UNIT 6 INTRODUCTION TO CHEMISTRY Chapter 21 Acids, Bases, and Salts

21A – ACIDS AND BASES

IONIZATION OF WATER Most acids and base are dissolved in water When substances are dissolved in water they become hydrated The substance is broken down into ions Can even happen to pure water and produces the hydronium ion and the hydroxide ion 2H2O H3O+ + OH- H3O+ + OH- 2H2O Constant but continuous change, reaches a dynamic equilibrium This equilibrium brings us to the definition of an acid and a base

ARRHENIUS DEFINITIONS Facet – Page 485 Arrhenius was the first to give simple and straightforward definitions for acids and bases – the Arrhenius Model Acid Any substance that produces hydrogen ions in a water solution Bare protons pulled from molecular compounds by the action of water molecules Highly unstable and instantaneously bond to water molecules to form hydronium ions Base Any substance that produces hydroxide ions in a water solution This model helped advance science during its time; however, it did not explain every kind of acid-base reaction His model assumed that they only existed in water solutions which is not true

BRONSTED-LOWRY DEFINITION Two chemist Johannes Bronsted and Thomas Lowry came up with the Bronsted-Lowry Model Acids are any compound that is a proton donor (H+) Base are compounds that accept protons Acids and bases do not have to be dissolved in aqueous (water) solutions Some compounds can behave as either acids or bases depending on the other reactants

POLYPROTIC ACIDS Acids can contribute different amounts of H+ ions depending on their chemical formula Those that only have one H+ ion are called monoprotic acids HCl Those that have two or more H+ ions are called polyprotic acids It is more difficult to remove H+ each time Look at table 21-1 on page 488

CONJUGATE ACIDS AND BASES Conjugate base The ion that remains after a molecular compound has donated a hydrogen ion Every acid compound therefore becomes its own conjugate base after donating a proton Conjugate acid The molecule that accepts the H+ ion H2O + H2O OH- + H3O+

PROPERTIES OF ACIDS & BASES Sourness of foods, due to the presence of hyronium ions You should never taste a chemical in the lab to see if it is an acid! Conducts electricity Corrodes metals Reacts with carbonates and bicarbonates to form carbon dioxide gas

PROPERTIES OF ACIDS & BASES Bitter taste Slippery feel because they react with the oils on your skin Do not taste or put a chemical on your skin to see if it is a base! Emulsify, break down, organic materials Oven cleaners Unclogging sink drains Severe burns Acids and bases cancel each other out Neutralization reactions

21B - SALTS

SALTS Ionic compounds produced from an acid-base reaction, or neutralization reaction Double-displacement reaction that produce a salt and water Example on next slide

VARIETY OF SALTS

21C – ACIDITY AND ALKALINITY

ACID STRENGTH Determined by the percentage of its first available hydrogen atoms that ionize

BASE STRENGTH Measured by the number of proton acceptors it produces in an aqueous solution.

STRENGTHS

THE pH SCALE pH scale 1-14 Less than 7 = acidic Greater than 7 = basic or alkaline Equal to 7 = neutral The lower the pH the more acidic The higher the pH the more basic

MEASURING SOLUTION pH pH indicators Change color in the presence of an acid or base Litmus One of the oldest know indicator compounds Red will change to blue if basic Blue will change to red if acidic Phenolphthalein Colorless in neutral or acidic Deep pink in basic Universal indicators Manufactured papers or solutions that can read the pH to the nearest tenth

pH METERS A more objective technique to measure pH values Measures voltage with a pH probe