Introduction to Acids and Bases

Introduction Organisms are often very sensitive to the effect of acids and bases in their environment. They need to maintain a stable internal pH in order to survive—even in the event of environmental changes. Many naturally occurring biological, geological, and man-made chemicals are capable of stabilizing the environment’s pH. This may allow organisms to better survive in diverse environments found throughout the earth.

Introduction Cont. Acids and bases are a way of classifying compounds based upon what happens to them when you place them in water. An acid is a substance that yields an excess of hydrogen ions when dissolved in water. A base is a substance that yields an excess of hydroxide ions when dissolved in water.

Definitions of Acids and Bases An acid is a substance that breaks into ions in an aqueous solution. A Base (is a substance that breaks into ions in an aqueous solution. Note: aqueous solution is any solution where is the solvent.

Common Acids Vinegar (acetic or ethanoic acid) Citric Acid

Characteristics of Acids Sour taste (for edible acids) May burn your skin Turn litmus paper from blue to red.

Other Examples of Acids HCl Hydrochloric Acid H2SO4 Sulfuric Acid CH3COOH Acetic Acid (Vinegar)

Common Bases Ammonia

Characteristics of Bases Bitter taste (for edible bases only) Can be poisonous and corrosive. Feels slippery Turn litmus from red to blue.

Other examples of Bases Sodium Hydroxide NaOH Potassium Hydroxide KOH Ammonia NH3

Three Definitions of Acids and Bases Svante Arrhenius (Swedish 1887) Bronsted- Lowry (Danish and English 1923 independently proposed definition) Gilbert Lewis (American 1920’s)

Arrhenius Definition of Acids and Bases Acids produce hydrogen ions (H+) in an aqueous solution, while bases produce hydroxide ion (OH-). Acid: HCl (aq) H+ (aq) + Cl- (aq) Base: NaOH(aq) Na+(aq) + OH-(aq)

Bronsted- Lowry Definition of Acids and Bases An acid is a proton (H+) donor. A base is a proton (H+) acceptor. Example: HF + H2O H3O+ + F- Acid Base H3O+ : hydronium ion

Lewis Definition of Acids and Bases An acid accepts a pair of electrons. A base donates a pair of electrons. This is a more general definition than the previous two. Example: :NH3 + H+  NH4+

Three Models of Acids and Bases Definition of Acid Definition of Base Arrhenius H+ producer OH- producer Bronsted-Lowry H+ donor H+ acceptor Lewis Electron-pair acceptor Electron-pair donor

Importance of WATER Water acts as an acid and as a base. It is amphoteric

Acids A dilute acid has lots of water and a small amount of acid A concentrated acid has lots of acid and not much water so must be handled carefully A strong acid releases lots of H+ A weak acid releases fewer H+

The extent to which a compound is acidic or basic is measured by the pH scale.

Measuring acid strength? To decide if something is an acid or a base we can use an indicator. Litmus and Universal Indicator are examples of indicators. They change colour depending on if they are in an acid or a base.

What is the pH scale? The pH scale measures how acidic or basic a solution is.

Identifying Acids and Bases Acids have a ph from 0-7 Lower pH value indicates a stronger acid. Bases have a pH from 7-14 Higher pH value indicates a stronger base.

Did we Miss something?? What happens when the pH of a substance is 7? Ans: A pH level of 7 indicates a Neutral Substance i.e: Water!

Why Learn about Acids & Bases? What do you think is the pH level of NYC tap water? The pH of a swimming pool must be checked periodically. Why? Is it important for Lakes & Rivers to maintain a certain pH?

Situations in which pH is controlled “Heartburn” Planting vegetables and flowers Fish Tanks and Ponds Blood Swimming pools

Acid Rain Pollution in the air (sulfur dioxide, carbon dioxide, nitrogen dioxide) combines with water to form various acids. .

pH in the Digestive System Mouth-pH around 7. Saliva contains amylase, an enzyme which begins to break carbohydrates into sugars. Stomach- pH around 2. Proteins are broken down into amino acids by the enzyme pepsin. Small intestine-pH around 8. Most digestion ends. Small molecules move to bloodstream toward cells that use them

Review

Acids Produce H+ (as H3O+) ions in water Produce a negative ion (-) too Taste sour Corrode metals React with bases to form salts and water

Bases Produce OH- ions in water Taste bitter, chalky Are electrolytes Feel soapy, slippery React with acids to form salts and water

pH of Common Substances Timberlake, Chemistry 7th Edition, page 335