pH

Exploring Acid Base Reactions. Measuring acidity.

Ch07

© Nick DeMello, PhD. 2007-2015

version 1.5

pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Ionization of Water

Water is amphoteric—it can act as an acid or a base. In water, ‣ H+ is transferred from one H2O molecule to another

‣ one molecule acts as an acid, while another acts as a base ‣ equilibrium is reached between the conjugate acid–base pairs

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H2O(l ) + H2O(l ) ⇄ H3O+(aq) + OH−(aq)

Ionization of Water

Water is amphoteric—it can act as an acid or a base. In water, ‣ H+ is transferred from one H2O molecule to another

‣ one molecule acts as an acid, while another acts as a base ‣ equilibrium is reached between the conjugate acid–base pairs

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H2O(l ) + H2O(l ) ⇄ H3O+(aq) + OH−(aq)

pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Writing the Ion Product Constant, Kw

In the equation for the dissociation of water, there is both a forward and a reverse reaction.

H2O (l) + H2O (l) ⇄ H3O+ (aq) + OH− (aq)

In pure water, the concentrations of H3O+ and OH− at 25 °C are each 1.0 × 10−7 M.

[H3O+] = [OH−] = 1.0 × 10–7 M

Kw = [H3O+] [OH−]

Kw = (1.0 × 10−7 M) (1.0 × 10−7 M) = 1.0 × 10–14 (at 25 °C)

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Base Acid Conjugate Conjugate Acid Base

Ion Product Constant, Kw

The ion product constant for water, Kw, is defined

• as the product of the concentrations of H3O+ and OH−

• as equal to 1.0 × 10−14 at 25 °C (the concentration units are omitted)

When • the [H3O+] and [OH−] are equal, the solution is neutral

• the [H3O+] is greater than the [OH−], the solution is acidic

• the [OH−] is greater than the [H3O+], the solution is basic

Using Kw to Calculate [H3O+], [OH−]

• If we know the [H3O+] of a solution, we can use the Kw to calculate the [OH−].

• If we know the [OH−] of a solution, we can use the Kw to calculate the [H3O+].

Pure Water Is Neutral

In pure water, the ionization of water molecules produces small but equal quantities of H3O+ and OH− ions. [H3O+] = 1.0 × 10−7 M

[OH−] = 1.0 × 10−7 M [H3O+] = [OH−] Pure water is neutral.

Acidic Solutions

Adding an acid to pure water • increases the [H3O+] • causes the [H3O+] to exceed 1.0 × 10−7 M • decreases the [OH−]

[H3O+] > [OH−] The solution is acidic.

Le Châtelier’s Principle

Basic Solutions

Adding a base to pure water • increases the [OH−] • causes the [OH−] to exceed

1.0 × 10−7 M • decreases the [H3O+]

[H3O+] < [OH−] The solution is basic.

Le Châtelier’s Principle

Comparison of [H3O+] and [OH−]

Calculating [H3O+]

What is the [H3O+] of a solution if [OH−] is 5.0 × 10−8 M?

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d

Milk of MagnesiaIf milk of magnesia has [OH−] of 2.0 × 10−3 M, what is the [H3O

+] of the solution?

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pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

What pH means.

‣ Mixtures that have a high H+ concentration have he properties of acids. They are acidic.

‣ Mixtures that have a low H+ concentration, have a high OH1- concentration. They have the properties of bases. They are basic.

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[H+] = .0000001 M

[H+] = .00000000000001 M

[H+] = .000000001 M

[H+] = .0001 M

[H+] = 1 M

= 10 -14 M

= 10 -9 M

= 10 -7 M

= 10 -4 M

= 10 -1 M

What pH means.

‣ pH stands for pouvoir hydrogen (power of hydrogen).

‣ It’s the negative power of the hydrogen ion concentration in a solution.

‣ pH is a measure of how acidic or basic that solution is. ‣ pH is defined as the negative log of the H+

concentration.

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[H+] = 10 -pH

pH = -log [H+]

The pH Scale

The pH of a solution

• is used to indicate the acidity of a solution

• has values that usually range from 0 to 14

• is acidic when the values are less than 7

• is neutral at a value of 7

• is basic when the values are greater than 7

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pH related to [OH1-]

‣ As pH increases ‣ [H+] Concentration goes up.

‣ [OH1-] Concentration goes down.

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Kw = [H3O+] [OH−] = 1.0 x 10-14

Acidic, Basic, or Neutral?

HCl with a pH = 1.5

pancreatic fluid, [H3O+] = 1 × 10−8 M

Sprite soft drink, pH = 3.0pH = 7.0

[OH−] = 3 × 10−10 M

[H3O+] = 5 × 10−12

ACIDIC

BASIC

NEUTRAL

ACIDIC

BASIC

ACIDIC

pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Acid-Base Indicators

‣ Some substances change color, depending on the pH of the solution they are in.

‣ A dye extracted from lichens, called litmus, is one example. ‣ Litmus turns red in acid.

‣ pH < 4.5

‣ Litmus turns blue in base. ‣ pH > 8.3

‣ Litmus is one of the oldest and most generally useful acid-base indicators.

‣ It was discovered in 1300 AD by the Spanish alchemist Arnaldus de Villa Nova.

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Acid-Base Indicators

‣ Some substances change color, depending on the pH of the solution they are in.

‣ Phenolphthalein is another example: ‣ Is colorless in acid.

‣ pH < 7.0

‣ Is light pink in base. ‣ pH > 7.0

‣ Is bright pink in stronger base. ‣ pH > 8.3

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Acid-Base Indicators

‣ Indicators are found in many common plants.

‣ The most useful indicators have more than one color change.

‣ Red cabbage for example produces an indicator with many different transitions.

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Acid-Base Indicators

‣ Universal indicator is a mixture of four indicators that produces a complete spectrum.

‣ It’s components are: ‣ methyl red

‣ bromothymol blue

‣ thymol blue

‣ phenolphthalein

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pH 1.0 = Cherry RedpH 2.0 = RosepH 3.0 = Reddish-OrangepH 4.0 = Orange-RedpH 5.0 = OrangepH 6.0 = YellowpH 7.0 = Yellowish-GreenpH 8.0 = GreenpH 9.0 = Bluish-GreenpH 10.0 = Blue

pH Paper

‣ Litmus, Universal Indicator, and other indicators are often soaked into paper strips to make them more convenient to use.

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pH Meters

‣ A pH meter is a device that directly measures the pH of a solution.

‣ It usually has a glass electrode plus a calomel reference electrode, or a combination electrode.

‣ Basic potentiometric pH meters simply measure the voltage between two electrodes and display the result converted into the corresponding pH value.

‣ Probes need to be kept clean of contamination as far as possible, and not touched by hand.

‣ Probes are best kept moist with a medium appropriate for the particular probe (distilled water, which can encourage diffusion out of the electrode, is undesirable) when not in use.

‣ For very precise work the pH meter should be calibrated before each measurement. For normal use calibration should be performed at the beginning of each day.

‣ Calibration should be performed with at least two standard buffer solutions that span the range of pH values to be measured. For general purposes buffers at pH 4.01 and pH 10.00 are acceptable.

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pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

pH Calculations

‣ The pH scale ‣ is a logarithmic scale that corresponds to the [H3O+] of aqueous

solutions.

‣ is the negative logarithm (base 10) of the [H3O+]

pH = − log [H3O+] ‣ To calculate the pH, the negative powers of 10 in the molar

concentrations are converted to positive numbers.

If [H3O+] is 1.0 × 10−2 M

pH = −log [1.0 × 10−2] = − ( −2.00 ) = 2.00

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pH Calculations

pH = − log [H3O+]

If [H3O+] is 1.0 × 10−2 M

pH = −log [1.0 × 10−2] = − ( −2.00 )

= 2.00

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+/−EE or Exp log1.0 2

pH Calculations

[H3O+] = 10 -pH

If pH is 7.0

[H3O+] = 10 -7.0

= 0.0000001

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+/−7.0 10x2nd

What is the concentration?

HCl with a pH = 1.5

pancreatic fluid, [H3O+] = 1 × 10−8 M

Sprite soft drink, pH = 3.0pH = 7.0

[OH−] = 3 × 10−10 M

[H3O+] = 5 × 10−12

ACIDIC

BASIC

NEUTRAL

ACIDIC

BASIC

ACIDIC

pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Significant Figures

‣ To determine the number of significant figures in the pH value, consider the following: ‣ The number of decimal places in the pH value is the same as the number of significant

figures in the coefficient of [H3O+].

‣ The number to the left of the decimal point in the pH value is the power of 10 (this is determined by counting how how far we move the decimal point,

it’s not part of the significant figures of the pH measaurement)

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What’s the pH of Aspirin?

‣ Aspirin, which is acetylsalicylic acid, was the first nonsteroidal anti-inflammatory drug used to alleviate pain and fever.

‣ If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M

‣ What is the pH of the solution?

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What’s the pH of Aspirin?

‣ Aspirin, which is acetylsalicylic acid, was the first nonsteroidal anti-inflammatory drug used to alleviate pain and fever.

‣ If a solution of aspirin has a [H3O+] = 1.7 × 10−3 M

‣ What is the pH of the solution?

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pH = − log [H3O+]

pH = − log 1.7 x10-3

+/−EE or Exp log1.7 3

pH = 2.76 9551079

Exact Two SFs

= 2.77

+/−

Find the pH

‣ Find the pH of a solution with a [H3O+] of 5.6 × 10−5.

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pH = − log [H3O+]

pH = − log 5.6 x10-5

+/−EE or Exp log5.6 5

pH = 4.251811

Exact Two SFs

= 4.25

+/−

What’s the [H+]?

‣ Determine the [H3O+] for solutions having each of the following pH values: pH = 3.0

pH = 3.42

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For pH values that are whole numbers, the [H3O+] can be written 1 × 10−pH.

[H3O+] = 1 × 10−3 M

pH

+/−3.42 10x2nd

= 3.801893963E−04

= 3.8 x10−4

d

Red WineA red wine has a pH of 3.2. What is the: (a) Concentration of [H3O+]?

(b) Concentration of [OH1-]?

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[H3O+] = 10 -pH kw = [H30+][OH1-]

[OH1-] = kw / [H30+]

= 1.0 x10-14 / [6.3 x10-4]

= 1.587301 x10-11

= 1.6 x10-11

= 10 -3.2

= 6.309573 x10-4

= 6 x10-4(a)

(b)

pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Reactions with Acid

‣ Acids react with ‣ metals to produce salt and hydrogen gas

‣ a reduction-oxidation reaction.

‣ bases to produce a salt and water ‣ a neutralization reaction.

‣ bicarbonate and carbonate ions to produce carbon dioxide gas ‣ a gas evolution reaction.

‣ A salt is an ionic compound that does not have H+ as the cation or OH− as the anion.

‣ Many of these types of reactions occur in the human body.

‣ Gastric acid contains HCl (aq) and is produced by parietal cells that line the stomach. When protein enters the stomach, HCl is secreted until the pH reaches 2, optimum for digestion.

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Acids and Metals

Acids react with active metals

• such as K, Na, Ca, Mg, Al, Zn, Fe, and Sn

• to produce hydrogen gas and the salt of the metal

2 K(s) + 2 HCl(aq) ➞ 2 KCl(aq) + H2(g)

metal acid salt hydrogen

Zn(s) + 2 HCl(aq) ➞ ZnCl2(aq) + H2(g)

metal acid salt hydrogen

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Acids and Metals

Write balanced equations and label the metal, acid, and salt for the following reactions:

‣ Magnesium metal with HCl (aq)

‣ Aluminum metal with HNO3 (aq)

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magnesium metal + hydrochloric acid → magnesium chloride + hydrogen gas

Mg(s) + HCl(aq) ➞ MgCl2(aq) + H2(g)

Mg(s) + 2 HCl(aq) ➞ MgCl2(aq) + H2(g)

metal acid salt

aluminum metal + nitric acid → aluminum nitrate + hydrogen gas

Al(s) + HNO3 (aq) ➞ Al(NO3)3 (aq) + H2 (g)

Al(s) + 6 HNO3 (aq) ➞ Al(NO3)3 (aq) + H2 (g)

Al(s) + 6 HNO3 (aq) ➞ 2 Al(NO3)3 (aq) + H2 (g)

2 Al(s) + 6 HNO3 (aq) ➞ 2 Al(NO3)3 (aq) + H2 (g)

2 Al(s) + 6 HNO3 (aq) ➞ 2 Al(NO3)3 (aq) + 3 H2 (g)

metal acid salt

Acids, Carbonates, Bicarbonates

Acids react

• with carbonates and hydrogen carbonates

• to produce carbon dioxide gas, a salt, and water

2 HCl(aq) + CaCO3(s) ➞ CO2(g) + CaCl2(aq) + H2O(l)

acid carbonate carbon salt water dioxide

HCl(aq) + NaHCO3(s) ➞ CO2(g) + NaCl(aq) + H2O(l) acid bicarbonate carbon salt water dioxide

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Acids, Carbonates, Bicarbonates

Write balanced equations for the following reactions:

‣ MgCO3(s) + HBr(aq) ➞

‣ HCl(aq) + NaHCO3(aq) ➞

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MgCO3(s) + HBr(aq) ➞ MgBr2(aq) + CO2(g) + H2O(l) MgCO3(s) + 2 HBr(aq) ➞ MgBr2(aq) + CO2(g) + H2O(l)

HCl(aq) + NaHCO3(aq) ➞ NaCl(aq) + CO2(g) + H2O(l)

Acids and Hydroxides: Neutralization

In a neutralization reaction, • an acid reacts with a base to produce salt and water • the salt formed is the anion from the acid and the cation from

the base

HCl(aq) + NaOH(aq) ➞ NaCl(aq) + H2O(l) acid base salt water

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pH & Reactions

‣ Water Ionization ‣ Water as an Acid-Base

‣ Amphoteric Property

‣ Ion Product Constant Kw

‣ Relating [OH1-] and [H+]

‣ pH Scale

‣ pH ‣ defined

‣ examples

‣ Relating pH ‣ [H+] & [OH1-]

‣ Measuring pH

‣ Calculations with pH ‣ Calculator Use

‣ Significant Figures

‣ Reactions ‣ Metals (gas evolution)

‣ Carbonates (gas evolvution)

‣ Hydroxides (neutralization)

‣ Titration ‣ Process

‣ Indicators & End Point

‣ Calculations

Ch07

Questions?

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