Acids Bases and pH Review

Acids and Bases: An Introduction

Acids are found in food, your stomach, and the environment. Bases are found in soap, household cleaners, and antacid tablets. Acids often taste sour, are corrosive, and will turn blue litmus paper (an indicator) red. Acids react with most metals to give off H2 gas (remember all those labs…).

Bases often taste bitter, feel slippery, are corrosive, and will turn red litmus paper blue.

Common acids:

Citrus fruits have citric acid.

Yogurt has lactic acid.

Vinegar is acetic acid.

Your stomach uses hydrochloric acid.

Car batteries use sulfuric acid.

Some soft drinks use phosphoric acid.

Nitric acid is used as a fertilizer.

Common bases:

Glass cleaner may contain ammonia.

Draino uses sodium hydroxide.

Tums uses calcium hydroxide.

Milk of magnesia uses magnesium hydroxide.

Baking soda is sodium bicarbonate.

What makes an acid an acid or a base a base can vary depending on definition being used. The first definition was created by Svante Arrhenius in 1883.

Arrhenius said compounds that will ionize to give off H+ ions (makes H3O+ - hydronium ions) are acids and

compounds that will ionize to give off OH- ions are bases. (Of course to ionize they must be dissolved in water - aquated!)

HCl → H+ + Cl-

NaOH → Na+ + OH-

HCl → H+ + Cl-

NaOH → Na+ + OH-

ACID

BASE

With the Arrhenius definitions only compounds with hydrogen can be acids and only compounds with hydroxide can be bases. This definition sometimes leaves out compounds that also seem to act like bases, so two other chemists came up with a new definition for a base. The Bronsted-Lowry definition for a base is that it is an H+ acceptor. The definition of an acid is still the same as for Arrhenius - an acid gives off H+

HCl + NH3 → NH4+ + Cl-

HCl + NH3 → NH4+ + Cl-

ACID BASE

In the above reaction the HCl gives off H+ and the NH3 takes the H+. Therefore, HCl is an acid and NH3 is a base. Now watch what happens when the reaction above is reversed:

NH4+ + Cl- → HCl + NH3

NH4+ + Cl- → HCl + NH3

ACID BASE

Now the NH4+ gives off H+ (is an acid) and Cl- accepts

the H+ (is a base). Notice how the HCl starts as an acid and makes Cl- which is a base, and NH3 starts as a base and makes NH4

+ which is an acid. This makes HCl and Cl- and NH3 and NH4

+ conjugate acid-base pairs.

Another interesting conjugate acid-base effect happens with water.

HCl + H2O → H3O+ + Cl-

NH3 + H2O → NH4+ + OH-

In the first reaction water acts as a base to accept the H+ from HCl, and in the second reaction water acts as an acid to give H+ to NH3. Substances that can act as both an acid and base are called amphoteric.

A third definition for acids and bases is often useful – a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donator.

H H H H

H – N : B – H → H – N – B - H

H H H H

A third definition for acids and bases is often useful – a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donator.

H H H H

H – N : B – H → H – N – B - H

H H H H

Base Acid

There are also acids and bases that can act as an acid or base more than one time. For example:

H3PO4 + H2O → H2PO4- + H3O+

H2PO4- + H2O → HPO4

-2 + H3O+

HPO4-2 + H2O → PO4

-3 + H3O+

Acids that can give off more than one H+ (protons – make more than one hydronium ion) are called polyprotic. H3PO4 is a triprotic acid, and H2SO4 would be a diprotic acid.

Strengths of Acid and Bases

The strength of an acid or base has nothing to do with the concentration. A strong acid is a compound where 100% of all the little molecules will ionize. A weak acid is a compound where much less than 100% of all the little molecules will ionize. Thus a strong acid falls apart completely and a weak acid still has some of the molecules stuck together.

A concentrated acid has a lot of acid dissolved in water, and a dilute acid has only a little. The concentration is usually measured in molarity (moles divided by liters). It is possible to have a concentrated strong acid (12M HCl) and a concentrated weak acid (12M HCN). It is also possible to have a dilute strong acid (0.1M HCl) and a dilute weak acid (0.1M HCN).

There is NO connection between concentrated/dilute and

strong/weak!

What is pH?

Because you cannot use the concentration to know the strength of the acid, and because you cannot use the strength to know the concentration of the acid, chemists have determined a way to get one all-inclusive number to use. This number is called pH. (pH stands for the “potential of Hydrogen”)

The pH scale goes from 0 to 14. 7 is considered neutral. Anything from 0-7 on the pH scale is an acid and anything from 7-14 is a base.

In addition, anything from 0-4 is a strong acid and anything from 4-7 is a weak acid. Anything from 7-10 is a weak base and anything from 10-14 is a strong base.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Strong Acid WeakAcid

WeakBase

Strong Base

To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s. Most indicators can only turn one color. For example, phenolphthalein is colorless in acids and pink-purple with bases.

To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s. Most indicators can only turn one color. For example, phenolphthalein is colorless in acids and pink-purple with bases. In order to quickly determine a pH, pH paper has many different indicators soaked into it. This allows pH paper to turn 5 or more colors so 5 or more pHs can be determined.

The best way to determine the pH is with a pH meter or calculation. pH = -log [H+], which means the negative logarithm of the concentration of the H+ ions. If the [H+] is known (in molarity), then a calculator can provide the pH number, but if the [H+] is not known, determining it without the use of technology is a long a grueling process.

pH meters have been created to do these calculations quickly and show the exact pH results almost immediately. Extreme care must be taken with pH meters as they are fragile and will not give correct results if not properly cared for.

Often it is useful with bases to calculate a pOH instead of a pH. For this pOH = - log [OH-]. Also a pOH scale could be created, and is literally the reverse of the pH scale.

On the pOH scale, low numbers are strong bases and high numbers are strong acids. The pH and pOH of a particular solution are inversely related, so for any solution, the pH + pOH = 14.

When water ionizes, it forms equal amounts of H+ and OH-. It has been measured that [H+][OH-] = 1 × 10-14 M2.

In acidic solutions [H+] > [OH-] and in basic solutions [H+] < [OH-], but for all solutions it still works that:

[H+][OH-] = 1 × 10-14 M2

[H+ ]

[OH- ]

pH

pOH

pH = -log [H+ ]

pOH = -log [OH- ][H+][

OH

- ] =

1 ×

10-1

4 M

2p

H +

pO

H =

14[H+ ] = 10-pH

[OH- ] = 10-pOH

Neutralization

When an acid and base are added together in appropriate amounts, the resulting pH is near 7 - neutral. This is why acids and bases are said to “neutralize” each other. In a neutralization reaction the acid and base (according to the Arrhenius definition) react to form water and a salt.

HCl + NaOH → HOH + NaCl

Water Salt

The salt does not have to be table salt (NaCl). A salt is almost any metal with an anion. What’s the salt in the below reaction?

H2SO4 + Ba(OH)2 → 2HOH + BaSO4

If the base contains carbonate ion instead of hydroxide, a salt and water are still formed, but so is carbon dioxide gas. Consider this volcano recipe:

HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2

If the base contains carbonate ion instead of hydroxide, a salt and water are still formed, but so is carbon dioxide gas. Consider this volcano recipe:

HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2

Vinegar Baking Soda

Neutralization has many purposes. If a lake becomes too acidic because of acid rain, then a base can be added to return the pH to the level fish need to live. If a base spills in lab an acid can be added to make the spill less dangerous to clean up.

Certain compounds in your blood neutralize the carbonic acid in your body to maintain homeostasis. Most importantly for chemists, if an unknown base is neutralized with a known acid, the amount of OH- in the base can be calculated. Likewise if an unknown acid is neutralized with a known base, the amount of H+ in the acid can be calculated.

Neutralizing an acid and base to determine an unknown amount of H+ or OH- is called titration. In a titration a small amount of indicator is added that will change color when the neutralization is complete.