Acids-and-bases.pdf, Schemes and Mind Maps of Marketing

Download Acids-and-bases.pdf and more Schemes and Mind Maps Marketing in PDF only on Docsity! 2014/01/01 1 Sour taste in lemons is caused by citric acid. Acids can ‘burn’ holes in metals or clothing. Acids can damage skin & cause blindness in the eye. Rain containing acid can very slowly dissolve cement, concrete & stone buildings. Acid rain from burning coal Acids & bases The Arrhenius model Arrhenius acids are regarded as substances that liberate H+ ions when dissolved in water. Hydrochloric acid: HCl → H+ + Cl- Nitric acid: HNO3 → H+ + NO3 - Sulphuric acid: H2SO4 → 2H+ + SO4 2- Phosphoric acid: H3PO4 → 3H+ + PO4 3- Acid Arrhenius model Arrhenius bases are regarded as substances that form OH- ions when dissolved in water. Sodium hydroxide: NaOH → Na+ + OH- Potassium hydroxide: KOH → K+ + OH- Calcium hydroxide: Ca(OH)2 → Ca2+ + 2OH- Although both acids, bases & water contain covalent molecules, the molecules are polar in nature & the acids & bases dissolve in water to form new ions in solution. Acid base song ∂+ ∂- H Cl Cl _ O H H Cl- O H H + H3O + There is a polar covalent bond in the HCl molecule as a result of the unequal sharing of the electron pair. The bond in the HCl breaks and new Cl- and H3O + ions are formed. These ions are then surrounded by the other water molecules. H Acids added to water When an acid reacts with a base there is an exchange of ions: H2SO4 + Ca(OH)2 → CaSO4 + 2H2O Expressed slightly differently – showing the ions in each solution: 2H+ + SO4 2- + Ca2+ + 2OH- → CaSO4 + 2H2O The - & + pairs in the solutions swop & we form a salt & water. This is called neutralisation. Whenever we add an acid & a base we always produce a salt & water. We say the base has neutralised the acid – or vice- versa. HCl + NaOH → NaCl + H2O H2SO4 + Ca(OH)2 → CaSO4 + H2O Acid + base → salt + water In order to tell if the one has neutralised the other, we need to use an indicator to see when there is a change in colour.. Acid and alkali reacting 2014/01/01 2 N.B. In the Lowry-Brønsted model, an acid is a proton donor and a base is a proton acceptor. Besides the alkalis (metal hydroxides) there are many other bases, such as metal oxides and metal carbonates etc. We thus need a wider definition of acids & bases. Lowry Bronsted theory for acids and bases Acid: HCl → H+ + Cl- Acid → proton + ion Base: NH3 + H+ → NH4 + base + proton → ion An acid is ∴ any substance that can donate a proton when reacting. A base is any substance that receives a proton when reacting. The reaction between an acid and a base is the donating and receiving of a proton. The following reaction illustrates an acid-base reaction: HCl + NH3 → Cl- + NH4 + Acid donating a proton Base receiving a proton Complete the following reactions & identify the acids & bases: H2SO4 + CaCO3 → NH3 + H2O → HCn + NH3 → HNO3 + NaOH → Conjugate acid base pairs An acid forms a conjugate base and a base forms a conjugate acid. Conjugate means ‘formed from.) HCl → H+ + Cl- Acid → proton + conjugate base NH3 + H+ → NH4 + base + proton → conjugate acid To differentiate between them we label them a1& b1, a2 & b2 where the 1& 2 represent the pairs. Conjugate acids and bases Conjugate acid base pairs e.g. HCl + NH3 → NH4 + + Cl- a1 b2 a2 b1 Similarly: NH4 + + Cl- → HC l + NH3 a2 + b1 a1 b2 The white and red arrows indicate the respective pairs. Conjugates are formed on the right hand side of the equation. In each pair there must be an acid & a base. Acid base pairs 2014/01/01 5 (c) M & S Marketing 25 The equilibrium constant for water is labelled by Kw. It is called the ionisation constant or Ionic product for water. Kw = [H+][OH-] The value of the ionic product is 1,0 x 10 -14 at 250C. If we consider the ionisation reaction of water, it is clear that there should be equal quantities of H+ and OH-. [H+] means ‘concentration’ of the ion or substance in the brackets. For pure water: [H+] = [OH-] = 10-7 And Kw will thus be 10-14 Kw for water H2O + H2O ⇆ 2[H+] + 2[OH-] Water ionises automatically by itself to form H+ and OH- ions according to the following equation: The one water molecule acts as an acid as it donates a proton, while the other acts as a base as it accepts a proton to form the respective ions. 27 Neutral Solution Acidic soln. Basic soln. [H+] : 10-7 mol/dm3 [OH-] : 10-7 mol/dm3 > 10-7 mol/dm3 < 10-7 mol/dm3 < 10-7 mol/dm3 > 10-7 mol/dm3 Note that in all cases: [H+][OH-] = 10-14 at 250C. Kw value To establish whether an alkali has neutralised an acid, add the indicator to the acid. Now slowly add the alkali – when the indicator changes colour – you have neutralised the acid & have thus added enough alkali. Burette Acid with litmus Add alkali Adding more alkali just makes the solution more alkaline & the indicator will go to a darker blue. Energy is released in process – exothermic reaction. Neutralising an acid The pH scale enables us to determine the degree of acidity of an acid or a base & goes from 0 to 14. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 N Increasing acidity Increasing basicity strongly acid: pH = 0, 1 or 2 weakly acid: pH = 4, 5 or 6 neutral soln.: pH = 7 weakly alkaline: pH = 8, 9 or 10 strongly alkaline: pH = 12, 13 or 14 Introduction to pH The pH scale is used in many instances – to ensure that solutions are not too acidic or alkaline. It is used in the following circumstances: Doctors checking blood for diseases. Safe drinking water should have a pH of 7. Steam used in power stations should not be too acidic or alkaline – will cause corrosion. Farmers check pH of their soil before planting. Swimming pools should have a pH of about 7,6 – otherwise chemicals become dangerous. Maintaining pH of a swimming pool Comparing pH of samples of water pH of soil in garden or on a farm 2014/01/01 6 Name of indicator Colour acid Colour base pH range Methyl Orange Red Yellow 3.1 – 4.4 3 - 4 Bromothymol Blue Yellow Blue 6 -7.6 - 8 Phenolphthalein Clear Red 8.3 – 10.0 - 10 Which indicator should be used depends upon which acid and which base are being used. Weak acid & weak base – pH range of 3 – Methyl orange Strong acid & strong base – pH range of 10 – phenolphthalein. Which indicator to use? pH stands for: ‘potential of hydrogen’ – one meaning at least 32 To calculate pH, the H+ concentration is needed. pH = -log [H+] Neutral Solution Acidic Basic pH: 7 <7 >7 pOH: 7 >7 <7 pH is measured by using acid-base indicators or pH meters. (c) M & S Marketing 33 [H2SO4] = 0,02 mol/dm3 Find its pH = ? Sulphuric acid is a strong acid that ionises completely in water forming 2 mol of H+ ions per ion of H2SO4. [H+] = 0,04 mol/dm3 pH = -log[H+] = -log 0,04 = 1,40 (c) M & S Marketing 34 [CH3COOH] = 0,02 mol/dm3 Ka = 1,8 x 10-5 CH3COOH CH3COO- + H+ COOH]CH COOCH[H 3 - 3 [ ]][ Ka = = COOH][CH ][H 3 2 Find the pH = ? (c) M & S Marketing 35 1,8 x 10-5 = 0,02 ][H 2 [H+] = 0,02 x 10 x 1,8 -5 = 0,0006 mol/dm3 pH = - log 0,0006 = 3,22 (c) M & S Marketing 36 [NaOH] = 0,02 mol/dm3 Find the pH = ? [H+][OH-] = 10 -14 [H+] = 0,02 10-14 = 5 x 10-13 pH = - log 5 x 10-13 = 12,30 pH calculation 2014/01/01 7 Hydrolysis of salts Most salts, when dissolved in water to form a solution, will produce solutions with a pH of 7, since there are the same number of H3O + & OH- ions. However, certain salts can yield either acidic or basic solutions when they dissolve in water. The water molecules either donate or receive H+ ions, thus making the solutions either acidic or basic. CO3 2- + H20 → HCO3 - + OH- ∴ solution basic NH4 + + H2O → NH3 + H3O + ∴ solution acidic NH4+ - strong or weak acid? An acid plus a base gives a salt and water. An acid plus a metal oxide gives a salt and water. An acid plus a metal gives a salt and hydrogen. An acid plus a carbonate gives a salt, CO2 and water. Reactions of acids Sulphuric acid - be careful! mA milli-ammeter Carbon rods Pour different liquids into a beaker and then connect up the electrical circuit as in sketch. Since all acids & alkalis contain ions, they will conduct electricity & there should be a reading every time you use an acid or an alkali. However the ability to conduct an electric current will differ quite a lot and depends upon the ability of the substance to form ions in solution. Conductivity during neutralisation Acids & bases are classified according to their ability to donate/receive protons & thus form new ions. Acids that donate protons completely to form new ions are said to be strong acids. Acids that only donate a few of their protons to form new ions are called weak acids. Strong acid: HCl + H2O → H3O + + Cl- ionises 100% Weak acid: HCOOH + H2O → H3O + + COOH- only ionises say 10% The ‘strong’ & ‘weak’ refer to the type of the acid/base & their ability to form ions. It does not refer to the concentration. Strong acids: HCl, HNO3, H2SO4 Strong bases: NaOH, KOH, Ca(OH)2 Weak acids: HCOOH, H2S, H2CO3 Weak bases: NO3 -, HSO4 -, SO4 2- Ionise completely Ionise partially A lot of methanoic acid (HCOOH) in a little water would be referred to as a concentrated, weak acid solution. Dissociation of strong and weak acids Volumetric analysis is the process of establishing the exact concentration of a solution by making use of neutralisation of an acid of unknown concentration with a base of known concentration (or vice–versa). A titration is the actual physical process whereby a an amount of acid is added from a burette into a 25 ml quantity of base (of known concentration) in a conical flask to exactly neutralise the base. Titration