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CHEMISTRY, M4 EQ-Bank 4 MC

Which of the following would involve an increase in entropy?

  1. \(\ce{H2O(l) → H2O(s)}\)
  2. \(\ce{2 Na2CO3(aq) + 3 CaCl2(aq) → 2 NaCl(s) + 3 CaCO3(s)}\)
  3. \(\ce{2 NH3(g) + 3 HCl(g) → 2 NH4Cl(s)}\)
  4. \(\ce{Na2CO3(s) → 2Na^+ + CO3^{2-}(aq)}\)
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\(D\)

Show Worked Solution

→ Dissolving a solid into an aqueous solution increases entropy, as the ions become separated from the ionic lattice and so more disordered in the solution.

\(\Rightarrow D\)

CHEMISTRY, M4 EQ-Bank 2 MC

Which of the following scenarios would lead to a decrease in entropy?

  1. A gas condensing into a liquid
  2. A solid dissolving into water
  3. One particle decomposes into two
  4. A solid breaking down into multiple particles
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\(A\)

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→ A gas condensing into a liquid results in a decrease in entropy because the molecules in the liquid phase are more organised and have less freedom of movement compared to the gas phase. 

→ This leads the system to a reduction in disorder.

\(\Rightarrow A\)

CHEMISTRY, M4 EQ-Bank 1 MC

Which of the following scenarios will result in a decrease in entropy?

  1. Water transitioning to steam
  2. Formation of silver chloride as a precipitate from a solution containing silver and chloride ions
  3. Ice melting into liquid water
  4. Ethanol dissolving in water
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\(B\)

Show Worked Solution

→ The formation of silver chloride precipitate involves the arrangement of silver and chloride ions into a solid structure, decreasing the system’s disorder.

→ This decrease in disorder corresponds to a reduction in entropy.

\(\Rightarrow B\)

CHEMISTRY, M4 EQ-Bank 4

Of the following state changes, explain which represents the smallest increase in entropy and which represents the largest.   (3 marks)

    1. Sublimation of ice to gas
    2. Melting of ice to liquid water
    3. Freezing of water to ice
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→ State change (3) has the smallest entropy increase as solids have less entropy than liquids and the freezing of water indicates a decrease in entropy as order increases.

→ State change (2) has a larger increase in entropy than state change (3) as water has more entropy than the solid ice it is formed from.

→ State change (1) has the largest entropy increase as gas has the most entropy compared to solids or liquids.

Show Worked Solution

→ State change (3) has the smallest entropy increase as solids have less entropy than liquids and the freezing of water indicates a decrease in entropy as order increases.

→ State change (2) has a larger increase in entropy than state change (3) as water has more entropy than the solid ice it is formed from.

→ State change (1) has the largest entropy increase as gas has the most entropy compared to solids or liquids.


CHEMISTRY, M4 EQ-Bank 3

A 3.1g sample of \(\ce{CaCO3_{(s)}}\) decomposes into \(\ce{CaO_{(s)}}\) and \(\ce{CO2_{(g)}}\). Entropy values  for these chemicals are given below and the molar enthalpy for the reaction is 360 kJ/mol.

\begin{array} {|l|c|}
\hline
\rule{0pt}{2.5ex}\text{Substance}\rule[-1ex]{0pt}{0pt} & \text{Standard Entropy}\ (\Delta S) \\
\hline
\rule{0pt}{2.5ex}\ce{CaCO3}\rule[-1ex]{0pt}{0pt} & \text{92.88 J/K} \\
\hline
\rule{0pt}{2.5ex}\ce{CaO(s)}\rule[-1ex]{0pt}{0pt} & \text{39.75 J/K} \\
\hline
\rule{0pt}{2.5ex}\ce{CO2(g)}\rule[-1ex]{0pt}{0pt} & \text{213.6 J/K} \\
\hline
\end{array}

  1. Write a chemical equation for the above reaction.  (1 mark)

  2. Calculate the entropy change for this reaction.  (2 marks)

  3. Determine whether the reaction is spontaneous or non-spontaneous at room temperature.  (2 marks)

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  1. \(\ce{CaCO3(s) \rightarrow CaO(s) + CO2(g)}\)
  2. \(\ce{4.97 J K^{-1}}\)
  3. \(\text{Since}\ \Delta G > 0,\ \text{the reaction is not spontaneous.}\)
Show Worked Solution

i.    \(\ce{CaCO3(s) \rightarrow CaO(s) + CO2(g)}\)

ii.     \(\Delta S\) \(=\Sigma S_{\text{products}}-\Sigma S_{\text{reactants}}\) 
    \(= 213.6 + 39.75-92.88\)
    \(= 160.47\ \text{J mol}^{-1}\ \text{K}^{-1}\)

 
\(\ce{n(CaCO3)}= \dfrac{\text{m}}{\text{MM}} = \dfrac{3.1}{100.09} = 0.03097\ \text{mol} \)

\(\text{Entropy change}\ = 160.47 \times 0.03097 = 4.97\ \text{J K}^{-1}\)
 

iii.   \(\text{Room Temperature = 298.15 K}\)

\(\Delta G\) \(=\Delta H-T \Delta S\)  
  \(=360-(298.15 \times 0.16047) \)  
  \(= 312.179\ \text{kJ}\)  

 
\(\text{Since}\ \Delta G > 0,\ \text{the reaction is not spontaneous.}\)

CHEMISTRY, M4 EQ-Bank 2

In the following reactions, predict whether entropy will increase or decrease, giving reasons. (3 marks)

  1. \(\ce{N2(g) + 3H2(g) \rightleftharpoons 2NH3(g)}\)   (1 mark)
  2. \(\ce{CaCO3(s) \rightleftharpoons CaO(g) + CO2(g)}\)   (1 mark)
  3. \(\ce{2HCl(aq) + Zn(s) \rightleftharpoons ZnCl2(aq) + H2(g)}\)   (1 mark)
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i.     → Decrease

→ 4 moles of gas becomes 2 moles of gas on the product side, causing the reaction to become more ordered and decreasing in entropy.

ii.    → Increase

→ A solid decomposes into 2 moles of gas. This reaction becomes more disordered, and the phase change involves an increase in entropy.

iii.   → Increase

→ An aqueous and solid reactant becomes an aqueous and gaseous product. The phase changes overall involve an increase in entropy.

Show Worked Solution

i.     → Decrease

→ 4 moles of gas becomes 2 moles of gas on the product side, causing the reaction to become more ordered and decreasing in entropy.
 

ii.    → Increase

→ A solid decomposes into 2 moles of gas. This reaction becomes more disordered, and the phase change involves an increase in entropy.
 

iii.   → Increase

→ An aqueous and solid reactant becomes an aqueous and gaseous product. The phase changes overall involve an increase in entropy.