are the liquid and solid chloroform in equilibrium?
At freezing point, there is equilibrium between solid and liquid state.

4. The element Aluminium (Al) freezes at 660C and its molar enthalpy of fusion is DHfus = 10.7 kJ/mol.
a) When molten aluminium solidifies to Al(s) at its normal melting point, is DS positive or negative?

Entropy change is negative as there is less positional entropy in solid state than in liquid state.

b) Calculate the values of DS when 47.0 g of Al(l) solidifies at 660C.
Al(l) -> Al(s)
Temperature, T = 660+273=933 K
DHf = 10.7 kJ/mol
Change in molar entropy, DS = -DHf/T = -10.7/933 = -0.0115 kJ/mol K
Molar mass of Al = 27 g/mol
Mass of Al = 47 g
No of moles of Al, n = 47/27=1.741 mol
Change in entropy of 1.741 mol of Al = n*DS = 1.741*(-0.0115) = 0.02 kJ/K

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