Acids, Bases, Salts

ACIDS

Substances that release hydrogen ions (H+) when dissolved in water, resulting in a pH less than 7. These H+ ions make the solution acidic. (State that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH- ions)

Aqueous solutions of acids contain H+ ions

Eg. HNO3 (in aqueous solution) —-> H+ + NO3- Nitric acid in aq solution dissosciates into Hydrogen ions and nitrate ions. Hydrogen ions make solution acidic.

Describe the characteristic properties of acids in terms of their reactions with metals

Acid + Metal —-> Salt + Hydrogen Gas Eg. 2Hcl + Zn —-> ZnCl2 + H2

Describe the characteristic properties of acids in terms of their reactions with bases

Neutralisation reaction: Acid + Base —-> Salt + Water Eg. H2SO4 + 2KOH —-> K2SO4 + 2H2O (includes acidic oxides reacting with alkali)

Describe the characteristic properties of acids in terms of their reactions with carbonates

Acid + Carbonate —-> Salt + Water + Carbon Dioxide Eg. 2HNO3 + CaCO3 —-> Ca(NO3)2 + H2O + CO2

Describe acids in terms of their effect on litmus

Red

Describe acids in terms of their effect on thymolphthalein

Colorless

Describe acids in terms of their effect on methyl orange

Red

Strength of acid increases with the concentration of hydrogen ions, resulting in a lower pH

(Hydrogen ^ Stronger the acid, Lower the pH)

A strong acid

an acid that completely dissosciates in an aqueous solution, meaning all of its molecules break apart into ions. Eg. HCl —-> H+ + Cl (hydrochloric acid is a strong acid, as shown by HCl (aq) → H+ (aq) + Cl- (aq))

A weak acid

an acid that only partially dissosciates in an aqueous solution, meaning only some of its molecules break apart into ions. Eg. CH3COOH (aq) reversible → H+ (aq) + CH3COO- (aq)

Acidic oxides

Formed when non-metals react with oxygen, Examples: CO2 and SO2

Acidic oxides react with water

SO2 + H2O -> H2SO3 (sulfurous acid)

Acidic oxides react with alkalis

Acidic oxide + alkali → salt + water Eg: CO2 + 2NaOH → Na2CO3 + H2O

Amphoteric oxides

oxides that react with acids and bases to produce a salt and water

Examples of amphoteric oxides

Al2O3 and ZnO

BASES

Substances that release hydroxide ions (OH-) when released in water, resulting in a pH greater than 7. State that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH- ions)

Aqeous solutions of alkalis contain OH- ions

Alkalis

Specifically refer to bases that are soluble in water (they dissolve in water to produce OH- ions which make the solution alkaline). All alkalis are bases, but not all bases are alkalis.

KOH —-> K+ + OH-

Bases are oxides or hydroxides of metals and alkalis are soluble bases

Bases are usually oxides or hydroxides of metals. Eg. Metal oxides (Sodium Oxide, Na2O) or Metal hydroxides (eg. Sodium Hydroxide, NaOH)

Basic Oxides

typically formed by metals eg. Copper(II) oxide and Calcium Oxide (CaO). Basic oxide + acid —-> salt + water

Describe the characteristic properties of bases in terms of their reactions with acids

Neutralsiation reaction : Base + Acid —-> Salt + Water Eg. NaOH + HCl —-> NaCl + H2O

Describe the characteristic properties of bases in terms of their reactions with ammonium salts

Base + Ammonium Salt —-> Salt + Water + Ammonia gas Eg. 2KOH + (NH4)2SO4 —-> K2SO4 + 2H2O + 2NH3

Describe alkalis in terms of their effect on litmus

Blue

Describe alkalis in terms of their effect on thymolphthalein

Blue

Describe alkalis in terms of their effect on methyl orange

Yellow

A higher concentration of hydroxide ions in a solution leads to a higher pH

(Hydroxide ^ Stronger the alkali, Higher the pH)

Universal indicator

A substance that changes color to show how acidic or alkaline a solution is.

Strength of acid increases with the concentration of hydrogen ions, resulting in a lower pH

(Hydrogen ^ Stronger the acid, Lower the pH)

A higher concentration of hydroxide ions in a solution leads to a higher pH

(Hydroxide ^ Stronger the alkali, Higher the pH)

Define acids as proton donors and bases as proton acceptors

Acids give away protons (H+ ions), and Bases take in protons (OH- ions.) Therefore, Acids are defined as Proton Donors and bases are defined as Proton Acceptors.

Hydrated substance

When salts are made, water can be trapped inside their crystals. A hydrated substance is a substance that is chemically combined with water. Eg. CuSO4°5H2O. This water is known as water of crystallization.

Water of crystallization

The water molecules present in hydrated crystals

Anhydrous substance

substance containing no water. Eg. NaCl form crystals without water (anhydrous) when they crystallize.

sodium, potassium and ammonium salts

are soluble

Nitrates

are soluble

Chlorides

are soluble, except lead and silver

Sulfates

are soluble, except barium, calcium and lead

Carbonates

are insoluble, except sodium, potassium and ammonium

Hydroxides

are insoluble, except sodium, potassium, ammonium and calcium (partially)

Acid with alkali by titration

1. Measure a known volume of alkali into a conical flask using a pipette.

2. • Add a few drops of indicator, such as methyl orange or thymolphthalein, to the alkali.

3. • Gradually add acid from a burette until the indicator changes colour, indicating neutralisation.

4. • Record the volume of acid used.

5. • Repeat the experiment without the indicator, using the recorded volume of acid.

6. Heat the resulting neutralised solution in an evaporating basin to partially evaporate the water.

7. Allow the solution to cool and crystallise.

8. Drain the excess solution and allow the crystals to dry.

Acid with excess metal

Preparation:

1. Add excess metal to the acid in a beaker.

2. The metal dissolves, and bubbles of hydrogen gas come out.

3. Allow the reaction to continue until no more gas is produced. This shows that all the acid has reacted with the metal.

4. Filter the mixture to remove any unreacted metal.

5. Evaporate the filtered solution to obtain the pure salt.

Acid with excess insoluble base

Preparation:

1. Add excess of the insoluble base to the acid in the beaker

2. Stir and heat the mixture until no more of the base dissolves.

3. filter the mixture to remove any unreacted base

4. Evaporate the filtered solution to obtainnthe pure salt.

Acid with Excess Insoluble Carbonate

1. Add an excess of the insoluble carbonate to the acid in a beaker

2. Stir and heat the mixture until no more of the carbonate dissolves

3. Filter the mixture to remove any unreacted carbonate

4. Evaporate the filtered solution to obtain the pure salt.

Precipitation

Mixing two soluble salts in solution which react to form an insoluble salt that precipitates out of the solution

Preparation:

1. First separately dissolve known amounts of the two soluble salts in distilled water in separate beakers.

2.  Mix the two solutions together. The insoluble salt will precipitate out of the solution as a solid

3. Filter the mixture to separate the solid insoluble salt (precipitate) from the liquid solution (filtrate)

4. Wash the precipitate with distilled water to remove any remaining soluble impurities.

5. Dry the purified precipitate in an oven.