Acids and Bases Chapter 19 DHS Chemistry

Definition

Definition - Acids An acid is a molecule or ion that acts as a proton (hydrogen ion) donor. HCl + H 2 0  H Cl - acid

Definition - Base A base is a molecule or ion that acts as a proton (hydrogen ion) acceptor. HCl + NH 3  NH Cl - base

Examples - Acids An acid is basically a compound that produces hydronium (H ) ions when dissolved in water. Typically acids start with hydrogen in their formulas. Ex. HCl, H 2 S0 4, HNO 3

Examples - Bases A base is basically a compound that produces hydroxide ( OH - ) ions when dissolved in water. Most bases (not all) have OH in their chemical formulas. Ex. NaOH, KOH, Mg(OH) 2, NH 3

Properties - Acids sour taste –Like orange juice or vinegar contain hydrogen and most react with active metals to produce hydrogen gas

Properties - Acids turn blue litmus paper red (pink) react with bases to produce salt and water (neutralization)

electrolytes (may be strong or weak) © Prentice Hall Properties - Acids

bitter taste feel slippery turn red litmus paper blue react with an acid to produce salt and water (neutralization) electrolytes (may be strong or weak) Properties - Bases

Litmus Paper Aciase Relue B D

Common Ones

A. Common Acids AcidFormulaMajor Uses sulfuric acid H 2 SO 4 Car batteries, production of metals phosphoric acid H 3 PO 4 Found in soft drinks Nitric acid HNO 3 Production of explosives hydrochloric acid HClCleaning of metals

Common Bases BaseFormulaMajor Uses ammoniaNH 3 Refrigerant, household cleaners sodium hydroxide NaOHDrain cleaner potassium hydroxide KOHTo increase pH of acidic soils

Strength of Acids and Bases

One way to measure the strength of an acid or a base is to measure how much of the original molecule remains after it has been added to water. If little remains, the acid or base is strong. If a lot of the original molecule remains, the acid or base is weak.

Acids acids will form ions (electrolytes) when dissolved in water in a process called ionization. Ex: HCl(g) + H 2 O(l)  H 3 O + (aq) + Cl - (aq)

Bases Bases also will form ions (electrolytes) when dissolved in water. Ex: NaOH(s)  Na + (aq) + OH - (aq)

Strength Strong acids: Completely ionize. Weak acids: Partially ionize. REMEMBER STRONG AND WEAK ELECTROLYTES !

Do not confuse the terms strong and weak with the terms concentrated and dilute. Strength refers to what % of the original molecules convert to ions in water. Concentrated or dilute refer to how many total moles there are in water.

Strong Acids HCl, HBr, HI, HNO 3, H 2 SO 4 These acids ionize almost completely in water. *memorize those that are highlighted

Weak Acids HF, HCN, HC 2 H 3 O 2, H 2 CO 3 These acids weakly ionize in water. *memorize those that are highlighted

The strength of a base depends on its solubility in water.

Strong bases NaOH, KOH, Ca(OH) 2, LiOH, CsOH, RbOH, Sr(OH) 2 These bases ionize almost completely in water. *memorize those that are highlighted

Weak bases CH 3 N 2, NH 3, NaCN, Mg(OH) 2 These bases weakly ionize in water. *memorize those that are highlighted

A. Neutralization Reactions  When an acid and a base react, they are neutralized and a salt and water are the produced: Acid + Base  Salt + Water

Generic Neutralization Reaction (not in your notes) HA + BOH  B A + H 2 O ex. HCl(aq) + NaOH(aq)  NaCl (aq) + H 2 O(l) acid basesaltwater Cation Anion

Add this to your notes! All neutralization reactions are double-replacement reactions

 The salt is a combination of the cation from the base and the anion from the acid. Water is produced if a hydroxide base is used. (This is different from table salt!) A. Neutralization Reactions

Ex. The salt is highlighted in each case. HCl + NaOH → NaCl + H 2 O HC 2 H 3 O 2 + KOH  KC 2 H 3 O 2 + H 2 O H 2 SO NH 4 OH  (NH 4 ) 2 SO 4 + H 2 O HC 2 H 3 O 2 + NH 4 OH  NH 4 C 2 H 3 O 2 + H 2 O Strong acid plus strong base Weak acid plus strong base Strong acid plus weak base Weak acid plus weak base

Practice 1. Complete and balance these reactions involving acids and bases. a) nitric acid + potassium hydroxide b) calcium hydroxide + carbonic acid (H 2 CO 3 ) c) acetic acid + sodium hydroxide HC 2 H 3 O 2 (aq) + NaOH(s)  NaC 2 H 3 O 2 (aq) + H 2 O(g) HNO 3 (aq) + KOH(aq)  KNO 3 (aq) + H 2 O(l) Ca(OH) 2 (aq) + H 2 CO 3 (aq)  CaCO 3 (aq) + 2H 2 O(l)

B. pH Scale

Hydronium vs Hydroxide HydroniumH 3 O + –A hydrogen ion in water –H + + H 2 O  H 3 O + –H + and H 3 O + used interchangeably –For acids HydroxideOH - –For bases

1. Background Any aqueous solution contains both hydronium ions and hydroxide ions. This stems from the fact that water will ionize to a very small amount:

Note: for pure water, the number of hydronium ions is equal to the number of hydroxide ions. H 2 O(l) + H 2 O(l)  H 3 O + (aq) + OH - (aq)

when an acid is added to water, the number of hydronium ions increases HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq)

when a base is added to water, the number of hydroxide ions increases H 2 O(l) NaOH(s)  Na + (aq) + OH - (aq)

it is the ratio of hydronium ions to hydroxide ions that determines whether a solution will be an acid, a base, or neutral

acid: contains more hydronium ions than hydroxide ions base: contains more hydroxide ions than hydronium ions neutral: the # of hydronium ions is equal to the # of hydroxide ions

2. pH Scale a measure of the number of hydronium or hydroxide ions is the pH scale it is based on the concentration of hydrogen and hydroxide ions in solution pH is defined as the negative logarithm of the hydrogen ion concentration

[ ] = concentration (molarity) notice pH is based on a base 10 scale pH = -log([H + ]) = -log([H 3 O + ])

pH Scale the typical pH scale runs from 0 to 14.

a pH of 7 is considered neutral which means that the concentration of hydrogen ions and the concentration of hydroxide ions are equal

as you go down on the pH scale (< 7), solutions are considered acidic solutions with pH’s greater than 7 are considered basic

Summary of the pH scale pHCategoryConcentration of ions < 3strong acidmany H 3 O + 3-7Weak acidH 3 O + > OH - 7neutralH 3 O + = OH Weak baseH 3 O + < OH - > 11strong basemany OH -

Practice 1. Determine whether the following are a strong acid, a weak acid, a strong base, a weak base, neutral solution a. pH = 2.5 b. lots of hydroxide ions, hardly any hydronium ions c. little more hydroxide ions than hydronium ions Strong Base Weak Base Weak Acid

There is also something called pOH, which is a measure of the concentration of hydroxide ions. pH and pOH are related by the following: 14 = pH + pOH

Again, for strong, single hydroxide bases, [OH-] = molarity of the base. Ex: What is the pOH of KOH if the pH is 14? pH + pOH = pOH = 14 pOH = 0

IV. Titrations

A. Titrations The concentration of an acid (or base) in solution can be determined by performing a neutralization reaction. acid + base  salt + water

An indicator is used to show when neutralization has occurred. An indicator is a substance that forms different colors in different pH solutions. Phenolphthalein is a common indicator used in acid- base titrations. It will change from colorless in acidic environments to pink in basic environments.