Question

What is the oxidation state of carbon in $CaC_{2}$ and $K_{2}C_{2}O_{4}$ respectively?

• A. -2 and +6
• B. -1 and +3
• C. +2 and +2
• D. -2 and +3

Hint:

Logic Tip: The $C_2^{2-}$ ion in calcium carbide is the acetylide ion, featuring a carbon-carbon triple bond ($[:C \equiv C:]^{2-}$). Since the two identical carbon atoms equally share the $-2$ charge, each carbon naturally carries a formal oxidation state of $-1$.

Answer 1

The Correct Option is B

Concept:
The oxidation state is calculated by assigning the standard known oxidation numbers to the more electronegative or electropositive elements in the compound and setting the sum of all oxidation states equal to the net charge of the molecule or polyatomic ion (which is zero for neutral compounds). - Alkaline earth metals (like Ca) always have an oxidation state of +2 in compounds. - Alkali metals (like K) always have an oxidation state of +1. - Oxygen (O) generally has an oxidation state of -2.
Step 1: Calculate the oxidation state of Carbon in $CaC_2$.
Let the oxidation state of Carbon be $x$. In Calcium carbide ($CaC_2$), Ca is an alkaline earth metal with an oxidation state of $+2$. Since the molecule is neutral, the sum of oxidation states is zero: $$(+2) + 2(x) = 0$$ $$2x = -2$$ $$x = -1$$ The oxidation state of Carbon in $CaC_2$ is $-1$.
Step 2: Calculate the oxidation state of Carbon in $K_2C_2O_4$.
Let the oxidation state of Carbon be $y$. In Potassium oxalate ($K_2C_2O_4$), K is an alkali metal ($+1$) and Oxygen is ($-2$). The sum of oxidation states is zero: $$2(+1) + 2(y) + 4(-2) = 0$$ $$2 + 2y - 8 = 0$$ $$2y - 6 = 0$$ $$2y = +6$$ $$y = +3$$ The oxidation state of Carbon in $K_2C_2O_4$ is $+3$.
Step 3: Combine the results.
The oxidation states of carbon in $CaC_2$ and $K_2C_2O_4$ are $-1$ and $+3$ respectively. This matches option (B).