Environmental Chemistry Lesson # 2 Acids & Bases

Properties of Acids & Bases Acids and bases are water soluble compounds that share some properties with ionic compounds (like sodium chloride) and molecular compounds (like hydrocarbons).

Test/Property Acid Base Contains Hydrogen Ions (H+) Hydroxide Ions (OH-) Taste Sour Bitter Feel Sticky Slippery Reaction with Proteins NR Denatures (breaks down) Reaction with Active Metals Create H2 gas – bubble & fizz Reaction with Carbonates Create CO2 gas – bubble & fizz Litmus Paper Red Blue Phenolphthalein Stays Colourless Turns Pink Bromothymol Blue Turns Yellow Stays Blue Conductivity Conductor Solubility Ionizes in Water

Ionization Sulfuric Acid in Water: Aluminum Hydroxide in Water:

Strong and Weak Acids Most acids are highly soluble in water, and the more soluble they are, the more they can conduct electricity. This is due to ionization. Hydrochloric acid is a better conductor of electricity than acetic acid because every HCl molecule completely dissolves (ionizes) in water to form hydrogen and chlorine ions. In acetic acid, only just over 1% ionizes. Because of the difference in ionization, hydrochloric acid is considered a strong acid, and acetic acid to be a weak acid. The same holds true with bases.

Concentration vs. Strength The concentration of a substance depends on the amount of solute dissolved in the solvent. A dilute solution has a relatively small amount of solute per unit volume of solution. A concentrated solution has a relatively large amount of solute per unit volume of solution. Both that strong and weak acids can be concentrated or dilute – there is a difference between the amount of solute and the dissociation/ionization of the solute.

pH The pH of a solution is the concentration of hydrogen ions in a solution. The pH scale runs from 0 to 14 at room temperature, with acids have a relatively low pH, bases have a high pH, and water having a neutral pH in the middle of the scale around 7. pH is expressed as the negative of the exponent to the base 10 of the hydrogen ion concentration. For example, a solution with a pH of 3 would have a concentration of 10-3 mol/L. A solution with a pH of 8 would have a concentration of 10-8 mol/L. pH = -log [H+] [H+] = 10 -pH

Example 1 What is the pH of a solution with a concentration of 2.5 x 10-4 mol/L?

Example 2 What is the pH of a solution with a concentration of 9.3 x 10-11 mol/L?

Example 3 What is the concentration of a solution with a pH of 3.7?

Example 4 What is the concentration of a solution with a pH of 12.5?

Acid-Base Neutralization When a strong acid and strong base are mixed, a neutralization reaction occurs. Neutralization is a double displacement reaction, where the products are always water, and a soluble salt. The hydrogen from the acid and the hydroxide from the water combine to form water. If the acids or bases are weak, a complete neutralization does not occur.

Example 5 Sulfuric Acid + Sodium Hydroxide →

Example 6 Aluminum Hydroxide + Phosphoric Acid →

Acid-Base Titration A titration is used to determine the concentration of a substance in solution. It is a method of quantitative chemical analysis. A known volume of the sample to be analyzed (either an acid or a base) is put into an Erlenmeyer flask. A buret contains a solution of an accurately known concentration, called a standard solution, or titrant. If you are analyzing an acid, then a base would be in the burette, and vice versa. Alternatively, the known solution can be in the flask, and the unknown can be in the buret. The titrant is added drop by drop until the reaction between the two chemicals is judged to be complete – this is usually determined by a colour change. This is called the endpoint. At least three trials of titration should be doing to accurately determine the concentration of the unknown solution. Knowing the mole ratios from the balanced equation, and the amounts (in L) of each substance used determines the concentration of the unknown solution.

Titration Setup

Example 7 A 10.0 mL sample of hydrochloric acid is titrated with a standardized 0.2 mol/L solution of sodium hydroxide. 13.05 mL of sodium hydroxide was used to neutralize the acid. What is the concentration of the hydrochloric acid?

Example 8 A 10.0 mL standard solution of 0.150 mol/L nitric acid is used to determine the concentration of sodium carbonate. 12.80 mL of sodium carbonate was used in the titration. What is the concentration of sodium carbonate?