Week Ten Lesson 2 PDF

WEEK TEN LESSON 2  Neutralization An acid-base reaction (also called a neutralization reaction) occurs when an acid reacts with a base. Such reactions involve the transfer of a proton between reactants. Volumetric Analysis is an aspect of quantitative analysis, whic...

WEEK TEN LESSON 2  Neutralization An acid-base reaction (also called a neutralization reaction) occurs when an acid reacts with a base. Such reactions involve the transfer of a proton between reactants. Volumetric Analysis is an aspect of quantitative analysis, which involves measuring the volumes of solutions of reactants in a chemical reaction, so as to determine the amounts of reactants in such solutions. The most popular aspect of volumetric analysis is the acid-base (neutralization) reactions. The technique of determination is by titration. Other aspects are redox and precipitation reactions.  Acid-Base Titrations Acid-base titration is the technique employed to determine the amount of acid required to neutralize completely a given amount of base, and vice versa. In an acid-base titration, the solution of acid is gradually added into the solution of a base, in the presence of a suitable indicator, until the reaction is complete as signaled by the sudden change in colour of the indicator. This is the endpoint or neutralization point or equivalence point of the reaction.  Acid-Base Titrations The equivalence point occurs when the amount (mol) of H+ ions in the original volume of acid has reacted with the same amount (mol) of OH- ions from the base: Amount (mol) of H+ = amount (mol) of OH- The end point occurs when a tiny excess of OH- ions changes the indicator permanently to its basic colour. During the course of an acid-base titration, a neutralization reaction occurs between the hydrogen ions from the acid and the hydroxide ions from the base to form water.  Acid-Base Indicators An acid-base indicator is either a mixture of weak organic acids or bases that change its colour, depending on the concentration of hydrogen ions, pH of the solution. Indicators are used in acid-base titrations to detect the end points of the neutralization reactions and to test whether a liquid is acidic, alkaline or neutral.  COLOUR IN pH Transition INDICATOR ALKALI ACID NEUTRAL PK RANGE 1. Litmus Blue Red Purple 7.0 6.0-8.0 2. Methyl orange Yellow Pink Orange 3.7 3.1-4.4 3. Methyl red Yellow Pink Red 5.1 4.4-6.3 4. Phenolphthalein Pink Colourless Colourless 9.1 8.3-10.0  Buffers A buffer is a solution that resists a change in the pH on the addition of a small quantity of a strong acid or base. A buffer is either an aqueous mixture of a weak acid and its salt (acid buffer) or a aqueous mixture of a weak base and its salt (alkaline buffer). Aqueous mixture of ethanoic acid and sodium ethanoate is an example of an acid buffer, while an aqueous mixture of ammonia solution and ammonium chloride is an alkaline buffer. Buffer solutions are used in biological and biochemical processes, in order to maintain constant pH values.  Choice of Indicators in Acid-Base Titrations The choice of an indicator for a particular set of an acid-base titration depends on the strengths (strong or weak) of the acid and the base, and is governed by the pK of the indicator, and the pH at the endpoint. The accuracy of a titration depends on the use of the correct indicator. Acid-base Titration Example pH of solution at Suitable Indicator endpoint Strong acid 7 Methyl orange or Vs HNO3 Vs NaOH Phenolphthalein Strong base Strong acid H2SO4 5-6 Methyl orange Vs Vs Weak base Na2CO3 Weak acid H2C2O4 Vs KOH 8-9 Phenolphthalein Vs Strong Base Weak acid CH3COOH Variable No suitable indicator; use a pH Vs Vs meter Weak Base NH3 (aq)  Acid-Base Titration Apparatus The pieces of apparatus used in acid base titrations are burette, pipette, conical flask, retort stand with clamp, funnel, white tiles and beakers. Precautions during Titration: Sources of Error In order to obtain accurate results during titrations, the following precautions should be taken; if not, they are likely sources of error: Rinse burette with the solution of the acid; to avoid diluting it with the remains of water used in washing. Rinse pipette with the solution of the base; to avoid diluting it with the remains of water used in washing.  Precautions during Titration: Sources of Error Never rinse the conical flask with the solution of the base; to avoid using more solutions than required. Remove air bubbles from the burette or pipette, to obtain accurate volume of solution. Avoid parallax error in reading the meniscus level of the burette or pipette, to obtain accurate volume. Pipette to the mark to avoid using more or less solution than required.  Precautions during Titration: Sources of Error Avoid spillage of solution from the pipette when transferring; to avoid using less solution than required. Place the conical flask on a white surface; to avoid over-shooting the end point. Shake the conical flask during titration; to obtain a homogeneous solution. Read the burette reading to two decimal places for consistency and accuracy. Obtain at least two sets of burette readings; for consistency and accuracy.  Precautions during Titration: Sources of Error Do not blow the last drop at the tip of the pipette; to avoid using more solution than the pipette is constructed to deliver. Always use two or three drops (maximum) of indicator, so as to obtain a sharp endpoint. Clamp the burette in a vertical position; to avoid parallax error while taking the reading. The burette tap or clip must be tight to avoid leakage. Remove the funnel from the burette before taking the initial reading, to prevent remains of solution in it from dropping into the solution in the burette, thereby changing the burette reading.  Importance of Acid-Base Titrations Acid-base titrations are used for the following purposes: 1. To standardize a solution of an acid or base (Standardization). 2. To determine the molar mass of an acid or a base. 3. To establish the mole ratio of acid to base in a neutralization reaction. 4. To determine the percentage purity of an acid or base. 5. To estimate the percentage of water crystallization in an acid or base. 6. To estimate the solubility of a base. 7. To determine the degree of temporary hardness of a water sample. 8. To determine the amount of acid or base in excess, after a titration.  Metathesis Reactions An exchange (or metathesis) reaction is a reaction between compounds that, when written as a molecular equation, appears to involve the exchange of parts between the two reactants. In a precipitation reaction, the anions exchange between the two cations (or vice versa). The molecular equation for the reaction between Pb(NO3)2 and NaI shows the ions exchanging partners Such reactions are called double-displacement reactions, or metathesis (pronounced meh-TA-thuh-sis) reactions.