CHEM1312H - Autoionization of Water and the pH scale

water an amphoteric substance

-water is a unique substance in the sense that it can act as an acid and a base

-a substance that can act as an acid and a base is called an amphoteric or amphiprotic substance

-an amphoteric substance can accept or donate a proton

autoionization of water

-since water is an amphoteric species, it can act as an acid and a base with itself in a process called autoionization to produce H₃O⁺(aq) and OH^-(aq) at equilibrium

-since water dissociates at equilibrium, we can write the equilibrium constant expression for water, K_w, also known as autoionization constant or ion-product for water

-note that the autoionization of water is an endothermic process (see example on slide)

autoionization (H₃O⁺and OH^-)

-in pure water, the concentrations of H₃O⁺ and OH^- are equal and therefore the solution is neutral

-in an acidic solution, the concentration of [H₃O⁺] > [OH^-]

-in a basic solution, the concentration of [H₃O⁺] < [OH^-]

the pH scale definition

-the pH scale is a concise way of describing the H₃O⁺ ion concentration and hence the acidity or basicity of a solution

-pH is defined as the negative log of the hydronium ion concentration

the pH scale and sig fig rules

-for logarithms such as pH, the number of sig figs is the number of digits to the right of the decimal including all zeros

-therefore, the number of decimal places in the pH value is equal to the number of sig figs in the original concentration

-for antilogs (10^-pH), the number of decimal places in the pH value is equal to the number of significant figures in the concentration

-a neutral solution has a [H₃O⁺] = 1.0 × 10^-7 at 25°C and a pH of pure water is 7 when calculated

-we can use the pH of water at 25°C to determine if a solution is acidic, basic, or neutral

pH values

-if the pH < 7 then the solution is acidic

-if the pH > 7 then the solution is basic

-if the pH = 7 then the solution is neutral

the pOH scale

-like the pH scale, the pOH scale is directly related to [OH^-]; pOH = -log[OH^-]

-if the pOH > 7 then the solution is acidic

-if the pOH < 7 then the solution is basic

-if the pOH = 7 then the solution is neutral

pH and pOH

if we take the negative log of K_w, pH + pOH = 14

the pH scale

-the pH scale is logarithmic, meaning that an increase or decrease of an integer value changes the concentration by a tenfold

-the pH range does not have an upper nor lower bound (pH can be a value below zero or above 14)