Consider the half reactions below for a chemical reaction.
[tex]\begin{array}{l}
Mg \longrightarrow Mg^{2+}(aq)+2 e^{-} \\
2 H^{+}(aq)+2 e^{-} \longrightarrow H_2(g)
\end{array}[/tex]
What is the overall equation for this chemical reaction?
A. [tex]Mg ( s )+2 H ^{+}( aq ) \longrightarrow Mg ^{2+}( aq )+ H _2(g)[/tex]
B. [tex]Mg ( s )+2 H ^{+}( aq ) \longrightarrow Mg ^{2+}( aq )+2 e ^{-}[/tex]
C. [tex]Mg ^{2+}( aq )+ H _2(g) \longrightarrow Mg ( s )+2 H ^{+}( aq )[/tex]
D. [tex]Mg ( s )+2 H \longrightarrow Mg ( aq )+ H _2(g)[/tex]
Answer (2)
Combine the oxidation and reduction half-reactions.
Ensure the number of electrons is balanced on both sides.
Add the half-reactions and cancel out the electrons.
The overall equation is M g ( s ) + 2 H + ( a q ) ⟶ M g 2 + ( a q ) + H 2 ( g ) .
Explanation
Understanding the Half-Reactions We are given two half-reactions:
Oxidation: M g ⟶ M g 2 + ( a q ) + 2 e − Reduction: 2 H + ( a q ) + 2 e − ⟶ H 2 ( g )
We need to combine these half-reactions to obtain the overall equation for the chemical reaction.
Electron Transfer In the oxidation half-reaction, magnesium ( M g ) loses two electrons to form magnesium ions ( M g 2 + ). In the reduction half-reaction, hydrogen ions ( H + ) gain two electrons to form hydrogen gas ( H 2 ).
Combining the Half-Reactions To obtain the overall equation, we add the two half-reactions together. Since the number of electrons released in the oxidation half-reaction is equal to the number of electrons consumed in the reduction half-reaction (both are 2), we can directly add the two equations:
M g ( s ) + 2 H + ( a q ) + 2 e − ⟶ M g 2 + ( a q ) + 2 e − + H 2 ( g )
Canceling Electrons Now, we cancel out the electrons that appear on both sides of the equation:
M g ( s ) + 2 H + ( a q ) ⟶ M g 2 + ( a q ) + H 2 ( g )
Final Overall Equation The overall equation for the chemical reaction is:
M g ( s ) + 2 H + ( a q ) ⟶ M g 2 + ( a q ) + H 2 ( g )
This corresponds to the first option provided.
Examples
This type of reaction is commonly seen in batteries and corrosion processes. For example, when magnesium is placed in an acidic solution, it will dissolve, forming magnesium ions and hydrogen gas. This principle is also used in some types of batteries where the oxidation of a metal is coupled with the reduction of hydrogen ions to generate electricity. Understanding redox reactions is crucial in many industrial and environmental applications, such as wastewater treatment and metal refining.
The overall equation for the reaction is M g ( s ) + 2 H + ( a q ) → M g 2 + ( a q ) + H 2 ( g ) , corresponding to option A. This reaction illustrates the oxidation of magnesium and the reduction of hydrogen ions. Redox reactions like this are crucial in various chemical processes, including those used in batteries.
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