Chemical Equilibrium: Acids and Bases

What are Acids?

Acid, nope not that kind of acid, these are acids which include those acids and more. Think more like citric acid or acetic acid (CH₃COOH or in laymen terms vinegar).

They are sour to taste, some are corrosive. They react with active metals such Aluminum (Al), Zinc (Zn), Iron (Fe), however, they do not react with Copper (Cu), Silver (Ag), or Gold (Au). They react with carbonates such as marble, baking soda, chalk, and limestone producing carbon dioxide CO₂.

Three different types of acids are binary acids, oxy acids, and carboxylic acids:

Binary Acids

Binary acids are acids which have hydrogens attached to a nonmetal atom. These acids include hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), and hydrofluoric acid (HF). Recognize the conventional naming of these acids which contain the prefix hydro-.

Oxy Acids

Oxy Acids are have acidic hydrogens attached to an oxygen atom. These include Sulfuric Acid (H₂SO₄), and Nitric Acid (HNO₃), the hydrogens here are bound to the oxygens of the sulphate, and nitrate atoms respectively.

Carboxylic Acids

Cabroxylic Acids haves a COOH group attached to them which are known as carboxy groups such as HC₂H₃O₂. Everyone, I have admit I am being a little sneaky here, because if you rearrange this molecule you will find that it is actually CH₃COOH otherwise known as acetic acid otherwise known as Vinegar. H₃C₆H₅O₇ is another example which is called 2-Hydroxytricarballyilic Acid.

What Are Bases?

Bases are also known as alkalis, and are the opposites of acids. For example, when an acetic acid loses its hydrogen it gains a conjugate base of CH₃COO^–. They have a bitter taste, they feel slippery when in solution. They also change the colour of dyes (i.e. litmus paper), however they turn red litmus blue. They react with acids to form ionic salts, which neutralizes the compounds.

Most ionic bases contain a hydroxyl ion (OH^–) such as NaOH and Ca(OH)₂. Some also contain CO₃^2- such as CaCO₃ and NaHCO₃. Molecular bases contain structure (mostly amine groups) which react with hydrogen ions (H⁺)

Arrhenius Theory

Arrhenius theory tells us that bases dissociate in water producing OH^– ions and cations. Ionic substances dissociate into water

NaOH_(aq) → Na⁺_(aq) + OH^–_(aq)

It also states that acids ionize in water, thus producing H⁺ ions and anions. Acids are unlike bases, and they are not made of ions. Therefore, they must be pulled apart (ionized) by the water in order to for, the H⁺ ions and anions.

HCl_(aq) → H⁺_(aq) + Cl^–_(aq)

The formula is written with the H⁺ in the front of the products.

HC₂H₃O_2(aq) → H⁺_(aq) + C₂H₃O₂^–_(aq)

These H⁺ that are produce are very reactive. Their reactivity is true on paper, but in real life are unable to exist.Whart actually happens is that these bare H⁺ ions react with water producing a Hydronium Ion H₃O⁺

H⁺ + H₂O → H₃O⁺

Arrhenius Acid-Base Reactions

When Acids and Bases comibne the H⁺ and the OH^– combine together to make water (H₂O). The cation from the base and the anion from the acid form a salt

HCL_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

acid + base → salt + water

Credit: Khan Academy

Arrhenius Theory does have its problems. For one, it doesn’t explain why molecular substances such as NH₃, still dissolve in water, but they do not contain any OH^– ions. Nor do some ionic compounds, such as Na₂CO₃ or Na₂O, dissolve in water to form basic solutions, even though they do not contain OH^– ions.

The same goes for acids uch as CO₂, which dissolve in water, but do n ot contain H⁺ ions.

It also does not explain for acid-base reactions that take place outside of aqueous solutions.

Brønsted-Lowry Acid-Base Theory

Brønsted-Lowry redefined what acids and bases do based on what happens in a reaction.

Any reaction that involves the transfer of H⁺ from one molecule to another (regardless if it is in an aqueous solution or if there is another OH^– present) is an acid-base reaction. Brønsted-Lowry definition contain all the reactions that fit the Arrhenius definition, plus many many more!

In the Brønsted-Lowry Acid-Base Reaction, an H⁺ is transferred. In this theory an acid is define as an hydrogen donor (H donor) because it donates its hydrogen; and in contrast a base is an hydrogen acceptor because the hydrogen is accepted (base) from the donor (acid) . The base structure must have an atom with an unshared pair of electrons.

H-A + :B ⇌ :A^– + H-B⁺

Any material that has atoms with lone pairs can potentially be a Brønsted-Lowry base becasue of the molecular structure. Often one atom in the molecule is more willing to accept a H⁺ transfer than others.

NH_3(aq) + H₂O_(l) ⇌ NH₄⁺_(aq) + OH^–_(aq)

Amphoteric Substances

Amphoteric Substances can act as either an acid or a base. They are able to do so because they have both a transferable H, and an atom with a lone pair of electrons. A great example of this is water. Water can either be an accepting base (H⁺ is accepted from HCl):

HCl_(aq) + H₂O_(l) → Cl^–_(aq) + H₃O⁺_(aq)

Or it is donated, as an acid, to NH₃:

NH_3(aq) + H₂O_(l) ⇌ NH₄⁺_(aq) + OH^–_(aq)

Brønsted-Lowry acid-base reactions are advantageous because the allow for reversible reactions

H-A + :B ⇌ :A^– + H-B⁺

When the product is formed, the original base as an extra hydrogen atom, and the original acid has a lone pair, overall allowing for the reaction to be reversed.

:A^– + H-B⁺ ⇌ H-A + :B

Conjugate Pairs

When the reactant acid becomes a base product, and the reactant base becomes acid product, they are known to be conjugate pairs of each other. Therefore, the original reactant base becomes a conjugate acid, and the original reactant acid becomes a conjugate base.

Is it Strong or Weak?

A strong acid is a strong electrolyte because practically all the acid ionizes the molecules. A single arrow is used →.

• The stronger the acid is, the more the acid is willing to donate its H-atom, as they practically donate all of their hydrogens.
• If the concentration of the strong acid is equal to the concentration of the hydronium ion formed post-reaction, thus, you have a strong acid.
• Examples of Strong Acids
  • Hydrochloric Acid (HCl)
  • Hydrobromic Acid (HBr)
  • Hyrdoiodic Acid (HI)
  • Nitric Acid (HNO₃)
  • Perchloric Acid (HClO₄
  • Sulfuric Acid (H₂SO₄ (diprotic)

A strong base is a strong electrolyte because practically all of the base dissociates and forms an OH^– . A single arrow is used →.

In contrast a weak acid partially ionizes, and a weak base partially dissociates. Use the equilibrium arrows ⇌

• Weak acids do not donate all of their hydrogen atoms, but less than 5% is ionized into water. The concentration will show a significantly less amount of hydronium ions post-reaction than the concentration of the weak acid.
• Examples of weak acids
  • Hydrofluoric Acid (HF)
  • Acetic Acid (HC₂H₃O₂)
  • Sulfurous Acid (H₂SO₃) (diprotic)
  • Carbonic Acid(H₂CO₃) (diprotic)
  • Phosphoric Acid (H₃PO₄)(triprotic)

Acids and Base strength can be measured by determining the equilibrium constant of a substances reaction with water.

The farther the equilibrium position lies toward the products, the stronger the acid or base. The position of equilibrium depends on the strength of attraction between the base form and the H⁺. A stronger attraction = stronger base/weaker acid.

The acid strength is measured by the size of the equilibrium constant when it reacts with H₂O. K_a represents the acid ionization constant. A larger K_a means a it is a stronger acid. Below is the equation to calculate the Acid Ionization Constant!

K_a=[A^–]x[H₃O⁺]/[HA]

Water is a very weak electrolyte, and it only requires a few ions present. Only about 2 out of every 1 billion water molecules form ions via autoionization.

H₂O ⇌ H⁺ + OH^–

H₂O + H₂O ⇌ H₃O⁺ + OH^–

Both Aqueous solutions both contain H₃O⁺ and OH^–, the concentration of H₃O⁺ and OH^– are equal in water. [H₃O⁺] = [OH^–] = 10^-7 M @ 25 ^oC

Ion Product of Water

The product of the H₃O⁺ and OH^– concentrations is always the same number. The number is called the Ion Product of Water and has the symbol K_W aka the dissociation constant of water

[H₃O⁺]x [OH^–]= K_w= 1.00 x 10 ^-14 @ 25^oC

As [H₃O⁺] increases the [OH^–] must decrease so the product stays constant.

Neutral solutions: [H₃O⁺] [OH^–]= 1.00 x 10^-7

Acidic solutions have a larger[H₃O⁺] than [OH^–]: [H₃O⁺] > 1.00 x 10^-7; [OH^–]< 1.00 x 10^-7

Basic solutions have a larger [OH^–] than [H₃O⁺]: [H₃O⁺] < 1.00 x 10^-7 ; [OH^–]> 1.00 x 10^-7

Acidity and basicity is measured as a pH using the equation pH= – log[H₃O⁺]. A pH > 7 is basic, a pH < 7 is acidic, and a pH = 7 is neutral.

To find the [H₃O⁺], use 10^-pH

The significant figures for a logs are the counted after the decimal point because the digits before the decimal point come from the exponent on 10

pOH is another way of expressing the acidity/basicity of a solution. This is done by calculating pOH= – log[OH^–], and the reverse [OH^–]= 10^-pOH

A pOH > 7 is acidic, a pH < 7 is basic, and a pH = 7 is neutral.

pH + pOH = 14

pK is another way of expressing the strength of an acid or base.

pK_a= -log(K_a), K_a= 10^-pK_a

pK_b= -log(K_b), K_b= 10^-pK_b

A stronger acid has a larger K_a , and a smaller pK_a

A stronger base has a larger K_b, and a smaller pK_b

For strong acids in solution, H₃O⁺ is present in both the acid and the water, and OH^– is present in the base and the water. This means that the contribution of the water to the total [H₃O⁺] and [OH^–] is negligible. The [H₃O⁺]_acid shifts the equilibrium K_w so far that the [H₃O⁺]_water is too small to be of any significance. Exceptions are made only in very dilute solutions < 1 x 10^-4 M

A monoprotic strong acid : [H₃O⁺]= [HA]

A strong ionic base [OH^–] =(number OH^–)([Base])

How to find the pH of a Weak Acid

When you have a weak acid there are also two sources of H₃O⁺. In the acid and in the water. However, finding the [H₃O⁺] is complicated because the acid undergoes partial ionization. To calculate this [H₃O⁺] requires solving an equilibrium problem for the reaction that defines the acidity of the acid.

HA + H₂O ⇌ A^– + H₃O⁺

Percent Ionization

The strength of an acid can also be determined by finding the percentage of acid molecules that ionize when dissolved in water. This process is called Percent Ionization.

Percent Ionization = molarity of ionized acid/ initial molarity of acid x 100%

In terms of H₃O⁺ the equation looks like this:

Percent ionization = [H₃O⁺]_equil/ [HA]_initial x 100%

The H₃O⁺ doesn’t keep up the increase in HA because according to Le Châtelier’s Principle, if we reduce the concentrations of all the _(aq) components, the equilibrium will shift to the right to increase the total number of dissolved particles.

We can reduce the _(aq) by using a more dilute initial acid concentration. This will give an increase in the concentration of hydronium ions in dilute solutions compared to the inital acid concentration, and a larger percent ionization will be apparent.

Strong Bases

A stronger base is more willing to accept a Hydrogen

Ionic bases practically are all dissociated into OH^– or accept Hydrogens

Weak Bases

In contrast, weak bases only accept a fraction of hydrogens.

They are weak electrolytes, most weak bases do not take H from water

much less than 1% ionization in water

[OH^–] << [Weak base], finding the pH of a weak base is similar to finding the pH of a weak acid.

The equilibrium constant K_b is the base ionization constant.

K_b= [OH^–]x[H:Base⁺]/ [:Base]

A larger K_b means a larger base

Acid-Base Properties of Salts

Salts are water-soluble ionic compounds

Salts that contain a cation of a strong base and an anion that is a conjugate base of a weak acid are basic. NaHCO₃ solutions are basic. Na⁺ is a the cation of the strong base NaOH, and HCO₃^– is the conjugate base of the weak acid H₂CO₃

Salts that contain cations from a conjugate acid of a weak base and an anion of a strong acid are acidic: NH₄⁺ is the conjugate acid of the weak base NH₄⁺, and Cl^– is the anion of the strong acid HCl

Every anion can be thought of as a conjugate base of an acid, therefore every anion can potentially be a base. The stronger the acid is, the weaker the conjugate base is. AN anion that is the conjugate base of a strong acid is pH neutral, and an anion that is the conjugate base of a weak acid is basic.

K_b values can be found from K_a values by adding the equations and multiplying the K’s.

Metal Cations, are small, highly charged metals, and they are weakly acidic. Alkali metal cations and alkali earth metal cations are pH neutral. Cations are hydrated.