Question

Write an equation to show how the following acids or bases behave in water. (a) \(\mathrm{NaOH}(s)\) (b) \(\mathrm{CN}^{-}(a q)\) (c) \(\mathrm{HNO}_{3}(l)\)

Short answer

The equations representing how acids and bases behave in water are: \(\mathrm{NaOH}(s) \rightarrow \mathrm{Na^{+}}(aq)+\mathrm{OH^{-}}(aq)\), \(CN^{-}(aq) + H2O(l) \leftrightarrow HCN(aq) + OH^- (aq)\), and \(\mathrm{HNO}_{3}(l)+H_{2} O(l) \rightarrow H_{3} O^{+}(aq)+\mathrm{NO}_{3}^{-}(aq)\)

Step-by-step solution

Identify the type of substance

Check whether the substance is an acid or a base. NaOH is a base because it is a hydroxide (has an OH group); CN- is a base because it is capable of accepting a proton; HNO3 is an acid because it is capable of donating a proton.

Write equation for NaOH

For the base NaOH, write the equation showing how it dissociates in water to give sodium ions and hydroxide ions: \(\mathrm{NaOH}(s) \rightarrow \mathrm{Na^{+}}(aq)+\mathrm{OH^{-}}(aq)\)

Write equation for \(CN^{-}\)

For the base \(CN^{-}\), write the equation showing how It accepts a proton from water molecule to form hydrocyanic acid and hydroxide ions: \(CN^{-}(aq) + H2O(l) \leftrightarrow HCN(aq) + OH^- (aq)\)

Write equation for HNO3

For the acid HNO3, it donates a proton to a water molecule to form hydronium ion and nitrate ion: \(\mathrm{HNO}_{3}(l)+H_{2} O(l) \rightarrow H_{3} O^{+}(aq)+\mathrm{NO}_{3}^{-}(aq)\)

Key concepts

Chemical Equations

Chemical equations are vital tools in chemistry that represent the reactants and products in a chemical reaction. They not only convey the substances involved in a reaction but also illustrate the conservation of mass through stoichiometric coefficients. When acids or bases react with water, the equations portray the unique ways in which these substances transform.

An example of a chemical equation is the dissociation of sodium hydroxide (\textnormal{NaOH}) in water, which can be written as: \[\textnormal{NaOH}(s) \rightarrow \textnormal{Na}^{+}(aq) + \textnormal{OH}^{-}(aq)\] This shows that the solid sodium hydroxide dissociates into sodium and hydroxide ions when it encounters water. For a balanced equation, ensure stoichiometry is accurate; while \textnormal{NaOH} dissociates into one each of \textnormal{Na}^+ and \textnormal{OH}^- ions, other reactions may require different coefficients to maintain balance.

Acid and Base Behavior

Acids and bases exhibit distinctive behaviors in water due to their ability to donate or accept protons (\textnormal{H}^+ ions). Acids, such as hydrochloric acid (\textnormal{HCl}), are proton donors and in water, they typically form \textnormal{H}_{3}\textnormal{O}^{+} ions (hydronium ions). An example is the dissociation of nitric acid (\textnormal{HNO}_{3}), written as:\[\textnormal{HNO}_{3}(l)+\textnormal{H}_{2} \textnormal{O}(l) \rightarrow \textnormal{H}_{3} \textnormal{O}^{+}(aq) + \textnormal{NO}_{3}^{-}(aq)\]Conversely, bases such as sodium hydroxide (\textnormal{NaOH}) and the cyanide ion (\textnormal{CN}^-) are proton acceptors. In the case of the cyanide ion, the reaction with water is reversible and is represented as:\[\textnormal{CN}^{-}(aq) + \textnormal{H}_{2}\textnormal{O}(l) \rightleftharpoons \textnormal{HCN}(aq) + \textnormal{OH}^{-} (aq)\]In this equilibrium, the cyanide ion accepts a proton from the water, forming hydrocyanic acid and hydroxide ions, which indicates base behavior.

Dissociation of Electrolytes

The dissociation of electrolytes in water is a crucial process in acid-base chemistry. Electrolytes are substances that, when dissolved in water, break apart into ions and become capable of conducting electricity. The process of breaking down into ions is known as dissociation. Strong electrolytes, such as strong acids and strong bases, dissociate completely in water, while weak electrolytes partially dissociate forming a dynamic equilibrium between the undissociated species and the ions produced.

For instance, sodium hydroxide (\textnormal{NaOH}) is a strong base and a strong electrolyte; it completely dissociates as shown below:\[\textnormal{NaOH}(s) \rightarrow \textnormal{Na}^{+}(aq) + \textnormal{OH}^{-}(aq)\]In contrast, the weak base cyanide (\textnormal{CN}^-) only partially associates with water to form hydrocyanic acid, which can be represented as a reversible reaction:
\[\textnormal{CN}^{-}(aq) + \textnormal{H}_{2}\textnormal{O}(l) \rightleftharpoons \textnormal{HCN}(aq) + \textnormal{OH}^{-} (aq)\]Understanding electrolyte behavior in water is crucial for predicting the outcomes of acid-base reactions and their impacts on solutions' physical properties like electrical conductivity.