NOTES 10 – Acids, Bases, & pH

Ion Review An ion is an atom that gains or loses electrons giving it an overall positive or negative charge Lose electrons = positively charged ion Gain electrons = negatively charged ion Sometimes a compound acts like an ion Hydrogen atoms have 1 proton and 1 electron Compounds that take hydrogen’s electron can become negative ions (more electrons than protons) The leftover hydrogen proton becomes a positive hydrogen ion

Water Molecules Form Ions Water molecules break apart into ions and then reform into molecules Water forms two different ions Hydrogen Ions = H+ Hydroxide Ions = OH- H2O  H+ + OH- When water breaks into ions, it is called dissociation (Water dissociates into ions) In pure water there are equal numbers of hydrogen and hydroxide ions

Dissociation of Water In any sample of water, very few of the molecules are dissociated at any one time About 1 in 550 million  There is constant change As one hydrogen ion reattaches to a hydroxide ion to form a water molecule, another water molecule dissociates to replace the hydrogen ion and the hydroxide ion in solution

What are acids? Acid – a compound that increases the number of hydrogen ions (H+) when dissolved in water In water, acids dissociate producing H+ ions Acidic solutions (acid substance + water) have more hydrogen ions than hydroxide ions (More H+ than OH-) Acids taste sour Acids can cause burns Acids can be good conductors of electricity EX. acetic acid, citric acid, ascorbic acid

Strong Vs. Weak Acids Strong acids ionize completely Every molecule of the acid produces H+ ions and negative acid ions Strong acids have a pH in the range of 0 to 3 EX. Nitric acid in water HNO3  H+ + NO3- Weak acids do not ionize completely Some molecules form ions, some do not Weak acids have a pH above 3 EX. Acetic acid in water CH3COOH  CH3COO- + H+

Acids Are Useful But Can Be Dangerous Acids are used for cleaning, in batteries, making fertilizers, dyes, etc. Foods contain acids (vinegar & citrus fruits) Stomach acid helps digest your food Acids in concentrated forms can cause severe skin damage Acid vapors can damage the eyes, mouth, and lungs

What are bases? Base – a compound that increases the number of hydroxide ions (OH-) when dissolved in water In water, bases dissociate producing OH- ions Basic solutions (base substance + water) have more hydroxide ions than hydronium ions (More OH- than H+) Bases taste bitter Bases feel slippery and may cause burns EX. baking soda, bleach, ammonia

Strong Vs. Weak Bases Strong bases are ionic compounds that contain hydroxide ions (OH-) When dissolved in water, a strong base dissociates (separates) forming hydroxide ions EX. Sodium hydroxide in water NaOH  Na+ + OH- Weak bases do not contain hydroxide ions, but form them when mixed with water EX. Ammonia and water NH3 + H2O  NH4+ + OH-

Bases Are Useful But Can Be Dangerous Bases are used to make soaps, to clean drains, to make cement, fertilize soil, etc. Bases attack living tissue very quickly In concentrated form, bases can be very harmful Even in diluted form, some bases can be fairly dangerous

Acids, Bases, & Electricity Strong acids and strong bases are good conductors of electricity (electrolytes) Electrolyte – a substance that dissolves in water forming a solution that conducts electricity EX. A solution of sulfuric acid and water conducts electric current in car batteries

What is pH? pH – a number used to indicate how acidic or basic a solution is The pH scale goes from 0 to 14 Solutions with a pH below 7 are acids Lower number = stronger acid EX. A solution with a pH of 2 is a stronger acid than a solution with a pH of 5

What is pH? Solutions with a pH above 7 are bases Higher number = stronger base EX. A solution with a pH of 13 is a stronger base than a solution with a pH of 9 Pure water has a pH of 7 and is neither acid nor base

pH measures the relative amount of H+ ions in a solution

Neutralization To neutralize means to make the pH of a solution 7 Acids and bases can neutralize each other If the amounts are equal: strong base + strong acid = neutral solution strong base + weak acid = weaker base solution weak base + strong acid = weaker acid solution

Neutralization Reactions Acids and bases react to form water and salt Salt – an ionic compound that is a product of a neutralization reaction between an acid and a base The base provides the OH- and the acid provides the H+ OH- + H+  H2O The other ions from the acid and base form the salt EX. sodium hydroxide + hydrochloric acid NaOH + HCl  NaCl + H2O

What is acid rain? Normal rain is slightly acidic, with a pH over 5.7 Acid rain – rain with a pH below 5.5 Acid rain can harm plants Acid rain can harm animals, particularly those that live in the water Acid rain can damage buildings and other structures

How does acid rain form? Burning fossil fuels releases sulfur and nitrogen compounds into the atmosphere Sulfur dioxide reacts with moisture in clouds to produce sulfuric acid Nitrogen oxides react with moisture producing nitric acid The acids drop the pH of rainwater below 5.5 The largest natural cause of acid rain is the release of sulfur dioxide by volcanoes

Acids & Bases - Main Idea Acids and bases react in water to form ions Acid and base solutions contain large amounts of positive or negative ions These solutions are highly reactive When mixed, acid and base solutions can neutralize one another pH scale measures the strength of an acid or base