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Chemistry I.G.C.S.E. Notes – Chapter 8 – Acids, Bases and Salts

Acids

·         Acids from animal and plant material are called Organic acid

·         Acids that chemists have found ways of creating are mineral acids

·         Mineral acids are strong acids and organic acids are weak acids

·         Solutions of acids can be dilute or concentrated

·         An acid is a substance that releases hydrogen ions when dissolved in water

Properties:

·         Acids have sour taste

·         Change colour of indicators. Acids turn blue litmus paper red

·         Acids react with metals to produce hydrogen and salt of the metal

o        Some metals react slow (copper) and some react very fast (sodium)

Metal + acid à metal salt + hydrogen

Zinc + Sulphuric Acid à Zinc Sulphate + Hydrogen

Zn (s) + H₂SO₄ (aq) à H₂ (g) + ZnSO₄ (aq)

o        Metal replaces hydrogen of acid

·         Acids react with carbonates to give carbon dioxide, salt and water

     Magnesium Carbonate + Hydrochloric Acid à Carbon Dioxide + Magnesium Chloride + Water

             MgCO₃ (s) + 2HCl (aq) à CO₂ (g) + MgCl₂ (aq) + H₂O

·         Acids neutralize bases to form salt and water only

Bases

·         A substances that reacts with an acid to form a salt and water only

·         They neutralize acids, the product being a salt

Acid + Base à Salt + Water

Calcium Hydroxide + nitric acid à Calcium nitrate + Water

·         Alkalis are soluble bases

Sodium Hydroxide + hydrochloric acid à Sodium Chloride + Water

NaOH (aq) + HCl (aq) à NaCl (aq) + H₂O (l)

·         Limewater is an alkali (calcium hydroxide). The test for carbon dioxide turns it cloudy (Calcium carbonate)

                   Carbon Dioxide + Calcium Hydroxide à Calcium Carbonate + water

Properties:

·         Bases neutralize acids to form salt and water only

·         Soluble bases change colour of indicators e.g. turn red litmus blue

·         Soluble bases feel soapy (they convert some oil on your skin into soap). Decreasing agents; convert oil and grease into soap.

·         Solution of alkali in water contains hydroxide ions OH^- (aq). Because most metal hydroxides are insoluble, an alkali added to a solution of a metal salt will precipitate an insoluble metal hydroxide.

Weak and Strong Acids

·         Acids that contain more hydrogen ions (ionize to a larger extent) react faster (e.g. HCl)

·         This is a strong acid (nearly all acid molecules form ions)

·         In weak acids, only some of the acid molecules form ions

·         Strong acids have lower pH numbers than weak acids of the same concentration

Strong Acids                                         Weak Acids

Hydrochloric Acid                                   Ethanoic Acid

Sulphuric Acid                                        Citric Acid

Nitric Acid                                             Carbonic Acid

Weak and Strong Alkalis

·         A strong alkali exists almost completely as ions in solution

·         A weak alkali, only some of the molecules will form ions in solution

Strong Alkalis                                       Weak Alkalis

Sodium Hydroxide                                  Ammonia

Potassium Hydroxide

Calcium Hydroxide                    

pH/Universal indicator paper

·         pH expressed the level/degree of acidity or alkalinity in an aqueous solution or a liquid

·         universal indicator turns different colors in strongly acidic and weakly acidic solutions

·         comes in both liquid and solution

·         Acids: pH number less than 7

·         Alkali: pH number GREATER than 7

·         Neutral Substances = pH = &

Other Indicators

Acidity in Soil

·         Most plants grow best when pH is close to 7

·         Is soil too acidic, or too alkaline, plants grow badly or not at all

·         Chemicals are added to the soil to adjust pH

·         Most often, soil is too acidic so it is treated with quicklime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate) (ALL CHEAP BASES)

TYPES OF OXIDES

Metallic/Basic Oxides

·         Basic Oxide + Alkali à no visible reaction

·         Basic Oxide + Acid à Salt + Water Only

·         Basic Oxide + Water à alkalis

o        CaO, MgO, CuO, Na₂O, K₂O

·         Oxides of P + K form peroxides on heating, other decompose

Non-Metallic/Acidic Oxide

·         Acidic Oxide + Alkali à Salt + water only [SO₃(s) + 2NaOH(aq) à Na₂SO₄ (aq) + H₂O (l)]

·         Acidic Oxide + Acid à no visible reaction [NO₂ + HCl à no reaction]

·         Acidic Oxide + Water à Acid [2NO₂ + H₂O à HNO₃ + HNO₂] *exception: mixed anhydride

o        Exception: 2NO₂ + 2NaOH à NaNO₃ + NaNO₂ + H₂O *mixed anhydride

o        SO₂, SO₃, NO₂, P₂O₅ (pentaoxide), P₂O₃

Neutral Oxide

o        Non-metallic oxides (but not acidic)

o        CO (Carbon Monoxide), NO (Nitrogen Monoxide), N_­2O (Dinitrogen Oxide), H₂O (Water)

o        No reaction with anything

Amphoteric Oxides (both basic/acidic)

o        Ampho Oxide + Acid à Salt + Water only

o        Ampho Oxide + Alkali (+ water) à Complex Salt

o        ZnO (s) + 2NaOH (aq) à H₂O à Na₂Zn(OH)₄ (Sodium Zincate)

o        Al₂O₃ + 2NaOH + 3H₂O à 2NaAl(OH)₄ (Sodium Aluminate)

o        PbO + 2NaOH + H₂O à Na₂Pb(OH)₄ (Sodium Plumbate)

Preparation of Basic Oxides

o        For Pb, Cu, Zn, Fe, Al, HEAT THE METALLIC nitrate, hydroxide or carbonate

(a)    2Cu(NO₃)₂ (s)  --∆à 2CUO (s) + 4NO₂(g) + O₂ (g)

*identifiable with brown nitrogen dioxide gas

(b) Cu(OH)₂ (s) --∆à CuO (s) + H₂O (g/l)

(c) CuCO₃ (s) --∆à CuO (s) + CO₂ (g)

Salts

o        Substance formed when either all or part of the hydrogen ions in an acid is replaced by a metallic or ammonium ion.

o        Made when acid is neutralized by base (water is also formed)

o        Anhydrous – salts with no water

o        Hydrated – salts with a fixed % of water (water of crystallization or water of hydration) – important for shape and color of salt

o        Soluble salts dissolve to leave a clear solution

o        Insoluble salt precipitate

Soluble                                      Insoluble

All chlorides                               Silver chloride

                                                Lead (II) Chloride

All sulphates                              Barium Sulphate

                                                Calcium Sulphate

                                                Lead (II) Sulphate

All Nitrates

Sodium Carbonate

Potassium Carbonate                 All Carbonates

All Ethanoates

Sodium Salts

Potassium Salts

Ammonium Salts

Preparation of Soluble Salt

o        Acid + Metal à Salt + Hydrogen

Warm acid, add metal in excess and wait till no more hydrogen is evolved

o        Acid + Metal Oxide à Salt + Water

Add excess of metal oxide to acid. Wait till solution no longer turns blue litmus red

o        Acid + Metal carbonate à salt + water + carbon dioxide

Add excess of metal carbonate to acid. Wait until no more CO₂ evolved

o        Acid + Alkali à Salt + Water

o        Solid is added in excess to make sure no acid has been left over

After the above has been conducted, the excess solid in solution is filtered out. The filtrate is left to evaporate on the water bath. When crystals form on the rod, it can be taken off. Solution cools to crystals. Then solution can be separated by filtering, washing crystals with distilled water and then leaving crystals to dry.

Preparation of Insoluble Salt

o        All insoluble salts will go through double decomposition reaction

o        More reactive metals will displace less reactive metals in solution

Lead (II) Iodide, is an insoluble salt, will be prepared from Sodium Iodide. Lead (II) Nitrate is added to the solution (all nitrates soluble in water). A precipitate of Lead (II) Iodide forms as Sodium displaces Lead.

NaI (aq) + Pb (NO_3­)₂ (aq) à PbI₂ (s) + 2NaNO₃ (aq)

The solution with the precipitate is filtered and washed with distilled water to dissolve any NaNO_­3 still present. Residue is dried with over or dessicator to obtain insoluble salt.

QUALITATIVE ANALYSIS TO FIND THE COMPOSITION OF A SUBSTANCE

Cation                               NaoH (aq)                                         NH₃ (aq)

Aluminium (Al³⁺)                white ppt. soluble in excess                white ppt. insoluble excess

Ammonium (NH₄⁺)              ammonia produced on warming with NaOH

Calcium (Ca²⁺)                    white ppt. insoluble in excess            no ppt. or slight white

Copper (Cu²⁺)                      light blue ppt. insoluble in excess      light blue ppt. soluble excess to

                                                                                                give dark blue solution

Iron (II) (Fe²⁺)                     green ppt. insoluble in excess            green ppt. insoluble in excess

Iron (III) (Fe³⁺)                    red-brown ppt. insoluble excess         red-brown ppt. insoluble in

                                                                                                Excess

Zinc (Zn²⁺)                         white ppt. soluble in excess,             white ppt. soluble in excess

                                         giving colourless soln.                       giving colourless solution.

Anion                            Test                                                      Test Result

Cabonate (CO₃^2‑)            add dilute acid + limewater                    effervescence, CO₂ produced

Chloride (Cl^-)                 add dilute nitric acid and then silver       white ppt.

                                                                        Nitrate.

Iodide (I^-)                      acidify with dilute nitric acid, then         yellow ppt.

                                    Add aqueous lead (II) nitrate

Nitrate (NO₃^-)                add aqueous sodium hydroxide, Al          ammonia produced

                                    Foil, warm carefully.

Sulphate (SO^2-₄)             acidify, add barium nitrate or barium     white ppt.

                                    Chloride

Gases                           Test and Test Result

Ammonia (NH₃)              turn damp red litmus paper blue

Carbon Dioxide (CO₂)     turns lime water milky

Chlorine (Cl₂)                 bleaches damp litmus paper

Hydrogen (H₂)                ‘pops’ with a lighted splint

Oxygen (O₂)                   relights a glowing splint