Hydrogen Ions and Acidity

Hydrogen Ions from Water Water is highly polar – what does that mean? Water particles are in continuous motion If they possess enough energy, a H + can be transferred from one water molecule to another The particle that remains once the H + leaves is OH - The water molecule gaining the H + becomes hydronium (H 3 O + ) H 2 O (l) + H 2 O (l) → H 3 O + (aq) + OH - (aq)

Self-ionization of water Another way of writing the ionization of water is: H 2 O (l) ↔ H + (aq) + OH - (aq) Hydronium and hydrogen ions can be written interchangeably Both examples show water forming ions The equilibrium concentration for both hydrogen and hydroxide ions is very small at 25°C Each is only 1 x M When both [H + ] and [OH - ] are equal in concentration, the solution is said to be neutral

Ion Product Constant for Water In any aqueous solution, as hydrogen ion concentration increases, hydroxide ion concentration decreases If hydrogen ions were to be added, the equilibrium would shift and the hydroxide ion concentration would decrease The product of the hydrogen-hydroxide concentration is 1 x In other words, the K w = [H + ][OH - ] = 1 x

Sample Problem, p. 596 Complete 9, 10, p. 596

Acidic and Basic Solutions When acids form aqueous solutions, there is more H + ions than OH - (coming from the ionization of water) The [H + ] is greater than 1 x M Basic solutions are the opposite The hydroxide ion concentration is greater than the hydrogen ion concentration The [OH - ] is greater than 1 x M They are otherwise known as alkaline solutions

Calculating pH (power of hydrogen) Ranges from 0 – 14 Neutral solutions have a pH of 7 The closer the pH is to zero, the more acidic it is; the closer it is to 14, the more basic it is To calculate pH from the hydrogen ion concentration, pH = -log[H + ] Figure 19.9, p. 597

Calculating pOH pH + pOH = 14 pOH = -log[OH - ] Express concentrations in scientific notation When writing the pH or pOH, the concentration of the solution is equivalent to the same number of digits after the decimal of the value itself Ex. 1.0 x = 5.00 (two digits in the base of the scientific notation – two digits after the decimal)

When is pH not a whole number? Most of the time… As a result, it is not easy to make a simple mental calculation. Instead, use the logarithmic equation to convert from concentration to pH or pOH. Ex. [H+] = 4.2 x M pH = -log[[4.2 x ] = = 9.38 Questions 11-12, p. 599

Calculating [H + ] from pH Rearrange the equation pH = -log[H + ] [H + ] = - antilog pH Using K w, in addition to pH/pOH, [H + ]/[OH - ], you can solve for any unknown

Sample Problem 19.3, p. 600

Sample Problem 19.4, p. 601 Questions 15-16, p. 601