Write the equilibrium expression for the autoionization of water.
[tex]K _{ w }=\left[ H _3 O ^{+}\right][ OH ] /\left[ H _2 O \right]^2[/tex]
[tex]K _{ w }=\left[ H _2 O \right]^2 /\left[ H _3 O ^{+}\right]\left[ OH ^{-}\right][/tex]
[tex]K _{ w }=\left[ H _3 O ^{+}\right]\left[ OH ^{-}\right][/tex]
Answer (2)
The equilibrium expression for the autoionization of water is given by K w = [ H 3 O + ] [ O H − ] . This expression reflects the relationship between the concentrations of hydronium and hydroxide ions in water. Water's pure liquid state means it is not included in the expression, only the ions formed are considered.
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Autoionization of water involves the formation of hydronium and hydroxide ions.
The balanced chemical equation is 2 H 2 O ( l ) ⇌ H 3 O + ( a q ) + O H − ( a q ) .
The equilibrium expression excludes the concentration of liquid water.
The correct equilibrium expression is K w = [ H 3 O + ] [ O H − ] .
Explanation
Understanding the Problem The autoionization of water is a process where water reacts with itself to form hydronium ( H 3 O + ) and hydroxide ( O H − ) ions. We need to determine the correct equilibrium expression ( K w ) for this process from the given options.
Writing the Equilibrium Expression The balanced chemical equation for the autoionization of water is:
2 H 2 O ( l ) ⇌ H 3 O + ( a q ) + O H − ( a q )
The equilibrium expression, K w , is based on the concentrations of the products divided by the concentrations of the reactants, each raised to the power of their stoichiometric coefficients. However, since water is a liquid, its concentration is considered constant and is not included in the equilibrium expression.
Determining the Correct Expression Therefore, the equilibrium expression K w is given by the product of the concentrations of the hydronium and hydroxide ions:
K w = [ H 3 O + ] [ O H − ]
Final Answer Comparing this with the given options, the correct expression is:
K w = [ H 3 O + ] [ O H − ]
Examples
Autoionization of water is crucial in many chemical and biological processes. For example, it helps maintain the pH balance in our bodies, which is essential for enzymes to function correctly. Understanding this concept allows us to predict how different substances will behave in aqueous solutions, impacting fields from medicine to environmental science.
