Acids and Bases

Dissociation of Water On average, 1 out of every 550 million water molecules dissociates (breaks down) into a hydrogen (H+) ion and a hydroxide (OH–) ion. Water molecules are constantly dissociating and reforming The OH- ion and the H+ ion easily react to create acids and bases

What is neutral? When we say something is neutral, we mean that the concentration of H+ ions and OH- ions are equal. [H+] = [OH–] [ ] = concentration Water is a neutral compound, there are no excess H+ ions or OH- ions

What is an Acid? An acid is a compound that dissolves in water to make a solution that contains more H+ ions than OH– ions. Example: Hydrochloric Acid (HCl) When dissolved in water HCl produces the H+ ion, making it an acid

Properties of Acids Create a sour taste in foods – fruits and candy Dissolve metals and other materials Cause serious skin and tissue damage

Common Acids Stomach Acid – Hydrochloric Acid (HCl) Battery Acid – Sulfuric Acid (H2SO4) Citric Acid – Sour taste in fruits Acetic Acid – Vinegar

What is a Base? A base is a compound that dissolves in water to make a solution that contains more OH– ions than H+ ions. Example: Sodium Hydroxide (NaOH) When dissolved in water NaOH produces the OH- ion, making it an base

Properties of Bases Create a bitter taste in foods – herbs, coffee, nuts Feels slick and slippery when handled Cause serious skin and tissue damage

Common Bases Soap Baking Soda Bleach Ammonia – Fertilizer, cleaners Magnesium hydroxide – antacids and laxatives

Strong vs. Weak Acids & Bases Strong acids and bases A strong acid or base dissociates completely in water so that each molecule contributes one H+ ion (acids) or one OH– ion (bases). Weak acids and bases A weak acid or base only dissociates partially or produces relatively few H+ or OH– ions in solution.

Acids & Bases Summary Acids make more H+ ions and bases make more OH– ions. The more H+ and OH– ions that are made the stronger the acid or base Neutral substances have an equal amount of H+ ions and OH- ions Acids Neutral Bases [H+] > [OH-] [H+] = [OH-] [H+] < [OH-]

Alternate Definition of Acids & Bases For a molecule or ion to act like an acid by donating a proton, another molecule or ion must act like a base by accepting the proton! You cannot have something acting like an acid without something else acting like a base and vice versa.

Water One of the most important properties of water is its ability to act as both an acid and a base. In the presence of an acid, water acts as a base and accepts protons (H2O + H+ → H3O+) In the presence of a base, water acts as an acid and donates protons (H2O → H+ + OH–)

The pH Scale pH tells you whether a solution is an acid, a base, or neutral. The pH scale ranges from 0 to 14.

The pH Scale pH Range Description Water is neutral and has a pH = 7 0 – 3 Strong Acid 4 – 6 Weak Acid 7 Neutral 8 – 11 Weak Base 12 – 14 Strong Base Water is neutral and has a pH = 7 The further away you get from 7, the stronger the acid or the base becomes

pH Indicators- Litmus Paper Red Litmus Paper Stays red with an acid Turns blue with a base Blue Litmus Paper Turns red with an acid Stays blue with a base

pH indicator – pH paper When the paper comes in contact with a chemical it turns the color of the pH. Match the strip to the color key on the package

pH Indicator- Red Cabbage Natural Indicator – use the juice of red cabbage to determine the pH of any substances

Calculating pH for Acids To calculate pH, you must do the following: 1. Determine the molarity of H+ ions. 2. pH = –log(molarity of H+) Example: A solution of acetic acid (HC2H3O2) has an H+ concentration of 5 × 10–5 M. What is the pH of the solution?

Calculating pH for Bases To calculate the pH of bases you must do the following: Find the concentration OH– ions Use the equation pH = 14 + log[OH–] Example: Find the pH of a 0.012 M sodium hydroxide (NaOH) solution.

Calculators and solving for pH If the concentration is in scientific notation… Look for the EE button! Type 5 × 10–5 as 5E-5 OR Use parenthesis! Type (5 × 10–5)

Calculators and solving for pH 2. Using the log button… Some calculators have you enter concentration first, then press log. Others you press log first and then enter the concentration. Don’t forget that finding pH of an acid it is a negative log!

Acid- Base Reactions After the reaction is done both the acid and the base have been neutralized, and the solution now has a pH of 7