Water: A Polar Molecule

Before you can understand acids and bases, you need to understand polarity. Water is a polar covalent compound. Polar means that a molecule has partial charges (+ and -) at each end.

Atoms in a covalent compound share electrons The attraction between the positive nucleus and the negative electrons keep the electrons in orbitals around the atom.

But in many molecules, sharing of electrons isn't always equal. In the case of water, the ten electrons in the molecule are pulled closer to the oxygen nucleus (with 8 protons) and away from the twp hydrogen nuclei. Because of this the oxygen end of the molecule takes on a partial negative charge ( is the symbol for partial) while the hydrogen ends of the molecule take on a partial positive charge.

You can have some fun with a polar water molecule. When you comb your hair you strip electrons off your hair and give the comb a negative charge. Place the comb next to a narrow stream of water and the water is attracted to the comb. This only works on a cool, dry day since moisture in the air prevents your comb from becoming charged.

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Acids: Polar Molecules

Acids are covalent compounds made up of polar molecules. A common example is hydrogen chloride. Chlorine has 17 protons in its nucleus while hydrogen has only one proton. The electromagnetic attraction (attraction between opposite charges) between the negative electrons and positive protons pulls the 18 electrons closer to the chlorine end of the molecule, giving it a partial negative charge. The hydrogen end of the molecule has a partial positive charge.

All acids contain hydrogen, and hydrogen has only one proton. So the hydrogen end of the molecule always loses the tug-of-war for the electrons and becomes partial positive. The other end (where the electrons are pulled closer to) becomes partial negative.

So how can you tell if a compound is polar covalent, nonpolar covalent or ionic? Here's how:
Does the compound have a metal in it? If so it is ionic. Metal is cation, nonmetal is an anion.
If the compound doesn't have a metal, it's covalent. Covanlent compounds share electrons.
To tell if it is polar or nonpolar count up the number of protons in each element. If the difference is 5 or more it is polar. Four or less then it's nonpolar.

Examples:

• MgSO4 - ionic, magnesium is a metal
• HF - polar covalent, hydrogen has 1 proton, fluorine has 9.
• N2O - nonpolar covalent, nitrogen has 7 protons, oxygen has 8.

When you place hydrogen chloride in water, the two polar molecules attract each other. Since molecules are in constant motion, the moving water molecules collide with the hydrogen chloride molecule and pull on it. The HCl molecule is pulled apart. But chlorine (with a much greater pull on the electrons) keeps all of the 18 electrons and becomes a chloride ion with a -1 charge (Cl^-). The hydrogen proton is pulled off the HCl molecule and becomes a hydrogen ion with a +1 charge (H⁺).

Ionization is the process in which atoms in a molecule become ions. All acids ionize in aqueous solution (in a water solution) to produce H⁺ ions. A H⁺ ion is nothing more than a proton. An equation for this reaction could be written in one of two ways.

When placed in water, bases dissociate. You know that associate means - to "come together". Dissociate means the opposite - to "break apart". You cannot say that bases ionize in water since ionization occurs when molecules (made of atoms) produce ions. Bases already exist as ions (in a unit cell). When you place a base in water the polar water molecules pull the crystalline lattice apart producing individual ions in solution. This process of the crystalline lattice breaking apart into individual ions in solution is called dissociation.

An equation for the dissociation of sodium hydroxide in water would be:

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The Strong & The Weak

When you place an acid in water it ionizes to produce a hydrogen ion (H⁺) and an anion (negative ion). But not all acids are totally soluble in water. Weak acids, such as carbonic acid (H2CO3) and hydrosulfuric acid (H2S) are only slightly soluble in water - most of the molecules don't ionize to produce H⁺. Because of this they only produce a small number of hydrogen ions and are weak acids. The chart below shows you how to identify strong and weak acids.

When HCl gas is placed in water it ionizes completely. So if you put 100 molecules of HCl in water you would get 100 H⁺ ions.

But when you place H2S in water, only 1% of the dihydrogen sulfide molecules ionize - the other 99% remain as molecules. So for every 100 molecules of you place in water, only 1 H⁺ ion is produced.

 

Bases follow the same rule: a strong base is soluble in water while a weak base is only slightly soluble.

Strong Bases	Weak Bases
Very Soluble in water	Slightly Soluble in water
Dissociate 100% in water to produce lots of OH- ions.	Dissociate 1% - 30% in water to produce few OH- ions.
Contain an alkali metal.	Contain a metal between 2A and 4A on the periodic table.
Examples: NaOH, LiOH, KOH	Examples: Cu(OH)2, Al(OH)3, Zn(OH)2

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Neturalization Reactions Produce a Salt & Water

If you were to pour an acid into a base, a double replacement reaction would occur. The H +ion of the acid and the OH -ion of the base would combine to form HOH (water) which is commonly written as H 2O. Water is always formed in a reaction between an acid and a base because the hydrogen ion (found in all acids) combines with the hydroxide ion (found in all bases).

Once you get rid of the H +ion of the acid and the OH -ion of the base, what is left over? The answer: the cation (+ ion) of the base and the anion (- ion) of the acid. These combine to form a salt.

A salt is defined as: any ionic compound produced in the reaction between an acid and a base in which the cation of the base combines with the anion of the acid .A salt can be any ionic compound that is produced in this reaction. You know that NaCl (common table salt) is a salt. But KF, BaSO 4, Cu(NO 3)2and LiI are also salts. Sodium chloride would form in the reaction between hydrochloric acid and sodium hydroxide. Here is an equation for this reaction:

If you were given a salt, such as potassium sulfate, how could you determine which acid and base would produce this salt? Here's how:

• The H⁺ ion of the acid and the OH^- ion of the base formed water. So these ions aren't in the salt.
• The cation (potassium) came from the base. So the base was potassium hydroxide (KOH).
• The anion (sulfate) came from the acid. So the acid was sulfuric acid (H2SO4).

The reaction would look like this:

The double replacement reaction between an acid and a base is called a neutralization reaction because the salt solution is neutral (pH 7) since there are no longer any H+ or OH- ions present.

Summing it up:

• Water is a polar covalent compound. Polar compounds have partial charges at each end of the molecule.
• Acids ionize in water to produce hydrogen ions. A hydrogen ion is a proton.
• Hydrogen ions combine with water to produce hydronium ions (H3O+)
• Bases dissociate in water to produce hydroxide ion (OH-).
• The strength of an acid or a base is determined by how soluble the compound is.
• Salts + water form in a neutralization reaction between an acid and a base.

©D.Gilliland 2004