Acids and Bases - Lecture Notes

Acids and Bases

Lecture Objectives

Distinguish between acids and bases.
Describe the characteristic reactions of acids with metals and bases.
Outline the uses of acids and bases in daily life.
Explain experimental detection of acids and bases.

Identifying Acids and Bases in Everyday Items

Examples include tomatoes, tomato sauce, hand soap, toothpaste, and vinegar.

pH Scale

Expresses the strength of acids and bases.
$pH < 7$ : Acid
$pH > 7$ : Base
$pH = 7$ : Neutral

Identifying Acids and Bases Based on pH

Acids have a pH from 0 to 7; lower pH indicates a stronger acid.
Bases have a pH from 7 to 14; higher pH indicates a stronger base.

Quick Quiz: True or False

A solution with a pH of 7 is considered neutral. True
The pH scale ranges from 0 to 14. True
A pH of 2 is more acidic than a pH of 5. True
Lemon juice typically has a pH above 7. False
A pH of 14 indicates a highly acidic solution. False
Substances with a pH below 7 are considered basic. False

Important Terms of Acids and Bases

Arrhenius Definition: Refers to compounds dissolved in water.
- Arrhenius Acid: Molecule that donates $H^+$ or $H_3O^+$ (hydronium) ions when dissolved in water.
- Arrhenius Base: Molecule that gives $OH^-$ (hydroxyl) ions when dissolved in water.
Bronsted-Lowry Definition:
- Bronsted-Lowry Acid: Compound that donates $H^+$ in solution (not necessarily water); proton donor.
- Bronsted-Lowry Base: Atom or ion capable of accepting or bonding to a free proton in solution; proton acceptor.

Arrhenius Acids and Bases Summarized

Arrhenius acid produces $H^+$ ( $H_3O^+$ ) in water.
Arrhenius base produces $OH^-$ in water.

Properties of Acids

Produces hydrogen ions ( $H^+$ ) in water (Arrhenius theory).
Taste sour.
pH < 7.
Affect indicators:
- Acids turn blue litmus to red.
- Acids turn methyl orange to red.
Neutralize bases, producing a salt and water.
Proton donors (Brønsted–Lowry theory).

Neutralization Reaction

Acids react with bases to produce a salt and water.
The effect of the base is nullified by the acid, and vice versa.

Salt Hydrolysis

Interaction of salt and water produces an acid and a base.
$NaCl + H_2O \rightarrow HCl + NaOH$

Outcomes of Salt Hydrolysis

The solution from salt hydrolysis can be acidic, basic, or neutral, depending on the nature of the salt.
1. Salts of strong acid and strong base give neutral solution.
2. Salts of weak base and strong acid give acidic solution.
3. Salts of weak acid and strong base give basic solution.

Examples of Acids

Name of Acid	Chemical Formula	Where Can It Be Found?
Hydrochloric acid	$HCl$	In gastric juice in the stomach
Sulphuric acid	$H<em>2SO</em>4$	In car batteries
Nitric acid	$HNO_3$	In the preparation of fertilizers and explosives
Carbonic acid	$H<em>2CO</em>3$	In fizzy drinks
Citric acid	$CH<em>8O</em>7$	In oranges and lemons
Acetic acid	$CH_3COOH$	In vinegar

Acids in Everyday Life

Citric acid causes the tartness in lemons and oranges.
Formic acid ( $HCO_2H$ ) causes the sting of ants.

Strength of Acid

Depends on the extent of ionization.
- Strong Acid: All acid molecules become ions in water (e.g., Sulphuric acid, Hydrochloric acid, Nitric acid).
- Weak Acid: Only a few acid molecules become ions in water (e.g., Acetic acid, Citric acid, Carbonic acid).

Reactions of Acids

With Bases
- Sulphuric acid + copper(II) oxide → copper(II) sulphate + water
- $H2SO4(aq) + CuO(s) \rightarrow CuSO4(aq) + H2O(l)$
- Acid + metal oxide → metal salt + water
With Alkali
- Sulphuric acid + magnesium hydroxide → magnesium sulphate + water
- $H2SO4(aq) + Mg(OH)2(aq) \rightarrow MgSO4(aq) + 2H_2O(l)$
- Acid + metal hydroxide → metal salt + water

Reactions of Acids Continued

With Metals
- Sulphuric acid + magnesium → magnesium sulphate + hydrogen
- $H2SO4(aq) + Mg(s) \rightarrow MgSO4(aq) + H2(g)$
- Acid + metal → metal salt + hydrogen
With Carbonates
- Hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
- $2HCl(aq) + CaCO3(s) \rightarrow CaCl2(aq) + H2O(l) + CO2(g)$
- Acid + metal carbonate → metal salt + water + carbon dioxide

Acid Rain

Rainwater with a pH less than 5.6.
Acid rain lowers the pH of river water, making aquatic life survival difficult.

Properties of Bases

Produces hydroxide ions $OH^-$ in water (Arrhenius theory).
Taste bitter.
pH > 7.
Feel slippery.
Affect indicators:
- Bases turn red litmus to blue.
- Bases turn methyl orange to yellow.
Neutralize acids, producing a salt and water.
Proton acceptors.

Examples and Uses of Bases

NaOH - Sodium Hydroxide: Used in manufacturing soaps and detergents, and as a drain cleaner.
KOH - Potassium Hydroxide: Used in manufacturing liquid soaps and fertilizers, and in alkaline batteries.
NH3 – Ammonia: Household cleaner.
Mg(OH)2 - Magnesium Hydroxide: Used as an antacid.

Lab Tests on Acids and Bases

pH Indicators
- Measure the approximate pH of a solution.
- Change color at different pH values when reacting with acidic or basic solutions.
- Phenolphthalein is an example; it is colorless in neutral but alkaline.
pH Testing Strips
- Contain an indicator that changes to a variety of colors over a range of pH values.
pH Meters
- Electronic instrument with an electrode that senses the hydronium ion concentration in solution.
- More accurate than pH strips.
Olfactory Indicators
- Substances with different odors in acid and base solutions.
- Vanilla essence has a pleasant smell in acid solution and no smell in alkali solution.
- Other examples: onion, clove oil, etc.

Chemical Reactions

Reacting substances are converted to new substances.

Different Types of Chemical Reactions

Combination reactions
Decomposition reactions
Displacement reactions
Double-displacement reactions
Oxidation-reduction reactions
Precipitation reactions
Exothermic and endothermic reactions

Types of Chemical Reactions Explained

Combination Reaction: Two or more substances combine to form a new substance.
- $A + B \rightarrow C$
- Example: $H2 + Cl2 \rightarrow 2HCl$
Decomposition Reaction: A substance breaks down into two or more simpler substances.
- $A \rightarrow B + C$
- Example: $ZnCO3 \rightarrow ZnO + CO2$
Displacement Reaction: One part of a molecule is replaced by another.
- $X + YZ \rightarrow Y + XZ$
- Example: $Zn + 2HCl \rightarrow ZnCl2 + H2$
Double-Displacement Reaction: Two reacting ionic compounds exchange their corresponding ions.
- $WX + YZ \rightarrow WZ + YX$
- Example: $AgNO3 + NaCl \rightarrow AgCl + NaNO3$
Oxidation Reaction: Addition of oxygen, removal of hydrogen, and/or loss of electrons.
- Example: $Al(s) \rightarrow Al^{3+}(aq) + 3e^-$
- $P4(s) + 5O2(g) \rightarrow 2P2O5$
- $H2S(aq) + Br2(aq) \rightarrow 2HBr(aq) + S(s)$
Reduction Reaction: Removal of oxygen, addition of hydrogen, and/or gain of electrons.
- Example: $Fe2O3(s) + 3CO(g) \rightarrow Fe(s) + 3CO_2(g)$
- $H2S(aq) + Cl2(g) \rightarrow 2HCl(aq) + S(s)$
- $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$
Precipitation Reaction: One of the products is an insoluble substance (precipitate).
- Example: $AgNO3(aq) + KCl(aq) \rightarrow AgCl(s) + KNO3(aq)$
Exothermic and Endothermic Reactions: Reactions accompanied by the evolution or absorption of heat, respectively.
- Melting of ice is an endothermic reaction.
- Freezing of water is an exothermic reaction.

Questions

Acids break into $H^+$ ions, and bases break into $OH^-$ ions in an aqueous solution.
Vinegar (pH 2.4) is more acidic than cheese (pH 5).
A higher pH value indicates a stronger base.
Acids are proton acceptors. (False)
Bases are proton acceptors. (True)
Arrhenius acids donate $H^+$ ions in water; Arrhenius bases donate $OH^-$ ions in water.
Acids turn methyl orange to red, and bases turn methyl orange to yellow.
Neutralization reaction: Reaction of acid with base to form salt and water.
Salt hydrolysis: Interaction of salt with water to give an acid and a base.
Products of acid + carbonate: metal salt + water + carbon dioxide.
Products of acid + metal: metal salt + hydrogen.
Citric acid is present in lemon.
Formic acid is present in ants.
Antacids are mild bases used to relieve acidity and indigestion.
The substances which have different smells in acid and base solutions are known as olfactory indicators.
The compound that changes color in acid and base solutions is a pH indicator.