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. According to the Brønsted-Lowry Theory, an acid is an H+ ion (or proton) donor. Properties: Taste. Most dilute acids have a sour taste. This is true of the three common mineral acids sulphuric, hydrochloric and nitric, and many others. The sour taste of many unripe fruits, lemons and milk is caused by the acids in them. 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), N­2O (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 InsolubleAll chlorides Silver chloride Lead (II) Chloride All sulphates Barium Sulphate Calcium Sulphate Lead (II) SulphateAll Nitrates Sodium Carbonate Potassium Carbonate All CarbonatesAll 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. |}