Acids and Bases: A Comprehensive Guide to Acid-Base Chemistry, Summaries of Chemistry

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Acids and Bases

 Acids and bases are special types of solutions  Remember a solution is a mixture of a solute and a solvent  Acids and Bases are extremely useful to us in our everyday life.

What is an Acid and Base?

 Taste Sour  Electrolyte (Conduct Electricity)  Have no characteristic feel  Turns blue litmus red  Produces hydrogen gas when reacted with a certain metals  Produce carbon dioxide gas when reacted with carbonate compounds.  Corrosive

Properties of Acids

 Drain Cleaner – Sodium hydroxide  Baking soda – Sodium hydrogen carbonate  Soap – Sodium hydroxide  Glass cleaner – Ammonia  Antacid Tablets – Magnesium Hydroxide

Common Bases

 An ion that forms when a hydrogen ion (H

) gets added to a water molecule  Hydronium ion = H 3

O

 In chemical reactions involving acids and bases the hydronium ion is often shown as H

for simplicity  HCl + H 2

O → H

• Cl

• (does not take water into account) OR  HCl + H 2

O → H

3

O

• Cl

Hydronium Ion

1. Arrhenius Theory of Acids and Bases
   • Early theory proposed in 1884
   • aqueous solution (aq) – a solution in which water is the dissolving medium or solvent.
   • Acid is a substance that adds hydronium ions (H 3

O

) to an aqueous solution.

• Base is a substance that adds hydroxide ions (OH
  • ) to an aqueous solution
• HCl(aq) → H 3 O
  • (aq) + Cl - (aq) (HCl = acid)
• NaOH (aq) → Na + + OH - (NaOH = base)

Acid and Base Theories

 The strength of an acid is determined by its ability to dissociate in water to produce Hydrogen/Hydronium ions.  Therefore strong electrolytes are strong acids  For example Hydrochloric Acid (HCl) is a strong acid because every HCl molecule dissociates into H

 HF is a weak acid because it doesn’t dissociate well in water

Strong vs Weak Acids

 The strength of a base is based on the concentration of dissociated OH

• (hydroxide ions)  Therefore strong electrolytes are strong bases  Strong bases can also be described as having a high alkalinity  Those bases that are not soluble in water and don’t dissociate well are weak bases.  NaOH (sodium hydroxide) is a strong base because it dissociates well into Na

and OH

ions.  Ammonia (NH 3+ ) is a weak base because most of its molecules do not react with water to form ions.

Strong vs Weak Bases

 The pH scale is a number line that assigns number values from 0 – 14 to acids and bases.  pH is measuring the concentration of hydrogen ions (H

in the solution  As the concentration of H

increases the pH decreases  Although pH is a measure of H

ions you can also determine the OH

• concentration because if H + concentration is high the OH

concentration is low.  Therefore a pH of 0 has a high concentration of H

ions.

pH scale

 Acids:  pH value of 0- 6  The lower the number the stronger the acid  Bases:  pH value of 8- 14  The higher the number the stronger the base  Neutral:  pH value of 7  Water is neutral

 The pH would increase  By adding water to HCl you are decreasing the concentration of H

ions .  Since pH is inversely related to the concentration of H

ions if H

concentration decreases the pH increases.

How would the pH of HCl change if

water was added to it?

 An indicator is a chemical that change color when they come in contact with an acid or base.  Types of indicators

1. pH paper / Universal Indicator - This yellow paper will change color depending on whether it is an acid or base - The color corresponds to a specific pH so you can also determine the pH of the **substance

2. Litmus Paper** - Two pieces of paper where one is blue and one is red - Acid turns blue litmus paper red - Base turns red litmus paper blue

Indicators

 The Bronsted-Lowry definitions of acids and bases provide a basis for studying proton – transfer reactions.  Suppose that a Bronsted Lowry acid gives up a proton; the remaining ion or molecule can re-accept that proton and thus act as a base.  Such a base is known as a CONJUGATE BASE  Thus, the species that remains after a Bronsted-Lowry acid has given up a proton is the conjugate base of that acid.  The species that is formed when a Bronsted-Lowry base gains a proton is the CONJUGATE ACID of that base.  In general, Bronsted-Lowry acid-base reactions are equilibrium systems meaning that both the forward and reverse reactions occur.  They involve two acid-base pairs, known as conjugate acid-base pairs.

Conjugate Acids and Bases

 In this example:

 HCl is the acid in the forward direction because it

donates the H

while H

2

O is the base because it

accepts the H

 H

3

O

is the conjugate acid because it donates the

H

in the reverse direction, while Cl

is the

conjugate base because it accepts the H