Unit 7: Acid Base Chemistry — Short Questions

Arrhenius acids are substances which dissociate in water to give proton (H⁺).
Among the compounds, the Arrhenius acids are:
1. HF (Hydrofluoric acid)
2. H₂SO₃ (Sulphurous acid)
3. H₂S (Hydrogen sulphide)
4. H₂O (Water)

These compounds release H⁺ ions in solution.

When calcium metal reacts with dilute sulphuric acid (H₂SO₄), it undergoes a chemical reaction that produces calcium sulphate (CaSO₄), which is soluble in water and hydrogen gas (H₂), which is released as bubbles.
The overall reaction can be written as:
Ca₍ₛ₎ + H₂SO₄₍ₐq₎ → CaSO₄₍ₐq₎ + H₂₍g₎

This reaction is exothermic, meaning it releases heat.

When hydrochloric acid (HCl) reacts with barium carbonate (BaCO₃), the product of the reaction are barium chloride (BaCl₂), water (H₂O) and carbon dioxide (CO₂).
The overall reaction can be written as:
BaCO₃₍ₛ₎ + 2HCℓ₍ₐq₎ → BaCl₂₍ₐq₎ + H₂O₍ℓ₎ + CO₂↑₍g₎

So, the salt formed in this reaction is barium chloride (BaCl₂).

Hydrogen sulphate ion is a Bronsted-Lowry acid, we need to look at its ability to donate proton (H⁺ion).

According to the Bronsted-Lowry definition, an acid is a substance that can donate a proton to a base. When HSO₄⁻ donates a proton, it transforms into SO₄²⁻ (sulphate ion). The reaction can be represented as follows:
HSO₄⁻₍ₐq₎ ⟷ H⁺ + SO₄²⁻₍ₐq₎

In this reaction, HSO₄⁻ donates a proton (H⁺) and thus acts a Bronsted-Lowry acid.

The chemical name commonly given to soap as a compound is sodium stearate. Sodium stearate is the sodium salt of stearic acid which is a fatty acid. Soaps are typically formed through the saponification process, where a fat or oil reacts with an alkali resulting in the formation of fatty acid salts, which are what we refer to as soap.

Lemon, Orange and Grapefruit contain citric acid. All citrus fruits contain citric acid.

Mineral acids like HCl, H₂SO₄ and HNO₃ are useful in several ways:
i. Industrial Processes: H₂SO₄ is essential in manufacturing fertilizers and cleaning metals.
ii. Food Industry: Citric acid acts as a preservative and flavor enhancer.
iii. Cleaning Agents: Combination of acids are effective for removing rust and mineral deposits.
iv. Batteries: Sulphuric acid is crucial in lead-acid batteries.

Chloride ions exist in water mainly from the dissolution of salts like sodium chloride (table salt). When NaCl dissolves, it breaks into sodium ions (Na⁺) and chloride ions (Cℓ⁻).

The chloride ions are surrounded by water molecules, which stabilize them in the solution.

Because ammonium hydroxide is a weak base. That's why, it partially ionize in water.

NH₄OH₍ₐq₎ ⟷ NH₄⁺₍ₐq₎ + OH⁻₍ₐq₎

Two examples of Bronsted-Lowry bases that are not considered bases by Arrhenius definition:
i. Ammonia (NH₃)
ii. Bicarbonate (HCO₃⁻) ion.

Characteristic properties of acids i. Acids have sour taste. For example unripe citrus fruits or lemon juice.
ii. They turn blue litmus red.
iii. They are corrosive in concentrated form.
iv. Their aqueous solutions conduct electric current and can burn your skin.

Organic Acids (i) Acids which are obtained from natural sources are called natural or organic acids.
(ii) Some common organic acids are acetic acid (CH₃COOH) citric acid (C₆H₈O₇) and Ascorbic acid (C₆H₈O₆).

Mineral Acids (i) Mineral or man-made acids are prepared from minerals like sodium chloride or sodium nitrate.
(ii) Common examples of mineral acids are hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃).

Organic acid | Natural source
Acetic acid | Vinegar
Ascorbic acid | Amla, Guava
Citric acid | Lemon, Orange
Lactic acid | Sour milk, curd
Formic acid | Ant sting
Oxalic acid | Tomato
Tartaric acid | Tamarind

Metals oxides are basic in nature because they react with acids to form salt and water. For example, Na₂O is basic oxide because it contains oxide ion, O²⁻, which is a very strong base with a strong tendency to react with water to produce hydroxide ions.
Na₂O₍ₛ₎ + H₂O₍ℓ₎ → 2NaOH₍ₐq₎

Both acids and alkalis are known to cancel the properties of each other when mixed together in equal amounts. The reaction is called neutralization reaction. A salt and water are formed as a result of this reaction e.g.
HCℓ₍ₐq₎ + NaOH₍ₐq₎ → NaCℓ₍ₐq₎ + H₂O₍ℓ₎
Acid Base → salt+water

Oxalic acid (C₂H₂O₄) is the simplest organic diprotic acid. Its commercial uses include bleaching straw and leather and removing rust and ink stains from fabrics.

When metal oxides dissolve in water, resulting in metal cations and oxide ions in aqueous solution. Because oxide ions are unstable in water, they immediately accept protons from water molecules and become hydroxide ions.

Example Metal oxide + water → Metal hydroxide
MgO + H₂O → Mg(OH)₂

MgO → Mg²⁺ + O²⁻
O²⁻ + H-OH → 2OH⁻

An acid is that substance which dissociates in water to give proton(H⁺) or hydroxonium ion (H₃O)⁺. Some typical Arrhenius acids are HCl, HNO₃, H₂SO₄ and HCN.

HCl₍ₐq₎ ⟷ H₃O⁺₍ₐq₎ + Cl⁻₍ₐq₎

A base is that substance which dissociates in water to give hydroxyl ions (OH⁻). Some typical Arrhenius bases are NaOH, KOH and Ba(OH)₂.

NaOH₍ₐq₎ ⟷ Na⁺₍ₐq₎ + OH⁻₍ₐq₎

Role of water Water has an essential role to play in Arrhenius concept of acids and bases. Whenever an acid or a base dissociates in water, its molecules participate in reaction by surrounding the resultant proton (H⁺) and hydroxyl ion(OH⁻). Since proton is very small in size and its charge density is very high, it forms a strong bond with the lone pair of water molecule to give hydroxonium ion, H₃O⁺.

An acid that ionizes completely in aqueous solution is called a strong acid. For example, HCl, HNO₃, HNO₃, H₂SO₄etc are strong acids. They ionize almost completely in aqueous solution. All the molecules of strong acids ionize in water.

HNO₃(l) ⟷ H⁺₍ₐq₎ + NO₃⁻₍ₐq₎
H₂SO₄(l) ⟷ 2H⁺₍ₐq₎ + SO₄²⁻₍ₐq₎

Acids that do not ionize completely in aqueous solutions are called weak acids. Fewer molecules of weak acids ionize in water. For example, ethanolic acid (acetic acid) which is found in vinegar ionizes only up to 5% in water. So, ethanoic acid is a weak acid.

CH₃COOH₍l₎ ⟷ CH₃COO⁻₍ₐq₎ + H⁺₍ₐq₎

A base that ionizes completely in aqueous solution is termed as strong base. For example, NaOH and KOH, are strong bases.

NaOH₍ₛ₎ ⟷ Na⁺₍ₐq₎ + OH⁻₍ₐq₎
KOH₍ₛ₎ ⟷ K⁺₍ₐq₎ + OH⁻₍ₐq₎

A base that ionizes to a little extent is called a weak base. Such bases produce few OH⁻ ions in aqueous solution. For example, Al(OH)₃ and NH₃ are weak bases.

H₂O₍l₎ + NH₃₍ₐq₎ ⟷ NH₄⁺₍ₐq₎ + OH⁻₍ₐq₎

Limitations of Arrhenius concept i. This concept is applicable only in aqueous medium and does not explain nature of acids and bases in non-aqueous medium.
ii. According to this concept, acids and bases are only those compounds which contain hydrogen (H⁺) and hydroxyl (OH⁻) ions, respectively. It cannot explain the nature of compounds. For example, Na₂CO₃, K₂CO₃ and NH₃ do not contain any hydroxyl group which will get ionized by water but all these compounds behave as bases and yield OH⁻ in water.

HCl ionizes completely in aqueous solution that's why, HCl act as a strong acid.

HCl₍ₐq₎ ⟷ H⁺₍ₐq₎ + Cl⁻₍ₐq₎

Stomach acidity or hyperacidity conditions are a common problem. Most often the problem arises when a person takes fatty and spicy food which cause more acid to produce in the stomach than required.

Our stomach produces hydrochloric acid(HCl) to digest the food that we eat. Whenever we eat, cells within the lining of the stomach produce acid. Problem occurs when these cells produce more acid than your stomach needs. When it happens, the person suffers from stomach acidity. The common indication of such a condition is the feeling of burning sensation right belowour breast bones. A person may also feel sour taste in mouth and heart burn or pain near the heart area.

The uneasy condition may easily be cured by taking weak bases like calcium hydroxide and magnesium hydroxide commonly known as antacids. These antacids remove minor stomach disorders by neutralizing the stomach acid, but the concentration of hydroxyl ions in them is too low to harm the throat or stomach.

Bronsted-Lowry acid An acid is substance (molecule or ion) that can donate a proton (H⁺) to another substance. e.g HCl and CH₃COOH.

HCl₍g₎ + H₂O₍l₎ ⟷ H₃O⁺₍ₐq₎ + Cl⁻₍ₐq₎

Bronsted-Lowry Base A base is a substance that can accept a proton (H⁺) from another substance. e.g H₂O and NH₃.

NH₃₍g₎ + H₂O ⟷ OH⁻₍ₐq₎ + NH₄⁺₍ₐq₎

Ammonia (NH₃) act as a base in water. Ammonia is a gas at room temperature when it is dissolved in water, it can accepts proton (H⁺) from water and form ammonium (NH₄⁺) radical.

According to Bronsted-Lowry, an acid base reaction is that reaction in which a proton is transferred from a proton donor to its acceptor. This reaction may take place in gas phase or in the presence of any solvent.
For example:
HCl₍g₎ + H₂O₍l₎ ⟷ H₃O⁺₍ₐq₎ + Cl⁻

Arrhenius base A base is a substance which dissociates in aqueous solution to give hydroxide ions (OH⁻).

For Example NaOH₍ₐq₎ ⟷ Na⁺₍ₐq₎ + OH⁻₍ₐq₎

Bronsted-Lowry base Bronsted-Lowry base is a substance which can accept a proton (H⁺) from another substance.

For Example HCl₍g₎ + H₂O₍l₎ ⟷ H₃O⁺₍ₐq₎ + Cl⁻

According to Arrhenius concept, acids give(H⁺) ions and bases give (OH⁻) ions. During neutralization reaction, hydrogen ions (H⁺) combine with the equal number of hydroxide ions (OH⁻) and both neutralize each other to form water.

HCℓ + H₂O ⟷ H₃O⁺ + Cℓ⁻
NaOH ⟷ Na⁺+OH⁻

Water is an amphoteric specie because it acts as an acid as well as a base. When it react with base, it act as acid and act as base when react with acid.

Water acting as acid
H₂O₍l₎ + NH₃₍ₐq₎ ⟷ NH₄⁺₍ₐq₎ + OH⁻

acid Base Conjugate,acid Conjugate base

Water acting as base
HCl₍ₐq₎ + H₂O₍l₎ ⟷ H₃O⁺₍ₐq₎ + Cl⁻

acid Base Conjugate acid Conjugate base

Ammonia is Bronsted-Lowry base because it has the ability to accept a Proton (H⁺) but not Arrhenius base because it does not produce hydroxide ion (OH⁻) in aqueous solution.

A base which is soluble in water is called an alkali. This means that all the bases are not alkalis. On the other hand all the alkalis are bases. Many bases do not dissolve in water. For example, copper hydroxide Cu(OH)₂, aluminium hydroxide Al(OH)₃ and ferric hydroxide Fe(OH)₃.

When acids react with carbonates and bicarbonates, carbon dioxide (CO₂) gas evolves out.

Example CaCO₃(s)+2HCl₍ₐq₎ →CaCl₂(aq)+H₂O(l)+CO₂(g)

When acid react with metal (Mg, Zn) it form salts and evolve out hydrogen gas.

Mg(s) + 2HCℓ₍ₐq₎ → MgCl₂(aq) + H₂(g)
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq)+ H₂(g)

Alkalis react with ammonium salts to liberate ammonia gas: e.g;
NH₄Cl₍ₐq₎ + NaOH₍ₐq₎ → NaCℓ₍ₐq₎ + NH₃(g) + H₂O(l)

One of the ways to clean the drain is to pour half a cup of sodium bicarbonate solution into the drain followed by half a cup of vinegar. Cover the drain and wait for thirty minutes. Pour boiling water down the drain.

Caustic chemical drain cleaners are capable of dissolving grease, hair, food and other common blockages. Pour down the caustic cleaner into your drain. Wait for half an hour and then flush your drain with water.

When rain water has pH less than 5.6, it is called acid rain. Burning of fossil fuels releases harmful gases in air. These gases (SO₂ and NO₂) when mixed with moisture present in air form acid droplets. These droplets then fall on the ground as acid rain.

2SO₂(g)+O₂(g) → 2SO₃(g)
SO₃(g)+H₂O(l) → H₂SO₄(aq)

i. Effects on Soil
Acid rain makes soil more acidic. It dissolves and washes away nutrients present in the soil which are needed by plants. Many plants cannot live or grow in an acidic soil. It can damage vegetation and plants.

ii. Effect on Aquatic life
Acid rain can make water of the water bodies too acidic for aquatic animals to live in. Due to this, many lakes, rivers, ponds and streams no longer have fish.

iii. Effect on buildings
Acid drain and dry deposition of acidic particles damage buildings, statue, auto mobiles and other structures made up of stone and metal.

The chemical name for soap depends on its composition. Most soaps are salts of fatty acids.

For example

• Sodium stearate: Common in solid soaps.
• Potassium oleate: Found in liquid soaps.

Soap can be described as a salt of a carboxylic acid (e.g., sodium or potassium salts of long-chain fatty acids).

In the context of acid-base chemistry, water can be classified as both an acid and a base. This is because water has the ability to donate a proton (H⁺) to a base, making it an acid, and it can also accept a proton from an acid, making it a base.

This dual nature of water is described by the Bronsted-Lowry theory of acids and bases. When water donates a proton, it forms hydroxide ions (OH⁻) and when it accepts a proton, it forms hydronium ions (H₃O⁺).

So, water is considered amphoteric, meaning it can act as both an acid and a base depending on the circumstances.

Sodium carbonate (Na₂CO₃), behaves like a base in water due to its ability to dissociate into sodium ions (Na⁺) and carbonate ions (CO₃²⁻) when dissolved in water.

When sodium carbonate is added to water, it dissociates as follows:
Na₂CO₃ → 2Na⁺ + CO₃²⁻

The carbonate ions (CO₃²⁻) can react with water in a process called hydrolysis. During this reaction, carbonate ions can accept hydrogen ions (H⁺) from water, leading to the formation of bicarbonate ions (HCO₃⁻) and hydroxide ions (OH⁻):

CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻

The production of hydroxide ions (OH⁻) increases the pH of the solution, making it more basic. This is why, sodium carbonate is considered a basic compound when dissolved in water.

NaHCO₃ or sodium bicarbonate is generally considered a weak base. When dissolved in water, it can partially dissociate into sodium ions (Na⁺) and bicarbonate ions (HCO₃⁻).

NaHCO₃ → Na⁺ + HCO₃⁻

The bicarbonate ion (HCO₃⁻) can act as a base by accepting a proton (H⁺) from an acid, which can lead to the formation of carbonic acid (H₂CO₃):

HCO₃⁻ + H⁺ ⇌ H₂CO₃

This ability to accept protons is what gives sodium bicarbonate its basic properties. However, it can also act as a weak acid when it donates a proton, making it amphoteric.

Strong Acid • A strong acid is an acid that completely dissociates into its ions in water. This means that when a strong acid is dissolved in water, all of its molecules break apart into hydrogen ions (H⁺) and the corresponding anions.
• Examples of strong acids include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃).

Concentrated Acid • A concentrated acid refers to the amount of acid present in a solution relative to the amount of water. A concentrated acid has a high concentration of acid molecules. Concentration is typically expressed in moles per liter (M).
• For example, you can have a concentrated hydrochloric acid, which is a strong acid, but you can also have a diluted version of hydrochloric acid, which would still be a strong acid but with a lower concentration.