Acids and Bases In this lesson, you will learn the definitions for acids and bases and be able to explain how acids and bases interact.

Acids and Bases You may already know quite a bit about acids and bases. You know that acids can be corrosive. You know that they taste sour. You know that acids react with bases. If asked to define an acid or a base, what would you say? Scientists have disagreed on the definitions over the years, but two sets of definitions are now accepted.

Acids and Bases - Arrhenius According to the Arrhenius definition of an acid, an acid is any substance that dissociates in solution (when dissolved in water) to release H+ ions. These H+ ions join up with existing H2O molecules to form H3O, hydronium, ions. For example: HCl dissolves in water to form H+ and Cl- ions. The H+ ions meet up with H2O molecules. They join to form H3O.

Acids and Bases - Arrhenius According to the Arrhenius model, then: Acids must be aqueous solutions formed by ionic compounds with hydrogen in them. Acids have a high concentration of hydronium (H3O) molecules. Example: hydrogen chloride – When dry, HCl crystals do not qualify as an Arrhenius acid. When in an aqueous solution, they dissociate to increase the hydronium concentration, making the aquesous solution of HCl an Arrhenius acid.

Acids and Bases - Arrhenius Bases have a related definition, also involving solutions. According to the Arrhenius model, a base is any solution with a high concentration of OH- (hydroxide) ions. A base is formed when an ionic compound is dissolved in a solvent (generally water) into a metal (or H) and one or more hydroxide ions.

Acids and Bases - Arrhenius For example: NaOH dissolves in water to form Na+ and OH- ions. The presence of the hydroxide ions in the solution make that solution a base. By the Arrhenius model’s definition, acids and bases are always solutions. Acids have hydronium ions, bases have hydroxide ions.

Acids and Bases – Bronsted-Lowry Two scientists came up with another model for acids and bases. This model is complementary to the Arrhenius definition. Simply put, and acid is an H+ donor, while a base is an H+ acceptor.

Acids and Bases – Bronsted-Lowry Sometimes the Bronsted-Lowry definition of acids says it is a “proton donor”. That makes sense, as an H+ ion is a hydrogen atom (one proton, no neutrons, one electron) that has lost its electron. It is, therefore, just a proton. Go to http://www.visionlearning.com/library/module_viewer.php?mid=58 Read and do the tutorial.

Acids and Bases - Reactions An acid will react with most metals to release hydrogen gas. HCl + Mg  MgCl2 + H2 Why is the formula for magnesium chloride MgCl2? Balance that chemical equation. Write it down. Did you add just one coefficient?

Acids and Bases - Reactions 2HCl + Mg  MgCl2 + H2 What kind of reaction is that? Magnesium took the place of the hydrogen in the compound, because magnesium is more active. You conducted this reaction or a similar one in the single-replacement/activity series lab. Here is what happened: 1. The HCl dissociated into H+ and Cl- ions. 2. The magnesium bonded with the Cl- ions. 3. The hydrogen paired up, as they do. Why do they do that? 4. The hydrogen gas molecules bubbled up and out.

Acids and Bases - Reactions So, now you know that acids react with most metals. When that reaction happens, something is released. What is released? Acids also react with bases in a process called neutralization. A neutralization reaction always produces two products; a salt and water. Notice I said a salt. A salt is any ionic compound formed in an acid-base reaction. Acid + Base  Water + Salt

Acids and Bases - Reactions NaHCO3 +HCl  H2O + NaCl + CO2 The equation above summarizes the reaction of hydrochloric acid with baking soda. People used to mix up an aqueous solution of baking soda to treat an upset stomach. Upset stomach is usually caused by an excess of stomach acid, which is hydrochloric acid. The steps that actually happen are these: HCl is dissolved in water to produce H+ and Cl- ions. The excess of H+ ions in your stomach creates a burning sensation. NaHCO3 decomposes in water to form NaOH and CO2. The CO2 bubbles away.

Acids and Bases - Reactions 3. The NaOH dissolves in water to form NA+ and OH- ions. 4. The H+ ions from the acid meet up with the OH- ions from the base, forming water. Every H+ joins with an OH- to form a water molecule. 5. The Cl- ions from the acid bond with the Na+ ions from the base, forming a salt, sodium chloride HCl  H+ + Cl- Which step is this? NaHCO3   NaOH + CO2   Which step is this? Na+ + OH- + H+ + Cl-  H2O + NaCl Which step(s) is this? An acid reacts with a base to form ___________ and a _________ in a neutralization reaction.

Acids and Bases - Reactions You try: Sulfuric acid (H2SO4) reacts with calcium hydroxide. (You figure out the formula – use your PT of E and the chart of polyatomic ions on the back). Write the balanced chemical equation for the reaction. You know that one product is water. The other will be formed by the anion from the acid and the cation from the base.

Acids and Bases - Reactions H2SO4 +  Calcium hydroxide… hmmm… Ca has a 2+ charge. Hydroxide is OH-. H2SO4 + Ca(OH)2  H2SO4 + Ca(OH)2  H2O + Hmmm… The sulfate from the acid and the calcium from the base are not yet accounted for on the product side…

Acids and Bases - Reactions H2SO4 + Ca(OH)2  H2O + CaSO4 Oh, no! There is something wrong with that equation! What is wrong? Count everything. It’s not balanced. Balance the chemical equation fro the neutralization of sulfuric acid and calcium hydroxide. H2SO4 + Ca(OH)2  2H2O + CaSO4