Acids, Bases, and Salts

Acids, Bases and Salts

• Parents use tamarind or lemon juice to polish copper vessels.
• Pickles should not be stored in metal containers.
• Common salt and sugar serve as good preservatives.
• Bleaching powder and baking soda are frequently used in households.
• Drain, pipe, and window cleaners contain chemicals.
• Many cleaning items can be grouped as acids, bases, or salts.
• pH is a measure of acidity.

Learning Objectives

• Define acid, base, salt, and indicator
• Identify common household acids, bases, and salts
• Suggest suitable indicators
• Identify the properties of acids and bases
• Differentiate between strong and weak acids and bases
• State the role of water in the dissociation of acids and bases
• Describe the ionic product constant of water
• Define pH
• Link hydrogen ion concentration and pH with the acidic/basic nature of aqueous solutions
• Know the importance of pH in everyday life
• Define salts and describe salt preparation methods
• Link the nature of a salt with the pH of its aqueous solution
• Know the manufacture/uses of baking soda, washing soda, plaster of Paris, and bleaching powder

Acids and Bases

• For thousands of years, it has been known that vinegar, lemon juice, Amla, tamarind, and other food items taste sour due to the presence of acids
• The term "acid" comes from the Latin term 'accre,' which means sour
• Robert Boyle was the first to label substances as acids and bases in seventeenth century

Acids

• Taste Sour
• Corrosive to metals
• Change blue litmus to red
• Become less acidic on mixing with bases

Bases

• Taste bitter
• Feel slippery or soapy
• Change red litmus to blue
• Become less basic on mixing with acids

Svante Arrhenius and Definitions

• Robert Boyle characterized acids/bases, but could not explain their behavior via chemical structure
• Swedish scientist Svante Arrhenius accomplished this in the late nineteenth century
• Dissolving in water causes many compounds to dissociate, forming ions
• Ions dictate the properties of dissociated compounds
• Acids release hydrogen ions (H+) when dissolved in water: HCl (aq) → H+(aq) + Cl-(aq)

Acids - Examples of Acids

• Gastric juice contains hydrochloric acid (HCl)
• Soft drinks contain carbonic acid (H2CO3)
• Lemons and many fruits contain ascorbic acid (vitamin C)
• Oranges and lemons contain citric acid
• Vinegar is acetic acid
• Tea consists tannic acid
• Laboratories utilizes nitric acid (HNO3)
• Sulphuric acid (H2SO4) also used in laboratories

Bases

• Bases release hydroxide ions (OH¯) when dissolved in water
• Sodium hydroxide NaOH (aq) dissociates as: NaOH (aq) → Na+(aq) + OH-(aq)
• 'Alkali' is often used for water-soluble bases

Bases - Examples

• Sodium hydroxide (NaOH) or caustic soda which are typically found in washing soaps
• Potassium hydroxide (KOH) or potash which are typically found in bathing soaps
• Calcium hydroxide (Ca(OH)2) or lime water which are typically found in whitewash
• Magnesium hydroxide (Mg(OH)2) or milk of magnesia for controlling acidity
• Ammonium hydroxide (NH4OH) used in hair dyes

Indicators

• Turmeric, when applied to a stain, can act as a bases indicator
• Acid-base indicators show one color in an acidic medium and another in a basic medium
• Litmus, the earliest indicator, is a natural dye from lichens
• Litmus turns red in acidic solutions and blue in basic solutions
• Phenolphthalein and methyl orange serve as other indicators

Indicator colors in solutions

• Litmus is red in acidic solutions, purple in neutral solutions, and blue in basic solutions
• Phenolphthalein is colorless in acidic and neutral solutions and pink in basic solutions
• Methyl orange is red in acidic solutions, orange in neutral solutions, and yellow in basic solutions

Properties of Acids and Bases

• Characteristics categorize bases and acids

Properties of Acids

• Taste: Sour
• This is particularly true of dilute acids
• Lemon juice contains citric and ascorbic acid (vitamin C)
• Vinegar contains ethanoic acid (commonly called acetic acid)
• Tamarind contains tartaric acid
• Sour milk contains lactic acid

Action on Indicators

• Indicators change colors in the presence of acids and bases

Conduction of Electricity

• Aqueous acid solutions conduct electricity
• Acids dissolved in water produce ions, which conduct electricity
• This is known as dissociation
• Specifically, acids produce hydrogen ions (H+) with characteristic properties
• H+ ions do not exist alone, but combine with water molecules: H+ + H2O → H3O+

Hydronium Ions

• H3O+ ions are called hydronium ions
• They are also represented as H+(aq)
• Based on dissociation in aqueous solutions, acids are strong or weak

Strong Acids

• Acids which completely dissociate in water are called strong acids
• Nitric acid completely dissociates in water: HNO3(aq) → H+(aq) + NO3¯(aq)
• The seven strong acids are hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), perchloric acid (HClO4), chloric acid (HClO3), sulphuric acid (H2SO4), and nitric acid (HNO3)

Weak Acids

• Weak acids only dissociate partially in water
• It is depicted by double half arrows.: HF(aq) H+(aq) + F-(aq)
• The 'equilibrium' exists between undissociated HF(aq) and H+(aq) and F¯(aq) ions
• Examples: ethanoic (acetic) acid (CH3COOH), hydrofluoric acid (HF), hydrocyanic acid (HCN), and benzoic acid (C6H5COOH)

Acids that Contain Hydrogen

• Not all hydrogen containing compounds are acids
• Ethyl alcohol (C2H5OH) and glucose (C6H12O6) both contain hydrogen but do not produce H+ ions in water
• Their solutions do not conduct electricity
• They are therefore not acidic

Reaction of Acids with Metals

• Acids react with metals
• A metal combines with the acid's remaining part and forms a salt
• General formula: Acid + Metal → Salt + Hydrogen gas

Reaction of Acids with Carbonates

• Acids react with metal carbonates and hydrogen carbonates
• Formula: Metal carbonate + Acid → Salt + Water + Carbon dioxide
• Metal hydrogen carbonate + Acid → Salt + Water + Carbon dioxide

Reaction of Acids with Metal Oxides

• Acids react with metal oxides
• Formula: Metal oxide + Acid → Salt + Water

Reaction of bases with acids

• The process which results in the formation of salt and water.is called neutralization
• HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
• H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)
• The general formula: Acid + Base → Salt + Water

Corrosive Nature

• Corrosive nature enables acids to attack different substances, including metals, metal oxides, and hydroxides
• It is related to part of the acid, not its strength
• Hydrofluoric acid (HF) is a weak acid, but it is corrosive and dissolves glass
• Fluoride ions attack silicon atoms in silica glass; hydrogen ions attack the oxygen

Properties of Bases

• Taste and touch: have a bitter taste and feel soapy to the touch
• Color change of Indicators differs with bases

Indicators in bases

• Litmus is blue
• Phenolphthalein is pink
• Methyl orange is yellow

Electricity and Dissociation of Bases

• Aqueous (water) solutions of bases conduct electricity due to the formation of ions
• Bases dissociate and produce hydroxyl ions (OH¯), which are responsible for the characteristic properties

Aqueous solutions are called Alkalis

• All alkalis are bases, however, all bases are not alkalis
• Bades are stronger or weaker based on dissociation

Strong Bases

• Completely dissociate in water
• Form the cation and hydroxide ion (OH¯)
• Example: KOH(aq) → K+(aq) + OH¯(aq)
• Strong bases consist of hydroxides of elements in Groups 1 and 2

There are eight strong bases

• Lithium hydroxide (LiOH)
• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Rubidium hydroxide (RbOH)
• Cesium hydroxide (CsOH)
• Calcium hydroxide (Ca(OH)2)
• Strontium hydroxide (Sr(OH)2)
• Barium hydroxide (Ba(OH)2)

Weak bases

• Do not furnish OH- ions by dissociation
• They react with water to furnish OH¯ions
• NH3(g) + H2O(l) → NH4OH
• NH4OH(aq) ⇄ NH4+(aq) + OH-(aq)
• Reaction does not complete, solution has low OH- concentration
• Equilibrium is reached before the reaction has completed

Weak bases examples

• NH4OH
• Cu(OH)2
• Cr(OH)3
• Zn(OH)2

Reaction of with Metals

• React with metals in solutions
• Liberating hydrogen gas

Non-metal oxides

• Bases react with oxidates, such as CO2, SO2, SO3, P2O5 etc.
• Forming salt and water

Neutralization Reactions

• Reactions between acids and bases are called neutralization reactions
• Producing salt and water
• strong bases, such as potassium hydroxide, are corrosive to organic matter
• Breaking down proteins on skin and flesh into a pasty mass
• Called "caustic action"
• Sodium hydroxide is called "caustic soda", potassium hydroxide is called "caustic potash"
• Term "caustic" is not used for corrosive action of acids

Water - Importance

• Acids furnish H+ ions
• Bases furnish OH¯ ions
• Water is very important for these processes

Water - Dissociation

• Moistening with a drop of water color turns red.
• Acidic characteristics are only seen as color changes from blue litmus paper to red with water introduced
• Similar behavior exhibited in alkali
• The colour of red litmus paper immediately changes to blue when water is introduced

Exothermic

• Acid (like sulfuric acid) or a base (like sodium hydroxide) to water which causes the formation of a hotter solution
• Dissolution is exothermic
• Part of the thermal energy released breaks chemical bonds
• Forming H+(aq) and OH-(aq) ions.

H+ and OH- ions

• Solid-state ionic compounds consist of ions
• E.g. Sodium hydroxide (Na+ and OH¯ion); solvent presence weakens electrostatic forces

Self-dissociation of Water

• Water helps produce H+(aq) and OH-(aq) ions
• ‘Self-dissociation of water’ as dissociation in the water itself
• Only about two of every billion (109) water molecules are dissociated @ 25°C
• Concentrations for H+(aq) and OH–(aq) ions is extremely low, 1.0 × 10–7 mol L–1 @ 25°C (298K)
• In pure conditions, the square brackets denote molar quantities for each species

Pure water equals balanced ions

• In pure conditions, [H+] = [OH-]
• Product of both ions in water is constant
• Referred to as the product ion of water, (Kw)
• Can be written as Kw=(1.0 x 10-7)(1.0 x 10-7) which = 1.0*10-14

Solutions

• All the solutions produced are a result from equal numbers

Neutral solutions

• The species is equal between H+ and OH where [H+]=[OH-]