19 reactions of acids and bases

8/8/2019 19 Reactions of Acids and Bases

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C H A P T E R 1 9

Reactions of Acids

and Bases


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C H A P T E R 1 9 : R E A C T I O N S O F A C I D S A N D

B A S E S

19-1 The Self-ionization ofWater and pH


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The Self-ionization of Water

y In the self-ionization of water, 1 water moleculetransfers an H+ ion to a second water moleculetransfers a hydrogen atom to another water

molecule, forming an OH- ion and an H3O+ ion.y In other words, water acts as both an acid and a base

in the same reaction.

y In pure water at 25rC, both H3O+ ions and OH- ions

are found at concentrations of1.0 x 10-7 M.y This size is very small, however its both significant

and important.


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The Self-ionization of Water Cont.

y To better understand the self-ionization of water,first take a look at its equilibrium constant:

Keq = [H3O+][OH-] / [H2O]

y However, in both pure water and dilute solutions atconstant temperature, the concentration of H2O isconstant.

y For this reason, chemists usually multiply both sidesof the equation by [H2O] which gives:

Keq x [H2O] = [H3O+][OH-]


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y The left side of this equation is called the ion-productconstant for water, and is abbreviated with the symbolKw. In pure water, Kw equals 1.0 x 10^-14.

Kw = [H3O+][OH-]

= (1.0 x 10^-7)((1.0 x 10^-7)= 1.0 x 10^-14

y Kw is useful because it applies not only to pure water, butto every water solution at 25rC, even a solution in whichthe concentrations of H3O+ ions and OH- ions are not

equal.y When you are dealing with water, as long as you know

the concentration of one of the ions, you can find theconcentration of the other.


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y This means that acidic, basic, and neutral solutions areeven more complicated than you thought.

y An acidic solution contains not only H3O+ ions, but alsoa small number of OH- ions.

y In acidic solutions, [H3O+] > 1.0 x 10^-7 M.y A basic solution contains not only OH- ions, but also a

small number of H3O+ ions.

y In basic solutions, [OH-] > 1.0 x 10^-7 M.y In all solutions at 25rC, the product concentrations of

H3O+ ions and OH- ions equals 1.0 x 10^-14.y A solution is neutral when the concentrations of both

ions are 1.0 x 10^-7 M.


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The pH Scale

y The pH scale is a way to express the concentrationsof H3O+ ions. It is based on logarithms.

y The pH of a solution is -1 times the base 10 logarithm

of the H3O+ concentration in moles per liter:pH = -log[H3O+]

y A numbers base 10 logarithm is its exponent when10 is the base. For example, 10,000 is the same as

10^4, so log(10,000) is 4.

y You can estimate a numbers log by comparing it tonearby numbers that have the simpler logs.


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The pH Scale Cont.

y The smaller the pH, the more acidic the solution(and vice versa).

y Because the pH scale is a logarithmic scale, each one-

unit change in pH represents a 10-fold change in theconcentration of H3O+ ions.

y For example, when the pH of a solution drops from 4to 3, its concentration of H3O+ ions increases by a

factor of10.


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The pH Scale Cont.

y In calculating pH, you must be careful withsignificant digits.

y For any logarithm, the number of digits after the

decimal point should equal the number of significantdigits in the original number.

y Example: -log(8.7 x 10^-4) = 3.06


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Measuring pH

y There are two common ways: an acid base indicatorand a pH meter.

y Indicators are weak acids or bases that undergo a

color change when they gain or lose an H+ ion.y Each indicator has a different equilibrium constant

for a specific equation, which means that each has adifferent pH range for which it changes colors.

y A combination of indicators helps to pinpoint asolutions pH.

y pH meters are much more accurate.


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Buffers

y A buffer is a mixture that is able to release or absorb H+ ions,keeping a solutions pH constant.

y Most common buffers are mixtures of weak acids and theirconjugate bases.

y The amount of acid or base that a buffer can neutralize iscalled the buffer capacity.

y When the buffer capacity is exceeded, the ions will remain inthe solution changing the pH.

y The greater the concentration of buffer in the solution, the

greater the buffer capacity.y Most buffer solutions contain roughly equal concentrations of

acid and its conjugate base, which means the buffer has anequal capacity to neutralize wither H3O+ ions or OH- ions.


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C H A P T E R 1 9 : R E A C T I O N S O F A C I D S A N D

B A S E S

19-3 Acid-Base Titration


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Performing a Titration

y An acid-base titration is a carefully controlledneutralization reaction.

y To run a titration on a solution of unknown

concentration of an acid or a base, you need a secondsolution called a standard solution.

y A standard solution contains an acid or a base is aknown concentration. If the unknown solution is an acid, the standard solution will

always be a strong base. If the unknown solution is a base, the standard solution always

contains a strong acid.


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Performing a Titration Cont.

y Chemists typically also use an acid-base indicator in a titration.

y To run a titration, the standard solution is slowly added to the unknownsolution. As the two solutions mix, the acid in one solution neutralizesthe base in the other solution, a reaction that runs nearly to completion.

y

Eventually, enough standard solution is added to neutralize all the acidor base in the unknown solution. The point at which this occurs is theequivalence point.

y The point at which the indicator changes color is called the end point ofthe titration.

y If the indicator is chosen correctly, the end point is very close to theequivalence point.

y Therefore, at approximately the end point of the titration, the totalnumber of moles H+ ions donated by the acid equals the total number ofmoles H+ accepted by the base.


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Choosing the Proper Indicator

y So how do you choose the correct indicator? Theanswer depends on the strengths of the acid and baseinvolved in the titration. There are three important

combinations of acids and bases for titrations: Strong acid with a strong base

Weak acid with a strong base

Strong acid with a weak base

yChemists monitor the pH throughout the titration ina graph called a titration curve.


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Choosing the Proper Indicator Cont.

A STRONG ACID WITH A STRONG BASE

y The salts of strong acids and strong bases formneutral water solutions.

y So, in the titration of a strong acid and a strong base,the solution at the equivalence point has a pH of7.

y In this titration, the indicator should change color atthe pH of7.

y The pH also increases from 4 to 10 with only a fewdrops of standard solution.

y Therefore, a successful indicator can have a widerange of values for the pH at which it changes color.


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A WEAK ACID WITH A STRONG BASE

y The salts of weak acids and strong bases are basic.

y Thus, for this titration, the equivalence point should

come at a basic pHa pH greater than 7.y Titration curves start at different pH levels

depending on the concentration of H3O+.

y The choice of indicator is limited to a smaller pH

range in this type of titration.


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A STRONG ACID WITH A WEAK BASE

y The pH of this titration starts out high because theunknown solution is basic.

y The pH drops because the standard solution isacidic.

y The salts of weak bases and strong acids are acidic,which means that the equivalence point should come

at a pH below7.

19 reactions of acids and bases

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