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CHEMISTRY, M3 EQ-Bank 8 MC

Identify the compound in which chromium has an oxidation state of 6.

  1. \(\ce{CrCl3}\)
  2. \(\ce{Cr2O3}\)
  3. \(\ce{K2Cr2O7}\)
  4. \(\ce{Cr(OH)3}\)
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\(C\)

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Let \(x\) be the oxidation state of chromium in each of the following calculations. 

\(\ce{CrCl3}\) → \(x+3(-1)=0\ \ \Rightarrow \ \ x=3\)

\(\ce{Cr2O3}\) → \(2x+3(-2)=0\ \ \Rightarrow \ \ x=3\)

\(\ce{K2Cr2O7}\) → \(2(1)+ 2x+7(-2)=0\ \ \Rightarrow \ \ x=6\)

\(\ce{Cr(OH)3}\) → \(x+3(-1)=0\ \ \Rightarrow \ \ x=3\)

\(\Rightarrow C\)

CHEMISTRY, M3 EQ-Bank 6 MC

Which of the following species decreases in oxidation number?

\(\ce{Fe^{2+}(aq) + MnO4^-(aq) + 8H^+(aq) -> 5Fe^{3+} + Mn^{2+}(aq) + 4H2O(l)}\)

  1. \(\ce{Fe^{2+}(aq)}\)
  3. \(\ce{H2O(l)}\)
  4. \(\ce{H^+(aq)}\)
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\(B\)

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  • The oxidation number of  \(\ce{Fe^{2+}(aq)}\)  increases from +2 to +3.
  • The oxidation number of \(\ce{Mn}\) in \(\ce{MnO4^-}\) 
  •    \(\ce{Mn} +4(-2)=-1\ \ \Rightarrow\ \ \ce{Mn}=7\)
  • \(\ce{Mn}\) decreases from an oxidation number of 7 to 2 during the reaction.

\(\Rightarrow B\)

CHEMISTRY, M3 EQ-Bank 2 MC

What is the oxidation state of sulfur in the sulfate ion, \(\ce{SO4^{2-}}\)?

  1. \(+2\)
  2. \(+4\)
  3. \(+6\)
  4. \(-2\)
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\(C\)

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  • The oxidation number of a polyatomic ion is the charge of the ion.
  • Let \(x\) be the oxidation state of sulfur and the oxidation state of oxygen is \(-2\).
\(x +4(-2)\) \(=-2\)  
\(x-8\) \(=-2\)  
\(x\) \(=6\)  

 
\(\Rightarrow C\)

CHEMISTRY, M3 2015 VCE 6 MC

In which one of the following compounds is sulfur in its lowest oxidation state?

  1. \(\ce{SO3}\)
  2. \(\ce{HSO4–}\)
  3. \(\ce{SO2}\)
  4. \(\ce{Al2S3}\)
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\(D\)

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Let \(x\) equal the oxidation of sulfur in each of the following calculations.

In \(\ce{SO3}\):

   \(x+ 3 \times -2=0\ \ \Rightarrow \ \ x=6\)

In \(\ce{HSO4–}\):

   \(1 + x+ 4 \times -2=-1\ \ \Rightarrow \ \ x=6\)

In \(\ce{SO2}\):

   \(x+ 2 \times -2=0\ \ \Rightarrow\ \ x=4\)

In \(\ce{Al2S3}\):

   \(2 \times 3+ 3 \times x=0\ \ \Rightarrow\ \ 3x=-6\ \ \Rightarrow\ \ x=-2\)

  • \(D\) has the lowest oxidation number showing it is in the lowest oxidation state.

\(\Rightarrow D\)

CHEMISTRY, M3 2009 HSC 9 MC

One test used for random breath testing in NSW involved crystals of potassium dichromate reacting with ethanol. In this reaction the orange dichromate ion, \(\ce{Cr2O7}^{2-}\), changes to the green chromium ion, \(\ce{Cr^3+}\).

Which statement is true for this reaction?

  1. Chromium has lost electrons and reached a lower oxidation state.
  2. Chromium has lost electrons and reached a higher oxidation state.
  3. Chromium has gained electrons and reached a lower oxidation state.
  4. Chromium has gained electrons and reached a higher oxidation state.
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\(C\)

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  • Let \(x\) equal the oxidation number of Cr in \(\ce{Cr2O7}^{2-}\).
  •    \(2x+ 7 \times -2 = -2\ \ \Rightarrow \ \ 2x= 12\ \ \Rightarrow \ \ x=6\)
  • The Chromium has been reduced as the oxidation number has decreased from 6 to 3, thus it gains electrons and the oxidation state is lower.

\(\Rightarrow C\)

CHEMISTRY, M3 2017 HSC 11 MC

Consider the following redox reaction.

\( \ce{2K2Cr2O7}(aq) + \ce{2H2O}(l) + \ce{3S}(s) \rightarrow \ce{2Cr2O3}(aq) + \ce{4KOH}(aq) + \ce{3SO2}(g) \)

Which species is being oxidised?

  1. \( \ce{Cr^6+}\)
  2.  \( \ce{K^+} \)
  3. \( \ce{O^2-} \)
  4. \( \ce{S} \)
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\(D\)

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  • Sulfer has an oxidation number of 0 on the left hand side of the equation.
  • Let \(x\) equal the oxidation number of sulfur within \(\ce{3SO2}(g) \).
\(x+2 \times -2\) \(=0\)  
\(x-4\) \(=0\)  
\(x\) \(=4\)  

 

  • The increase in oxidation number shows the species, \( \ce{S} \), has undergone oxidation.

\(\Rightarrow D\)