# World of Chemistry

## Educators

Problem 1

How is oxidation defined? Write an equation showing an atom being oxidized.

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Problem 2

How is reduction defined? Write an equation showing an atom being reduced.

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Problem 3

For each of the following oxidation–reduction reactions of metals with nonmetals, identify which element is oxidized and which is reduced.
$$\begin{array}{l}{\text { a. } 6 \mathrm{Na}(s)+\mathrm{N}_{2}(g) \rightarrow 2 \mathrm{Na}_{3} \mathrm{N}(s)} \\ {\text { b. } \mathrm{Mg}(s)+\mathrm{Cl}_{2}(g) \rightarrow \mathrm{MgCl}_{2}(s)} \\ {\text { c. } 2 \mathrm{Al}(s)+3 \mathrm{Br}_{2}(l) \rightarrow 2 \mathrm{AlBr}_{3}(s)} \\ {\text { d. } 4 \mathrm{Fe}(s)+3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{Fe}_{2} \mathrm{O}_{3}(s)}\end{array}$$

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Problem 4

For each of the following oxidation–reduction reactions, identify which element is oxidized and which is reduced.
$$\begin{array}{l}{\text { a. } \mathrm{Mg}(s)+\mathrm{Br}_{2}(l) \rightarrow \mathrm{MgBr}_{2}(s)} \\ {\text { b. } 2 \mathrm{Na}(s)+\mathrm{S}(s) \rightarrow \mathrm{Na}_{2} \mathrm{S}(s)} \\ {\text { c. } \mathrm{Cl}_{2}(g)+2 \mathrm{NaBr}(a q) \rightarrow \mathrm{Br}_{2}(l)+2 \mathrm{NaCl}(a q)} \\ {\text { d. } 6 \mathrm{K}(s)+\mathrm{N}_{2}(g) \rightarrow 2 \mathrm{K}_{3} \mathrm{N}(s)}\end{array}$$

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Problem 5

What is an oxidation state? Why is this concept useful?

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Problem 6

Explain why, although it is not an ionic compound, we assign oxygen an oxidation state of $-2$ in water, $\mathrm{H}_{2} \mathrm{O} .$ Give an example of a compound in which oxygen is not in the $-2$ oxidation state.

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Problem 7

Why must the sum of all the oxidation states of the atoms in a neutral molecule be zero?

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Problem 8

What must be the sum of the oxidation states of all the atoms in a polyatomic ion?

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Problem 9

Assign oxidation states to all of the atoms in each of the following:
$$\begin{array}{ll}{\text { a. } \mathrm{NCl}_{3}} & {\text { c. } \mathrm{PCl}_{5}} \\ {\text { b. } \mathrm{SF}_{6}} & {\text { d. SiH }_{4}}\end{array}$$

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Problem 10

Assign oxidation states to all of the atoms in each of the following:
$$\begin{array}{ll}{\text { a. HBr }} & {\text { c. } \mathrm{Br}_{2}} \\ {\text { b. HOBr }} & {\text { d. } \mathrm{HBrO}_{4}}\end{array}$$

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Problem 11

Assign oxidation states to all of the atoms in each of the following:
$$\begin{array}{ll}{\text { a. HNO }_{3}} & {\text { c. HSO }_{4}^{-}} \\ {\text { b. HPO }_{4}^{2-}} & {\text { d. } \mathrm{O}_{2}^{2-}}\end{array}$$

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Problem 12

Assign oxidation states to all of the atoms in each of the following:
$$\begin{array}{ll}{\text { a. } \mathrm{CuCl}_{2}} & {\text { c. } \mathrm{HCrO}_{4}^{-}} \\ {\text { b. } \mathrm{CrCl}_{3}} & {\text { d. } \mathrm{Cr}_{2} \mathrm{O}_{3}}\end{array}$$

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Problem 13

Assign oxidation states to all of the atoms in each of the following:
$$\begin{array}{ll}{\text { a. } \mathrm{CH}_{4}} & {\text { c. } \mathrm{KHCO}_{3}} \\ {\text { b. } \mathrm{Na}_{2} \mathrm{CO}_{3}} & {\text { d. } \mathrm{CO}}\end{array}$$

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