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Reversible Reactions

Reversible Reactions

Some reactions can take place in two directions. In one direction the reactants combine to form the products. This is called the \(\color{orange}{\text{forward reaction}}\). In the other direction, the products react to form the reactants again. This is called the \(\color{purple}{\text{reverse reaction}}\). A special double-headed arrow ( \(\leftrightharpoons\)) is used to show this type of \(\color{blue}{\text{reversible reaction}}\):

\(\text{XY} + \text{Z}\) \(\leftrightharpoons\) \(\text{X} + \text{YZ}\)

Definition: A reversible reaction

A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions. In other words, the reactants and products of the reaction may reverse roles.

Fact:

Remember that \(\to\) implies that A goes to B and cannot become A again. \(\rightleftharpoons\) implies that the reaction is reversible. A can go to B and B can go to A.

So, in the following reversible reaction:

\[\overset{{\color{orange}{\textbf{forward}\longrightarrow}}}{\text{H}_{2}\text{(g)} + \text{I}_{2}\text{(g)}} {\color{blue}{\leftrightharpoons}} \underset{\color{purple}{\longleftarrow\textbf{reverse}}}{2\text{HI(g)}}\]

\(\color{orange}{{\text{H}}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}} \to {\text{2HI(g)}}}\) is the \(\color{orange}{\text{forward}}\) reaction.

The forward reaction is always taken from the given equation and written: left to right.

\(\color{purple}{{\text{2HI(g)}} \to {\text{H}}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}}}\) is the \(\color{purple}{\text{reverse}}\) reaction.

The reverse reaction is always taken from the given equation and written: right to left.

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