Acid-base reactions equivalence point

Acid-base indicator, 403-404q colors, 392-393 equivalence point and, 84 Acid-base reactions, 96-97q, 402q amino acids, 622-625 Brensted-Lowry model, 353-354 buffer systems, 383-391 equations for, 82-84 Lewis acid in, 410 Lewis base in, 410 types, 81-82... 

Method 1 The resultant solution at the equivalence point of any acid-base reaction contains only salt and water. 

In a titration, the equivalence point is the point at which chemically equivalent amounts of reactants have reacted, whereas the end point is the point at which an indicator changes color and a titration should be stopped. So, a chemist needs to be careful when choosing an indicator in an acid-base reaction, to be certain that the pH at which the indicator changes color is close to the pH at the equivalence point of the titration. 

Redox titrations are similar to acid-base titrations. The point at which the oxidation-rednction reaction is complete is called the equivalence point. 

The relative basicity of some Lux bases (OH-, C03-, S04-) in molten Nal at 700 °C was investigated by the potentiometric titration method with weights of sodium pyrophosphate from base to acid [165]. The first of these bases reacted with the acid-titrant in the proportion 2 1, the others were neutralized in the proportion 1 1. Depending on the value of the pO drop at the equivalence points the studied bases were arranged in the sequence OH--C0 --S04- with decreasing basicities. The constants of the acid-base equilibria were not determined. Among the results presented, the most doubtful is the validity of the process of sulfate ion decomposition by the Lux acid. The reason consists in the fact that S03 formed as a result of the acid-base reaction... 

Section 4.1 polar molecule (109) solvated (110) electrolyte (110) nonelectrolyte (112) Section 4.2 molecular equation (113) total ionic equation (114) spectator ion (114) net ionic equation (114) Section 4.3 precipitation reaction (115) precipitate (115) metathesis reaction (116) Section 4.4 acid-base reaction (117) neutralization reaction (117) acid (117) base (118) salt (119) titration (11 9) equivalence point (120) end point (120) Section 4.5 oxidation-reduction (redox) reaction (123) oxidation (124) reduction (124) oxidizing agent (124) reducing agent (124) oxidation number (O.N.) (or oxidation state) (124) Section 4.6 activity series of the metals (130)... 

An indicator is usually used to detect the equivalence point in an acid-base reaction or titration. The most common indicators used are weak organic acids or bases that change color in response to a change from acidic to basic medium or vice versa. The pH at which the color change occurs is characteristic of each indicator. For an acid-base reaction, the indicator is chosen based on the pH at which the equivalence point is expected to occur. Consider the hypothetical dissociation reaction of an indicator represented by HIn. 

We noted earlier that normality is a convenient concentration imit in analytical work. This is because of the fact pointed out in Section 16.8 The number of equivalents of all species in a reaction is the same. Consequently, for an acid-base reaction. 

A titration is the process of determining the quantity of a substance by adding measured increments of another substance, the titrant. The latter is almost always added as a standardised solution (or by electrolyte generation, as in a coulometric titration). The end-point of the titration, which should indicate the addition of an exact chemical equivalence, is recognized by a visual indicator or instrumentally. Titrations are based on acid-base reactions (for determination of acids or bases), redox reactions (for determining oxidants or reductants), chelating reactions (usually with EDTA-type compounds, for determination of metal ions) or precipitations (usually of halides or pseudohalides with silver ions). 

At the equivalence point, the moles of acetic acid initially present and the moles of NaOH added are identical. Since their reaction effectively proceeds to completion, the predominate ion in solution is CH3COO-, which is a weak base. To calculate the pH we first determine the concentration of CH3COO-. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

The objective of the titration is to determine the point at which reaction is complete, called the equivalence point. This is reached when the number of moles of OH- added is exactly equal to the number of moles of acetic acid, HC O originally present To determine this point, a single drop of an acid-base indicator such as phenolphthalein is used. It should change color (colorless to pink) at the equivalence point. 

As pointed out in Chapter 4, an acid-base indicator is useful in determining the equivalence point of an acid-base titration. This is the point at which reaction is complete equivalent quantities of acid and base have reacted. If the indicator is chosen properly, the point at which it changes color (its end point) coincides with the equivalence point To understand how and why an indicator changes color, we need to understand the equilibrium principle involved. 

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

In a titration, one solution is added slowly to the other until the equivalence point is reached. At the equivalence point of a neutralization reaction, the moles of acid and moles of base are equal. An indicator, placed in the reaction mixture, tells you by means of a color change, when the equivalence point has been reached. Your experimental data—the volume and molarity of the standard solution and the volume of the unknown acid or base solution—are all that you need to calculate the molarity of the unknown acid or base. 

When an acid and a base react together, an indicator can be used to show when the reaction is just complete, called the equivalence point. 

Since this is the titration of a strong base with a strong acid, KI is the solute present at the equivalence point and thus, the pH = 7.00. The titration reaction is ... 

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

In an acid-base titration you may either add acid to base or base to acid. This addition continues until there is some indication that the reaction is complete. Often a chemical known as an indicator will indicate the endpoint of a titration reaction, the experimental end of the titration. If we perform the experiment well, the endpoint should closely match the equivalence point of the titration, the theoretical end of the reaction. All the calculations in this section assume accurate experimental determination of the endpoint, and that this value is the same as the equivalence point. 

The addition of titrant from the buret must be stopped at precisely the correct moment—the moment at which the last trace of substance titrated is consumed by a fraction of a drop of titrant added, so that the correct volume can be read on the buret. That exact moment is called the equivalence point of the titration. In order to detect the equivalence point, an indicator is often used. An indicator is a substance added to the reaction flask ahead of time in order to cause a color change at or near the equivalence point, i.e., to provide a visual indication of the equivalence point. For example, the use of a chemical named phenolphthalein as an indicator for a titration in which a strong base is used as the titrant and an acid as the substance titrated would give a color change of colorless to pink in the reaction flask near the equivalence point. The color change occurring near, not exactly at, the equivalence point is usually not a concern. The reason will become clear in a later discussion. The point of a titration at which an indicator changes color, the visual indication of the equivalence point, is called the end point of the titration. As we will see, equivalence points can be determined in other ways too. 

In an acid-base titration, you carefully measure the volumes of acid and base that react. Then, knowing the concentration of either the acid or the base, and the stoichiometric relationship between them, you calculate the concentration of the other reactant. The equivalence point in the titration occurs when just enough acid and base have been mixed for a complete reaction to occur, with no excess of either reactant. As you learned in Chapter 8, you can find the equivalence point from a graph that shows pH versus volume of one solution added to the other solution. To determine the equivalence point experimentally, you need to measure the pH. Because pH meters are expensive, and the glass electrodes are fragile, titrations are often performed using an acid-base indicator. 

For example, suppose that you are titrating a strong acid solution against a weak base. At the equivalence point, the flask contains an aqueous solution of the salt that is formed by the reaction. The solution of a salt of a weak base and a strong acid is acidic, so methyl red may be an appropriate indicator. The pH at equivalence in a titration is the same as the pH of an aqueous solution of the salt formed. 

The two types of titration that you have encountered so far are acid-base and redox titrations. During a titration, the experimenter looks for a permanent colour change in the solution in the conical flask, usually due to the presence of an indicator. This is known as the end-point of the reaction. The equivalence point is the point at which the reaction is just complete. The ideal situation is when the equivalence point and the end-point are exactly the same. Choosing the correct indicator and carrying out titrations very carefully and accurately help to ensure that the equivalence point and the end-point are very close (see p. 37). 

In a neutralization reaction, moles of H+ ions equal moles of OH ions. This relationship is the basis for the procedure called titration, which you will use to standardize a base solution. Standardizing a base means determining its molar concentration. You will then use your standardized base to determine the molar mass of an acid. To determine when the moles of H+ equal the moles of OH, you will monitor the pH of an acid solution as a solution of base is added slowly. The pH will rise suddenly when the concentrations of the two ions are equal (the equivalence point). 

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