Chapter 25 Acids and Bases

Acids Contain at least one hydrogen atom that can be removed when the acid is dissolved in water Example: HCl (hydrochloric acid) You can recognize an acid by the chemical formula (starts with a H ion) Some acids can burn and are dangerous to handle Most acids in foods are safe to eat – Pickles, lemons

Properties of Acids An acid is a substance that produces hydrogen ions in a water solution Ability to produce H + ion is what gives acids their characteristic properties When acid dissolves in water, H + ions interact with water molecules to form H 3 O + ions, which are called hydronium ions

Properties of Acids Several common properties: – All acids taste sour Never use taste to determine the presence of an acid – Acids are corrosive “eat away” materials they come into contact with Can use an indicator to determine presence of an acid An indicator is an organic compound that changes color in an acid and base (Litmus paper turns red)

Common Acids Many foods contain acids Examples: citric acid in fruits, lactic acid in yogurt, and acetic acid in anything pickled Your stomach uses hydrochloric acid (HCl) to help digest food Sulfuric, phosphoric, nitric and hydrochloric acid are all vital to industry

Bases Bases feel slippery, like soapy water (soap is a base) Important in many cleaning materials Bases can be defined two ways: – Any substance that forms hydroxide ions (OH - ) in a water solution – Any substance that accepts H + from acids

Properties of Bases Opposites of acids Many are crystalline solids Feel slippery Bitter taste Strong bases are corrosive, and can be just as dangerous as acids React with indicators to produce a change in color (Litmus turns blue)

Common Bases Cleaning products in the home Shampoos, hand soap, laundry soap, oven cleaner, drain cleaner, etc.

Solutions of Acids and Bases When acids are added to water, the H + ion leaves the acid and joins the water to form a hydronium ion (H 3 O + ) Example: – HCl + H 2 O → H 3 O + + Cl - When bases are added to water, the OH - group completely dissociates (leaves) from the ion and the two become individual ions in solution Example: – NaOH (aq)→ Na + + OH -

Strength of Acids and Bases Chapter 22

Strong and Weak Acids and Bases Acids and bases can be classified as strong or weak Strong acids ionize (separate) completely in water Weak acids only partially ionize A strong base is going to completely separate in solution A weak base does not completely separate in solution

pH of a Solution pH of a solution is a measure of the concentration of H+ ions in the solution *Be careful not to confuse the terms “weak base/acid” and “strong base/acid” for pH The pH measures how acidic or basic the solution is To indicate pH a scale ranging from 0-14 has been created

pH of a Solution

Solution with a pH of lower than 7 are acidic The lower the value is the more acidic the solution is Solution with a pH greater than 7 are basic A solution with a pH of exactly 7 is neutral (ex: pure water)

pH of a Solution There are many ways to determine pH of a solution A universal indicator undergoes a color change in the presence of an acid or base The color of the pH paper is matched with colors in a chart to find the pH A pH meter can be used; operated by immersing a probe into the solution; convenient for testing in the field

pH of a Solution

The pH of blood must remain between Enzymes (act as catalysts) cannot work outside of this pH range Blood contains compounds called buffers, which allow you to eat foods outside of this pH range, but do not affect your body’s overall pH Buffers are solutions containing ions that react with additional acids or bases to minimize the effect on pH

pH of a Solution

Salts Chapter 22

Neutralization Ads for antacid tablets (Tums, Pepcid AC) describe how effectively these products neutralize stomach acid What does this mean? Normally your stomach is acidic (HCl), too much acid can cause discomfort Antacids contain bases that lower acid concentrations Bases and acids “equal” each other out

Neutralization Neutralization is a chemical reaction between an acid and a base that takes place in a water solution A salt is formed in a neutralization reaction A salt is a compound formed when the negative ions from an acid combine with the positive ions from a base Example: HCl + NaOH → H 2 O + NaCl Acid-base reaction: acid + base = water + salt

Salts Salts are essential for many animals Humans need salt, because they lose salt during perspiration (sweating) Many salts will form when acids react with metals in a single-displacement reaction The metal displaces the hydrogen from the acid, and a salt is formed along with a hydrogen molecule

Titrations Sometimes we will need to know the concentration of an acidic or basic solution To determine the concentration, we can use a process called titration Titration is a process in which a solution of known concentration is used to determine the concentration of another solution (concentration unknown)

Titration The solution in which the concentration is known is called the standard solution The standard solution is added slowly to the solution of unknown concentration to which an acid/base indicator has been added If the unknown solution is a base, an acidic standard solution is used If the unknown solution is an acid, a basic standard solution is used

Titration You begin by adding a few drops of indicator to the solution of unknown concentration Then, slowly add the standard solution one drop at a time Once you observe a color change in the unknown solution, you have reached the endpoint You can determine the concentration of the unknown solution by looking at how much of the standard solution it took before you reached the endpoint

Titration Many natural substances are acid-base indicators Red cabbage varies from a deep red color at pH 1 to lavender at pH 7 to yellow-green at pH 10 Grape juice Roses Radishes