department of chemistry chem1010 general chemistry ***********************************************

Department of Chemistry

CHEM1010 General Chemistry***********************************************

Instructor: Dr. Hong ZhangFoster Hall, Room 221

Tel: 931-6325

Email: [email protected]

CHEM1010/General Chemistry_________________________________________

Chapter 7. (L27)-Acids and Bases

• Today’s Outline..Review: How to write acid and base molecules in molecular formula ..Review: Common acids and bases; strong acids and strong bases; weak acids and weak bases..Review: Acidic and basic anhydrides..Acidity..pH scale to measure acidity


• How to write acid and base molecules in molecular formula

..Acids:

We commonly put H or proton on the left and the anion on the right (for inorganic acids).

Example: HCl, H2SO4, HNO3

..Bases:

We commonly put OH on the right and the cation on the left (for inorganic bases).

Example: NaOH, Ca(OH)2


• Common acids..Common acids (see Table 7.1 for more details):Name Molecular Formula Strengthhydrochloric acid HCl strong

sulfuric acid H2SO4 strong

nitric acid HNO3 strong

phosphoric acid H3PO4 moderate

hydrogen sulfate HSO4- moderate

carbonic acid H2CO3 weak

acetic acid CH3COOH weak

lactic acid CH3CHOHCOOH weak

boric acid H3BO3 very weak

hydrocyanic acid HCN very weak


• Common bases

..Common acids (see Table 7.2 for more details):

Name Molecular Formula Strength

sodium hydroxide NaOH strong

potassium hydroxide KOH strong

lithium hydroxide LiOH strong

calcium hydroxide Ca(OH)2 strong*

magnesium hydroxideMg(OH)2 strong*

ammonia NH3 weak

*very low solubility although classified as a strong base


• Acidic and basic anhydrides, nonmetal oxides: Acidic anhydrides

Nonmetal oxides can react with water to form acids.These nonmetal oxides are called acidic anhydrides.

Example:

SO3 + H2O = H2SO4

SO2 + H2O = H2SO3

CO2 + H2O = H2CO3 General form:

nonmetal oxide + H2O acidanhydride means without water.


• Acidic and basic anhydrides, metal oxides: Basic anhydridesMetal oxides can react with water to form bases.These metal oxides are called basic anhydrides.

Example:

CaO + H2O = Ca(OH)2

BaO + H2O = Ba(OH)2

Li2O + H2O = 2LiOH General form:

metal oxide + H2O base


• Acidity of acidsQuestion: How to know how much acid is present in a water solution?

In other words, how to know how much protons, H+, are present (acid is a molecule that can dissociate to give H+ in a water solution)

So, we need a quantity or parameter to tell us about the acidity of a solution.


• Acidity of acids

One way to express acidity can be concentration of H+: Molarity

Example:[H+] = 1 M or mole/L[H+] = 0.001 M or mole/L[H+] = 0.0001 M or mole/L[H+] = 0.0000001 M or mole/L


• Acidity of acidsHowever, in natural environments and biological bodies, the acidity is commonly quite small.

Example:Lake, river water [H+] = from 0.0000001M to 0.00000001MClean rain water[H+] = from 0.00001M to 0.001MBlood

[H+] = ~0.0000001M


• Acidity of acids

In science, we like to use small numbers and values to express quantities

Example: We use mole to represent 6.022×1023

molecules

Question, can we have some way to express those very small values of acidity?


• Acidity of acids: pH scale

In chemistry, we use pH scale to express very small values of acidity.

Definition of pH:pH = -log[H+] where the unit of [H+] is M or mole/LpH value has no unitcommon pH range: 1-14

pH scale was first proposed in 1909 by a Danish biochemist, S.P. L. Sorensen.


• Acidity of acids: pH scaleExpress acidity in pH scaleExamples:Lake water: [H+] = from 0.0000001M to 0.00000001M

pH = -log(0.0000001) = -log (10-7) = -(-7) = 7pH = -log(0.00000001) = -log (10-8) = -(-8) = 8

Clean rain water: [H+] = from 0.00001M to 0.000001MpH = -log(0.00001) = -log (10-5) = -(-5) = 5pH = -log(0.001) = -log (10-6) = -(-6) = 6

Blood: [H+] = ~0.0000001MpH = -log(0.0000001) = -log (10-7) = -(-7) = 7


• Acidity of acids: pH scaleExpress acidity in pH scale: A special featureBecause the definition of pH scale, the higher the concentration of H+, the lower the values of pH; So, the direction pH and concentration of H+ change is just opposite

Example:[H+] = 0.00001M < 0.001Mbut, correspondingly, pH = 5 > 3


• Acidity of acids: pH scaleConvert H+ concentration to pH scale:

Example: [H+] = 0.0000001MpH = -log(0.0000001) = -log (10-7) = -(-7) = 7General equation:pH = -log (10-x) = -log(-x) = x


• Acidity of acids: pH scaleConvert pH scale to H+ concentration:

Example: pH = 7[H+] = 10-7 M

General equation:pH = x = -log (10-x) [H+] = 10-x M


• Acidity of acids: pH scaleUse pH scale to define acidic solution and basic solution:

Acidic solutions: pH < 7Basic solutions: pH > 7

This definition is based on the pH of pure water, which is 7 (6.9975) at 25 ºC. So, -if an aqueous solution has the acidity higher than pure water, then its pH is lower than water’s pH, or 7-if an aqueous solution has the acidity lower than pure water, then its pH is higher than 7


• pH values of some common solutionsSolution pH valueLemon juice 2.1Vinegar (4%) 2.5Soda pop 2.0-4.0Rainwater (thunderstorm) 3.5-4.2 Milk 6.3-6.6Saliva 6.2-7.4Urine 5.5-7.0Pure water at 25 ºC 7.0Blood 7.4Fresh egg white 7.6-8.0Washing soda 12.0


Quiz Time

Which of the following is the definition of pH scale?(a) pH = log [H+]; (b) pH = [H+]; (c) pH = -[H+]; (d) pH = -log[H+].


Quiz Time

If a solution has a pH of 4, then its concentration of H+ is(a) [H+] = 0.4 M; (b) [H+] = 4 M; (c) [H+] = 0.0001 M = 10-3 M; (d) [H+] = 0.0001 M = 10-4 M .


Quiz Time

If a solution has a concentration of H+ being 0.00001 M, then its pH is(a) pH = 1; (b) pH = 0.00001 M; (c) pH = -5; (d) pH = 5.


Quiz Time

If a solution has a concentration of H+ being 10-6, then its pH is(a) pH = 10; (b) pH = 0.6; (c) pH = -6; (d) pH = 6.

department of chemistry chem1010 general chemistry ***********************************************

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