Acids and BasesAcids and Bases

Chp 16Chp 16

Old DefinitionsOld Definitions

ClassicClassic– Acids taste sourAcids taste sour– Bases taste bitterBases taste bitter

Arrhenius modelArrhenius model– Acids produce hydronium ions (HAcids produce hydronium ions (H33OO++) in solution) in solution

– Bases produce hydroxide ions (OHBases produce hydroxide ions (OH--) in solution) in solution

Current DefinitionCurrent Definition

Bronsted – Lowry ModelBronsted – Lowry Model– Acids donate protonsAcids donate protons– Bases accept protonsBases accept protons

Conjugate Acid/Base PairsConjugate Acid/Base Pairs

When an acid donates a proton, it forms a When an acid donates a proton, it forms a conjugate baseconjugate base– HCl donates and becomes ClHCl donates and becomes Cl--

When a base accepts a proton, it forms a When a base accepts a proton, it forms a conjugate acidconjugate acid– OHOH-- accepts and becomes H(OH) accepts and becomes H(OH)

These differ by only ONE hydrogen atomThese differ by only ONE hydrogen atom If the acid is strong, it’s conjugate base is If the acid is strong, it’s conjugate base is

weak (and vice versa)weak (and vice versa)

Acid-Base ReactionsAcid-Base Reactions

Can occur in either direction, but both sides Can occur in either direction, but both sides compete for the free H ions so 1 direction compete for the free H ions so 1 direction often dominatesoften dominates

HCl + HHCl + H22O HO H33OO++ + Cl + Cl--

AcidAcid base conj. acid conj. basebase conj. acid conj. base

Some TermsSome Terms

AmphotericAmphoteric– can be acids or bases, depending on the can be acids or bases, depending on the

circumstancecircumstance– Would be weak in both casesWould be weak in both cases– Water is the most common amphoteric Water is the most common amphoteric

substancesubstance BufferBuffer

– A weak acid or base that can be added to a A weak acid or base that can be added to a solution to help it resist changes in pHsolution to help it resist changes in pH

IndicatorsIndicators

Substances that change color when exposed to an Substances that change color when exposed to an acid or a baseacid or a base

Occurs because of reactions with the hydrogen Occurs because of reactions with the hydrogen ionsions

Many substances are natural pH indicators (ex. Many substances are natural pH indicators (ex. Blueberries, red cabbage, ammonia)Blueberries, red cabbage, ammonia)

Some are commonly used in labsSome are commonly used in labs– Litmus paper (bases turn red paper blue, acids turn blue Litmus paper (bases turn red paper blue, acids turn blue

paper red)paper red)– Phenalthalien turns red when basicPhenalthalien turns red when basic

ConcentrationsConcentrations

Can be measured with molarity, but can be Can be measured with molarity, but can be misleading because some acids have multiple H misleading because some acids have multiple H ions to donateions to donate

Use Normality instead:Use Normality instead:N = N = mols (equivalents)mols (equivalents)

LitersLiters Equivalents = how many H ions (or OH ions) that Equivalents = how many H ions (or OH ions) that

acid or base hasacid or base has Ex. 0.5 mol HEx. 0.5 mol H33POPO44 dissolved in 1.5 L has what dissolved in 1.5 L has what

normality?normality?N = N = 0.5(3)0.5(3) == 1 N1 N 1.51.5

NeutralizationNeutralization

Adding just enough acid to completely react with a Adding just enough acid to completely react with a base (or vice versa)base (or vice versa)

Need to have every proton usedNeed to have every proton used Creates water and a salt (ionic compound)Creates water and a salt (ionic compound) Can determine the amount of acid or base in a Can determine the amount of acid or base in a

reaction using this in a process known as titrationreaction using this in a process known as titration– Adding a little of a known acid or base until the Adding a little of a known acid or base until the

equivalence point is reached as shown by a pH indicatorequivalence point is reached as shown by a pH indicator

NNacidacidVVacidacid = N = NbasebaseVVbasebase

An ExampleAn Example

What volume of 0.10 M NaOH is required to What volume of 0.10 M NaOH is required to completely neutralize 50.0 mL of 0.20 M Hcompletely neutralize 50.0 mL of 0.20 M H22SOSO44??

0.10 M NaOH = 0.10 (1) = 0.10 N NaOH0.10 M NaOH = 0.10 (1) = 0.10 N NaOH

0.20 M H0.20 M H22SOSO44 = 0.20 (2) = 0.40 N H = 0.20 (2) = 0.40 N H22SOSO44

SOSO

0.40 (50) = 0.10 (V)0.40 (50) = 0.10 (V)

V = 200 mlV = 200 ml

[H+] and [OH-] Ions [H+] and [OH-] Ions

Acids create HAcids create H++ ions when mixed in water because ions when mixed in water because they donate H they donate H ++ to it to it

Bases create OHBases create OH-- ions when mixed in water ions when mixed in water because they take a H because they take a H ++ away from it away from it

If [H If [H ++] > [OH] > [OH--], the solution is acidic], the solution is acidic If [H If [H ++] < [OH] < [OH--], the solution is basic], the solution is basic If [H If [H ++] = [OH] = [OH--], the solution is neutral], the solution is neutral However, for any solution [H However, for any solution [H ++] [OH] [OH--] = 1 x 10] = 1 x 10-14-14

An exampleAn example

If a solution has a [OHIf a solution has a [OH--] = 3.4 x 10] = 3.4 x 10-9 -9 M, what M, what is the concentration of [His the concentration of [H++]?]?

[H[H++] (3.4 x 10] (3.4 x 10-9-9) = 1 x 10) = 1 x 10-14-14

[H[H++] = 2.9 x 10] = 2.9 x 10-6-6 M M Is the solution acid or basic?Is the solution acid or basic?

since [Hsince [H++] > [OH] > [OH--], it is acidic], it is acidic

(remember that negative exponents mean (remember that negative exponents mean more zeros after the decimal, so a smaller more zeros after the decimal, so a smaller #)#)

pH ScalepH Scale

An easier way to measure acids and bases An easier way to measure acids and bases because the concentrations we work with because the concentrations we work with are so smallare so small

0-7 acidic0-7 acidic 7 neutral7 neutral 7-14 basic7-14 basic The closer to 7 the weaker the substance is, The closer to 7 the weaker the substance is,

the further away the stronger it isthe further away the stronger it is Each step is actually a factor of 10 Each step is actually a factor of 10

– So pH 2 is 100 X stronger than pH 4So pH 2 is 100 X stronger than pH 4

pH continuedpH continued

pH measures the concentration of H ions (or pH measures the concentration of H ions (or hydronium ions) in solutionhydronium ions) in solution

pH = -log [HpH = -log [H++]] pOH = -log [OHpOH = -log [OH--]] pH + pOH = 14pH + pOH = 14 To get [HTo get [H++] from pH:] from pH:

[H[H++] = inverse log (-pH)] = inverse log (-pH)

An exampleAn example

What is the pH of a solution that has a [HWhat is the pH of a solution that has a [H++] ] of 1.3 x 10of 1.3 x 10-11-11M?M?pH = -log (1.3 x 10pH = -log (1.3 x 10-11-11) = 10.88) = 10.88

Is it an acid or a base? Is it an acid or a base?

basic since the pH is greater than 7basic since the pH is greater than 7

Another ExampleAnother Example

What is the [HWhat is the [H++] if the pH of a solution is ] if the pH of a solution is 2.3?2.3?

[H[H++] = inverse log (-2.3) = 5.01 x 10] = inverse log (-2.3) = 5.01 x 10-3-3 M M What would the pOH of this solution be?What would the pOH of this solution be?

2.3 + pOH =142.3 + pOH =14

pOH = 11.7pOH = 11.7