GCSE Science/Acids & Bases, Salts and Electrolysis
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- Acids from animal and plant material are called Organic acid - Acids that chemists have found ways of creating are mineral 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
> Some metals react slow (copper) and some react very fast (sodium) Metal + acid -> metal salt + hydrogen Zinc + Sulphuric Acid -> Zinc Sulphate + Hydrogen Zn (s) + H2SO4 (aq) -> H2 (g) + ZnSO4 (aq) > Metal replaces hydrogen of acid
- Acids react with carbonates to give carbon dioxide, 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 salt and water only - Acids have H+ ions which combine with OH- ions in bases to form H20 as a result of the neutralization reaction
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 -> 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). The test for carbon dioxide turns it cloudy (Calcium carbonate) Carbon Dioxide + Calcium Hydroxide -> 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
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 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 -> no visible reaction
· Basic Oxide + Acid -> Salt + Water Only
· Basic Oxide + Water -> 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 -> no visible reaction [NO2 + HCl à no reaction]
· Acidic Oxide + Water -> Acid [2NO2 + H2O à HNO3 + HNO2] *exception: mixed anhydride
o Exception: 2NO2 + 2NaOH -> 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), N2O (Dinitrogen Oxide), H2O (Water)
o No reaction with anything
Amphoteric Oxides (both basic/acidic)
o Ampho Oxide + Acid -> Salt + Water only
o Ampho Oxide + Alkali (+ water) à Complex Salt
o ZnO (s) + 2NaOH (aq) -> H2O à Na2Zn(OH)4 (Sodium Zincate)
o Al2O3 + 2NaOH + 3H2O -> 2NaAl(OH)4 (Sodium Aluminate)
o PbO + 2NaOH + H2O -> 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) -∆-> 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
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 -> Salt + Hydrogen
Warm acid, add metal in excess and wait till no more hydrogen is evolved
o Acid + Metal Oxide -> Salt + Water
Add excess of metal oxide to acid. Wait till solution no longer turns blue litmus red
o Acid + Metal carbonate -> salt + water + carbon dioxide
Add excess of metal carbonate to acid. Wait until no more CO2 evolved
o Acid + Alkali -> 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 (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, white ppt. soluble in excess
giving colourless soln. 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 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 (SO2-4) acidify, add barium nitrate or barium white ppt.''Italic text
