YEAR 12 CHEMISTRY ACIDS AND BASES Acids are compounds that: have a low pH (below 7) taste sour turn blue litmus paper red react with bases to neutralize them and produce salts release H2 gas in reactions with active metals aqueous solutions conduct electricity furnish H+ burn the skin if strong dissolve carbonates These are some common acids. What others can you think of? Make a list in your notes. ACIDS AND BASES Bases are compounds that: have a high pH (above 7) taste bitter turn red litmus paper blue react with acids to neutralize them and produce salts are slippery feeling aqueous solutions conduct electricity burn skin if strong react with fats to form soap furnish OH- What other bases/alkalis do you know? ACIDS AND BASES Some common laboratory acids HCl HBr HI hydrochloric acid hydrobromic acid hydroiodic acid HClO4 HNO3 H2SO4 perchloric acid nitric acid sulfuric acid Five of these acids are classified as monoprotic acids. They only have one hydrogen which they are able to donate. Sulfuric acid is classified as a diprotic acid because it has two acidic hydrogens that it can donate. Similarly, an acid which has three donatable hydrogens would be classified as triprotic. ACIDS AND BASES Some common laboratory bases NaOH KOH NH3 sodium hydroxide potassium hydroxide ammonia Ca(OH)2 Mg(OH)2 Na2CO3 calcium hydroxide magnesium hydroxide sodium carbonate Bases are often found in everyday products such as many cleaning products (sodium hydroxide), antacid products (magnesium hydroxide )and fertilisers (ammonia). It is a common misconception that bases are not as dangerous as acids. In fact, many bases can be as much or more corrosive than many acids. pH Indicators A simple explanation of pH is that it is a measure of acidity/basicity (more indepth explanation to follow) Many substances change colour as they are exposed to different pH levels. These can be used to “indicate” the pH of substances when the colour ranges are known Some natural products such as litmus, cabbage, grapes and tea are natural indicators while others such as phenolphthalein and methyl orange are synthetic pH indicators are themselves acids or bases as they donate or accept protons (more on this later) Notice the variety of ranges where different indicators change colours. Some have more than one change. pH of some common substances pH Indicators Some specific examples: 1. 2. 3. 4. Litmus Phenolphthalein Methyl orange Bromothymol blue red5 8blue colourless8.3 10red red3.1 4.4yellow yellow6 7.6blue Note: colour changes within these ranges are gradual A pH problem Indicator Colour Litmus Blue Phenolphthalein Colourless Methyl orange Yellow Bromothymol blue Blue An unknown solution produces the colours above. What is the pH range of this solution? 8-8.3 Uses of Indicators Soil testing Most plants cannot survive outside pH 5.5-7.5 Different plants require different pH level 2 methods for testing soil pH Mix small sample with universal indicator and sprinkle BaSO4 powder on top – read colour Mix soil with water in a test tube and add indicator – read colour Changing soil pH Too acidic – add NH3, CaO (lime), CaCO3 Too basic – add manure, pine bark, peat Uses of Indicators Pool acidity Pool should be pH of 7.4 Tested with a meter or an indicator such as phenol red Changing the pH Too acidic – add CaOCl2 (pool chlorine), Na2CO3 Too basic – add HCl soln, NaHSO4 Non-metal Oxides Sources Atmospheric O2 is very reactive and reacts with many substances to form oxides Natural formation 1. 2. 1. 2. 3. CO2 – from respiration (“burning” sugars for energy) NO2 – from lightning strikes (N2 + 2O2 in the air 2NO2) SO2 – released from volcanoes or H2S + O2 SO2 + H2O (H2S produced by bacterial decomposition of organic matter) Human causes (bushfires and burning fossil fuels) 3. 1. 2. 3. 4. 5. CO2 – fossil fuel combustion product NO – high temperature combustion product NO2 – NO is easily oxidised in the air (NO + O2 NO2) SO2 – burning coal that contains S as an impurity SO3 – SO2 is easily oxidised in the air Acidic Non-metal Oxides Many non-metal oxides react with water in the atmosphere to produce acids; CO2 + H2O H2CO3 (carbonic acid) SO2 + H2O H2SO3 (sulfurous acid) SO3 + H2O H2SO4 (sulfuric acid) 2NO2 + H2O HNO3 + HNO2 (nitric and nitrous acid) These non-metal oxides are all gases Their acidic products all contribute to the acidity of rain Oxide Trends in the Periodic Table Oxides tend to increase in acidity from left to right In general: Metal oxides are basic (left side) Non-metal oxides are acidic (right side) Exceptions: Amphoteric oxides (i.e. Al, Be, Ga, Sn, Pb) Why this trend? This is due to electronegativity increasing from left to right (see following slides for more details) Oxides on the left side of the PT Electrons are transferred to the O2 This is due to the ionic nature of these bonds because of a large difference in electronegativities, therefore ions are formed in solution For example: element electronegativity Na 0.93 O 3.44 This means: Na2O(aq) Na+ + O2- and O2- + H+ OH- (readily) This overall consumption of H ions leads to an increase of pH (i.e. Basic) Oxides on the right side of the PT Electrons are shared with the O2 This is due to the covalent nature of these bonds because of a small difference in electronegativities, thus no ions are formed For example: element electronegativity S 2.58 O 3.44 This means: Due to a partially positive S central atom, SO3(aq) + H2O H+ + HSO4- This overall production of H ions leads to an decrease of pH (i.e. acidic). (See following slide for details of this reaction) Sulfur trioxide forms an acid O S O - O OH- S O Sulfur trioxide H+ water O O H+ OH Sulfuric acid solution Industrial Pollution Research The Industrial Revolution brought about many changes in the 19th Century. One of these changes involves an increase in the release of sulfur dioxide SO2 and oxides of nitrogen such as NO2. Summarise the industrial origins of SO2 and NO2 and evaluate reasons for concern about their release into the environment 2. Consider health issues, visibility issues, effects on the environment (rivers, soil, plants), and effects on buildings 1.