Acids, Bases, pH and Buffers

Acids

• An acid dissolves in water to donate H+

• In water the H+ reacts with water to make the hydronium ionH+ + H2O H3O+

• Acids therefore produce hydronium ions in waterHCl + H2O H3O+ + Cl-

Bases

• A base is something that accepts an H+

• Examples NaOH and NH3

NaOH + H+ H2O + Na+

• Or more accurately ….NaOH + H3O+ 2H2O + Na+

NH3 + H2O NH4+ + OH-

Neutralization

• When an acid reacts with a base an H+ reacts with an OH- to make water

• HCl + NaOH H2O + NaCl• If we write the ionic substances as ions• H++Cl-+Na++OH- H2O+Na++Cl-

• In a complete neutralization the amount of H+ = OH-

pH=-log[H+]

• The pH of a solution tells us if it is acidic, basic or neutral

Measuring pH

• With a pH meter• With an indicator (red cabbage here)• pH is actually measuring [H+] • In pure water • Kw = [H+][OH-] = [10-7M][10-7M] = 10-14

• In acid or base [H+][OH-] = 10-14

• Example, if an acid [H+] = 10-4M

• pH = -log[H+] = -log[10-4] = 4

Buffers

• Buffers maintain the pH ([H+]) of a solution• Example blood must be maintained between pH = 7.35-7.45• Buffers can work as both an acid or a base so they can

neutralize small amounts of acid and base• pH of blood is maintained by bicarbonate HCO3

-

• HCO3- + H+ H2CO3

• HCO3- + OH- H2O + CO3

2-

• In this experiment we will study the effect of an acid and base on a buffer and compare it with a non-buffer

pH color using red cabbage indicator

• 13 test tubes arranged in a rack• Pour 3-4mL of of each buffer into a separate test tube• label the pH of each buffer on each test tube

Add 2-3 mL of the red cabbage indicator to each test tube and describe the color of each

• Keep as a reference

Measuring pH

• 3-4 mL of the samples (shampoo, conditioner, mouthwash, detergents, fruit juice, aspirin etc) into a test tube

• Add 2-3 mL of the red cabbage solution and estimate the pH from the color

• Measure the pH with the pH meter

Buffers

Adding AcidIn four separate test tubes add 10.0mL of the following• H2O, 0.1M NaCl, a buffer with a high pH, a buffer with a low

pH• Add 2-3 mL of cabbage indicator to each• Add 5 drops 0.1 M HCl and stir• Determine the pH• Determine the change in pH if anyAdding Base• Repeat the above instead adding 5 drops 0.1M NaOH