GCSE Science/Acids & Bases, Salts and Electrolysis

Acids

• Acids from animal and plant material are called Organic acids.
• Acids that chemists have found ways of creating are Mineral acids.
• Solutions of acids can be either dilute or concentrated.
• An acid is a substance that releases hydrogen ions when dissolved in water.

Properties:

• Acids have a sour taste.
• Acids change the colour of indicators: Acids turn blue litmus paper red.
• Acids react with metals to produce hydrogen and salt of the metal.

> Some metals react slowly, for example copper, and some react very fast, such as sodium.

Metal + acid -> metal salt + hydrogen

Zinc + Sulphuric Acid -> Zinc Sulphate + Hydrogen

Zn (s) + H₂SO₄ (aq) -> H₂ (g) + ZnSO₄ (aq)

> Metal replaces the hydrogen contained in acid.

• Acids react with carbonates to give carbon dioxide, a salt and water.

Magnesium Carbonate + Hydrochloric Acid -> Carbon Dioxide + Magnesium Chloride + Water

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

• Acids neutralize bases to form a salt and water only.
• Acids have H⁺ ions which combine with OH^- ions in bases to form H₂O as a result of the neutralization reaction.

Bases

• A base is a substance that reacts with an acid to form a salt and water only.

Acid + Base -> Salt + Water

Calcium Hydroxide + Nitric acid -> Calcium nitrate + Water

• Alkalis are soluble bases

Sodium Hydroxide + hydrochloric acid -> Sodium Chloride + Water

NaOH (aq) + HCl (aq) -> NaCl (aq) + H₂O (l)

Limewater is an alkali (calcium hydroxide). It goes milky white when Carbon Dioxide is bubbled through it. The precipitate is Calcium Carbonate.

Carbon Dioxide + Calcium Hydroxide -> Calcium Carbonate + water

Properties:

• Bases neutralize acids to form salt and water only.
• Soluble bases change the colour of indicators (e.g. turns red litmus paper blue)
• Soluble bases feel soapy (they convert some oil on your skin into soap). Decreasing agents; convert oil and grease into soap.
• A solution of an 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 is the level of acidity or alkalinity in an aqueous solution or a liquid.
• Universal Indicator turns different colors in strongly acidic and weakly acidic solutions.
• It comes as both a liquid and as a solution.
• Acids have a pH value of less than 7.
• Alkalis have a pH value of more than 7.
• Neutral Substances have a value of pH 7.

Other Indicators

Indicator	Acidic Colour	Neutral Colour	Alkaline Colour
Litmus	Red	Purple	Blue
Phenolphtalein	Colorless	Colorless	Pink
Methyl Orange	Red	Yellow	Yellow

Acidity in Soil

• Most plants grow best when the pH of the soil is close to 7.
• If the soil is too acidic, or too alkaline, plants will grow badly or not at all.
• Chemicals are added to the soil to adjust the pH level.
• Most often, soil is too acidic so it is treated with quicklime (calcium oxide), slaked lime (calcium hydroxide) or chalk (calcium carbonate). These are all cheap bases.

TYPES OF OXIDES

Metallic/Basic Oxides

• Basic Oxide + Alkali -> no visible reaction
• Basic Oxide + Acid -> Salt + Water Only
• Basic Oxide + Water -> alkalis
• CaO, MgO, CuO, Na2O, K2O
• Oxides of P + K form peroxides on heating, others decompose.

Non-Metallic/Acidic Oxide

• Acidic Oxide + Alkali à Salt -> water only [SO₃(s) + 2NaOH(aq) -> Na₂SO₄ (aq) + H₂O (l)]
• Acidic Oxide + Acid -> no visible reaction [NO₂ + HCl -> no reaction]
• Acidic Oxide + Water -> Acid [2NO₂ + H₂O -> HNO₃ + HNO₂] *exception: mixed anhydride
• Exception: 2NO₂ + 2NaOH -> NaNO₃ + NaNO₂ + H₂O *mixed anhydride
• SO₂, SO₃, NO₂, P₂O₅ (pentaoxide), P₂O₃

Neutral Oxide

• Non-metallic oxides (but not acidic)
• CO (Carbon Monoxide), NO (Nitrogen Monoxide), N_­2O (Dinitrogen Oxide), H₂O (Water)
• No reaction with anything

Amphoteric Oxides (both basic/acidic)

• Ampho Oxide + Acid -> Salt + Water only
• Ampho Oxide + Alkali (+ water) -> Complex Salt
• ZnO (s) + 2NaOH (aq) -> H₂O -> Na₂Zn(OH)₄ (Sodium Zincate)
• Al₂O₃ + 2NaOH + 3H₂O -> 2NaAl(OH)₄ (Sodium Aluminate)
• PbO + 2NaOH + H₂O -> Na₂Pb(OH)₄ (Sodium Plumbate)

Preparation of Basic Oxides

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

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

• identifiable with brown nitrogen dioxide gas

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

(c) CuCO3 (s) -∆-> CuO (s) + CO2 (g)

Salts

• Substance formed when either all or part of the hydrogen ions in an acid is replaced by a metallic or ammonium ion.
• Made when acid is neutralized by base (water is also formed)
• Anhydrous – salts with no water
• Hydrated – salts with a fixed % of water (water of crystallization or water of hydration) – important for shape and color of salt
• Soluble salts dissolve to leave a clear solution
• 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

• Acid + Metal -> Salt + Hydrogen

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

• Acid + Metal Oxide -> Salt + Water

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

• Acid + Metal carbonate -> salt + water + carbon dioxide

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

• Acid + Alkali -> Salt + Water

• 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

• All insoluble salts will go through double decomposition reaction

• 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 (NO3­)2 (aq) -> PbI2 (s) + 2NaNO3 (aq)

The solution with the precipitate is filtered and washed with distilled water to dissolve any NaNO­3 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, giving colourless soln.	white ppt. soluble in excess, giving colourless solution.

Anion	Test	Test Result
Cabonate (CO32‑)	add dilute acid + limewater	effervescence, CO2 produced
Chloride (Cl-)	add dilute nitric acid and then silver nitrate.	white ppt.
Iodide (I-)	acidify with dilute nitric acid, then add aqueous lead (II) nitrate	yellow ppt.
Nitrate (NO3-)	add aqueous sodium hydroxide, Al Foil, warm carefully.	ammonia produced
Sulphate (SO2-4)	acidify, add barium nitrate or barium	white ppt.