Windsor University School of Medicine PREPARATION & COLLECTION OF NON-METAL COMPOUNDS UNLESS YOU TRY TO DO SOMETHING BEYOND WHAT YOU HAVE ALREADY MASTERED, YOU WILL NEVER GROW. RALPH WALSO EMERSON Ch19. J.C. Rowe Collection of gases Downward displacement of water Upward delivery Downward delivery Collecting 5 important gases (table) Downward displacement of water Downward displacement of water Suitable for collecting gases which are insoluble in water. e.g. hydrogen, oxygen, carbon dioxide. Not suitable for hydrogen chloride gas and ammonia gas which are very soluble in water Collection of a gas by the displacement of water Upward delivery A gas which is less dense than air can be collected by upwards delivery. Ex: ammonia & hydrogen A gas jar (inverted) is connected by a tube to the flask which is giving off the gas Downward delivery A gas which is more dense than air can be collected by downward delivery. Ex: carbon dioxide, chlorine, hydrogen chloride & sulfur dioxide. A gas jar (upward) is connected by a tube to the flask which is giving off the gas. Downward vs. Upward collections Downward delivery : Gas jar is upward Gas is more dense than air. Upward delivery: Gas jar is inverted (downward) Gas is less dense than air. You need to know how to collect gases produced in reactions by upward & downward delivery, over water using a gas syringe. You need to be able to explain the method of collection if you are given information about the density & solubility of the gas concerned. Collecting 5 important gases Hydrogen Oxygen Carbon dioxide Ammonia less dense than air? yes yes Upward delivery yes yes denser than air? slightly Downward delivery Chlorine yes yes yes yes Solubility in water? very low low low over water yes yes yes very high moderate yes Laboratory preparation of gases Ammonia, NH3 Carbon dioxide, CO2 Chlorine, Cl2 Hydrogen, H2 Nitrogen, NO2 Oxygen, O2 Sulphur dioxide, SO2 Ammonia, NH3 Ammonia gas can be produced in laboratory by heating an ammonium salt (ammonium chloride) with a strong alkali (sodium hydroxide). NH4Cl (s) + NaOH (aq) ……. NH3 (g) + NaCl (aq) + H2O (l) Identification of gas Ammonia turns damp red litmus blue; & in the presence of HCl gas white fumes of ammonium chloride are formed. Carbon dioxide, CO2 Reacting any carbonate with any acid forms carbon dioxide; For example calcium carbonate & hydrochloric acid : CaCO3 + 2HCl ……….. CaCl2 + CO2 + H2O Identification of gas Carbon dioxide turns lime water “milky” Chlorine, Cl2 Chlorine is formed by oxidation of concentrated hydrochloric acid using either manganese dioxide or potassium permanganate as the oxidising agent. MnO2 + 4HCl …… MnCl2 + Cl2 + H20 2KMnO4 + 16HCl ……. 2KCl + 5Cl2+2MnCl2 + 8 H2 O Identification of the gas. Chlorine is a pale green gas, which turns moist blue litmus paper red & then bleaches it. Hydrogen, H2 Any reactive metal (a meta above hydrogen in the reactivity series) will displace hydrogen from a dilute acid except for nitric acid. Zn (s)+ HCl (aq) …………. ZnCl2 (aq) + H2 (g) Identification of the gas. Hydrogen gives a “popping” sound when mixed with oxygen & ignited. Nitrogen dioxide, NO2 Nitrogen dioxide is formed during the decomposition of metal nitrates on heating: 2Pb(NO3) 2 (s) …… 2PbO (s)+ 4NO2 (g) + O2 (g) Sodium nitrate & potassium nitrate are exceptions as they produce the corresponding nitrite & oxygen gas instead. 2NaNO3 (s) ………. 2NaNO2 (s)+ O2 (g) Identification of the gas. Nitrogen dioxide is a brown gas, which turns moist blue litmus red but does not bleach it. Oxygen, O2 Oxygen is produced by the decomposition of hydrogen peroxide (H2O2). Hydrogen peroxide naturally decomposes slowly to form oxygen & water on exposure to light. 2 H2O2 (l) ……….. 2 H2O (l) + O2 (g) Identification of gas. Oxygen relights a glowing splint. Sulphur dioxide, SO2 Sulphur dioxide is formed by gently heating a sulphite salt or hydrogen sulphite with dilute hydrochloric acid or sulphuric acid: Na2SO3 (s) + 2HCl (aq) ……. 2NaCl (aq) + SO2 (g) + H2O (l) Na2SO3 + H2SO4(aq) ……. Na2SO4 (aq) + SO2 (g) + H2O (l) Identification of gas. When sulphur dioxide is bubbled through acidified potassium dichromate, the color of the solution changes from orange to green. Michael Faraday A natural law regulates the advance of science. Where only observation can be made, the growth of knowledge creeps; where laboratory experiments can be carried on, knowledge leaps forward.