Empirical Definitions of Acids and Bases

CHEMISTRY 20

Notes and problems
Unit 4: Acids and Bases

pH = -log [H3O+(aq)]
pOH = -log [OH-(aq)]
pH + pOH = 14
Prepared by Daniel Veraart
CIS Abu Dhabi

Chemistry 20

Day 29

Date: ______Name: ______

Empirical Definitions of Acids and Bases:

ACIDS

Names for acids refer to aqueous solutions of the pure substance.

For example: HCl(g) is called hydrogen chloride while the aqueous HCl(aq) is hydrochloric acid.

The most common acids are given on P.11 of your booklet. There are two types of acids:

1. Strong acid: The first 6 acids of the table: HCl , HBr , HI, H2SO4, HNO3 and HClO4.

2. Weak acids: All the other acids of the table. (CH3COOH , HOOCCOOH , …).

To find if an acid is strong or weak, we can check the conductivity of the solution

-If the solution conducts electricity greatly  Strong acid

-If the solution conducts electricity slightly  Weak acid

BASES:The common bases are positive ions bonded to hydroxide ions (OH-). When bases dissolve in water, they dissociate to produce a metallic ion and the hydroxide ion.

Properties of acids and bases

Properties /

Acids

/ Bases
Taste
Color with litmus
Reaction with metals
PH
Neutralize what?
Conductivity
Color with phenolphtalein
Color with bromothymol blue
Feel

Common Acids and bases:

ACIDS
Products / Acids in the product
Acetic acid = CH3COOH
Salicylic acid = C6H4(OH)COOH
Carbonic acid = H2CO3
Phosphoric acid = H3PO4
Citric acid = C6H8O7
Ascorbic acid = C6H8O6
BASES
Products / Bases in the product
Sodium hydroxide = NaOH
Magnesium hydroxide = Mg(OH)2
Ammonia = NH3

The pH

Chemistry 20

Homework 29

Date: ______Name: ______

Exercices

1. Classify the following solutions as acids, bases or neutral.

a) bleach, pH = 12.4: ______b) urine, pH = 6.0 : ______c) coffee, pH = 5.0: ______

d) sugar water, pH = 7.0 : ______e) egg white, pH = 7.8 : ______

2. Which of the following substance would turn litmus red?

A. Water B. Ammonia C. Pop D. Bleach

3.Mr. Clean probably has a pH

A. of 7 B. of 0 C. lower than 7 D. greater than 7

4.The pH of blood is 7.4 and the pH of urine is 5.4. Which of the following statement is true?

A. Blood is more basic than urineB. Blood is more acidic than urine

C. Both blood and urine would turn red litmus blueD. Both blood and urine are examples of acids.

5.A teacher gives you an unknown substance, how could you identify if this substance is acidic, basic or neutral?

A. By tasting it

B. By testing if the substance is corrosive

By putting the substance in water and testing it with an universal indicator
By looking at the substance, acids are colourless solutions while bases are white solids

6.Which of the following properties does not apply to a base?

A. pH greater than 7B. React with magnesium

C. Turn red litmus blue D. Can be neutralised with vinegar

7. When a pool manager tested the water with blue litmus, it turned pink. In order to neutralize the water in the pool, the pool manager should add

hydrochloric acid diluted (acid)B. sodium carbonate diluted (base)

C. distilled water (neutral)D. chlorine (disinfectant)

8.After using an oven cleaner, some people rinse the oven with a dilute solution of vinegar. What is the purpose of the vinegar rinse?

A. To change the smell of the ovenB. To clean what the oven cleaner wasn’t able to clean

C. To produce a saltD. To neutralize the oven cleaner that remains in the oven

9.Which of the following properties does not apply to an acid?

A. pH greater than 7B. Sour taste

C. Turn blue litmus redD. Can be neutralised with baking soda

10.Hydrogen is produced when magnesium reacts with

A. CH3OH(aq)B. C2H5OH(aq)C. CH3COOH(aq) D. C6H12O6(aq)

11.An acidic solution

A. does not conduct electricity B. reacts with zinc

C. neutralizes CH3COOH(aq)D. taste bitter

12. Which of the following substance would turn litmus blue?

A. WaterB. Hydrochloric acid C. Lemon juiceD. Bleach

Use the following information to answer the next two questions

13. The basic solution is

A. Solution 1 B. Solution 2C. Solution 3D. Solution 4

14. The student made an error in its observation for

A. Solution 1 B. Solution 2C. Solution 3D. Solution 4

15. True or False. Justify the false statements.

A. ______: Soap is an example of base.

______: Lemon juice tastes bitter.

______: A solution of vinegar has a pH of approximately 0.

D. ______: A solution of baking soda taste bitter because it’s pH is lower than 7.

E. ______: The solution we find in a car battery would react with magnesium.

F. ______: Aspirin is an acid and would then turn bromothymol blue to yellow.

G. ______: Litmus paper can help you finding the exact pH a solution.

H. ______: Many household cleaners would react with magnesium.

I. ______: A pH of 9 is more acidic than a pH of 5.

J ______: A solution with a pH of 5.2 is considered very acidic.

K. ______: Concentrated acids and bases are corrosive.

L. ______: A solution of vinegar is a strong acid.

M. ______: A solution with a pH of 4 is more acidic than a solution with a pH of 2.

Launch Lab

The colour of your breath

Introduction:Bromothymol blue is an acid-base indicator used to test if a substance is neutral, basic or acidic. In this lab, we will try to find out if your breath is neutral, basic or acidic.

Materials:

- Bromothymol blue- NaOH(aq)- Straw-Erlenmayer

- Stopwatch

Procedure:

Add 100 mL of distilled water in the Erlenmayer flask. Add five drops of bromothymol blue. Record the color.
Add drops of NaOH(aq), swirling the solution after each addition, until the solution turns blue (it might be after one drop).
Start the stopwatch. Using the straw, blow into the solution (pick the student with the worst breath) until the solution changes color.
Record how long it takes for a color change to occur.
Repeat steps 2, but hold your breath for 30-45 s before step 3.Record time to change color.

Analysis:

Make a table of observations.
Based on your prior knowledge, what type of substance was produced when you blew into the solution. Explain your answer.
How did holding your breath for 30-45 seconds affect your observations, if at all?

Chemistry 20

Day 34

Date: ______Name: ______

How can we measure the pH?

We can measure the pH of a solution using a pH meter or a universal indicator.
pH meter:Electronic device used to measured with precision the pH of a solution.
Universal indicator:A universal indicator is a mixture of several indicators that turns a different colour for each number on the pH scale. It is usually sold as a roll of paper and is cheaper than a pH meter.

Lab #2

Finding the pH of acids and bases

Problem:Using a pH meter and a universal indicator to find the pH of 10 solutions.

Design:Test all solutions for the pH with the pH-meter and the universal indicator. Record observations in the table below.

Solution / pH with pH-meter / pH with universal indicator
Tide
Cream of tartar
Aspirin
Baking soda
Antacid
Vinegar
Drain cleaner
Rubbing alcohol
Ammonia

Lab #3

Purple cabbage as a Universal indicator

Problem:Using purple cabbage as a universal indicator

Design:Prepare 11 solutions with pH between 2 to 12 and add cabbage solution to them.

Keep a sample of each solution. Test the 4 unknown solutions for the pH. Record observations in the tables below.

PH / Color of solution
2
3
4
5
6
7
8
9
10
11
12
Solution / Color of solution / pH of the solution
1
2
3
4

Chemistry 20

Day 30

Date: ______Name: ______

Acids and Bases in Solution

Svante Arrhenius (1859-1927) was one of pioneer in the study of acids and bases. He won a Nobel prize in 1913 for Chemistry for his work.

Arrhenius’s explanation of the properties of acids and bases is the Arrhenius’s theory of acids and bases.

Arrhenius Theory of Acids and Bases
An Arrhenius acid is a substance that ionizes to form hydrogen ion (H+) concentration in aqueous solution.
Ex. : HCl(g)  HCl(aq)  H+(aq) + Cl-(aq)
An Arrhenius base is a substance that dissociates to produce hydroxide ion (OH-) in aqueous solution.
Ex. : NaOH(s)  NaOH(aq)  Na+(aq) + OH-(aq)

The hydrogen ion is really only a proton (with no electron). If such a particle comes near polar water molecules, it is likely to bond strongly to it and become what we call a hydronium ion.

H+(aq) + H2O(l) H3O+(aq)

Using the Arrhenius theory, we could infer that acids must contain a source of H+(aq) and bases must contain a source of OH-(aq).

Examples

Classify the following solutions as acidic, basic, or neutral.

a) HNO3(aq) : ______b) Ca(OH)2(aq) : ______

c) HBr(aq) : ______d) LiOH(aq) : ______

e) NaCO3(aq) : ______f) NH3(aq) : ______

Does the Arrhenius theory successfully predict acidic or basic properties in all cases?

New and improved Arrenius’ definitions

Evidence from the examples above indicates the limited ability of Arrhenius's theory to predict acidic or basic properties of a substance in aqueous solution.

Only four substances followed the definition: HNO3(aq), HBr(aq), Ca(OH)2(aq)and LiOH(aq)

The other substances did not follow that definition; therefore, the Arrhenius’s definitions of acids and base needed to be revised.

A modified Arrhenius theory
An Arrhenius acid reacts with water to produce hydronium ions H3O+(aq) in aqueous solutions.
Examples:HCl(aq) + H2O(l)  H3O+(aq)+ Cl-(aq)
CH3COOH(aq) + H2O(l) H3O+(aq)+ CH3COO-(aq)
An Arrhenius base dissociates or reacts with water to produce hydroxide ions OH-(aq) in aqueous solutions.
Examples:NaOH(s)  Na+(aq) + OH-(aq)

Example #1: Ammonia is a base because of the reaction of ammonia with water that produces the hydroxide ion.

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

Example #2: Sodium carbonate in water produce a basic solution:

First, sodium carbonate dissociate: Na2CO3(s)  2 Na+(aq) + CO32-(aq)

Second, the carbonate ion reacts with water: CO32-(aq)+ H2O(l) OH-(aq) + HCO3-(aq)

Example #3: Sodium hydrogen sulfate in water produces an acidic solution:

First, sodium hydrogen sulfate dissociate: NaHSO4(s)  Na+(aq) + HSO4-(aq)

Second, the hydrolysis: HSO4-(aq)+ H2O(l) H3O+(aq) + SO42-(aq)

Example #4: Carbon dioxyde in water produces an acidic solution:

First, carbon dioxyde reacts with water: CO2(s) + H2O(l)  H2CO3(aq)

Second: H2CO3(aq)+ H2O(l) H3O+(aq) + HCO3-(aq)

Exercises:

1. Explain why the following substances are considered as acids or bases (According to Arrhenius)

a)Na3PO4(s), produces a solution with a pH = 11 when dissolved in water.

b)NH4Cl(s) produces a very weak acidic solution when dissolved in water.

c)Baking soda produces a very weak basic solution when dissolved in water.

d) Nitric acid is a very acidic solution.

Chemistry 20

Homework 30

Date: ______Name: ______

Exercises:

1.Write a chemical equation to show how the following acids would react in water, according to the modified Arrhenius theory of acids and bases.

a) HCl(aq)

b) NH4Cl(aq)

c) NaHSO4(aq)

d) H2SO3(aq)

2.Write a chemical equation to show how the following bases would dissociate or react in water, according to the modified Arrhenius theory of acids and bases.

a) Ca(OH)2(aq)

c) Na3PO4(s)

b) NaS2 (aq)

c) LiOH(aq)

Chemistry 20

Day 31

Date: ______Name: ______

Strong and Weak Acids and Bases

According to the modified Arrhenius theory of acids and bases, all acids produce hydronium ions in water and all bases produce hydroxide ions in water. Not all acids, however ionize to the same degree.

Strong acid
Strong acid: An acid that ionizes nearly 100% in water.
For example: HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq) (100% reaction)

Weak acid
Weak acid: An acid that reacts very little in water. Only a small percentage of the acid molecule form ions in water.
For example: CH3COOH(g) + H2O(l)  H3O+(aq) + CH3COO-(aq) (1% reaction)

Notice the double arrow-headed equilibrium arrow in the equation above. This arrow indicates that the solution is in equilibrium.

 The acid molecules are constantly reacting with water to form the ions and

the ions are constantly reacting to form the acid molecules and water.

Strong base
Strong base: A strong base dissociate completely into ions in water. All oxides and hydroxides of alkali metals and Earth alkali metals are strong bases.
For example: NaOH(s)  Na+(aq) + OH-(aq) (100% reaction)
Weak base
Weak base: A weak base is a substance that reacts very little in water to produce a very small amount of hydroxide ions.
For example: NH3(g) + H2O(l)  NH4+(aq) + OH-(aq) (1% reaction)

Only about 1% of ammonia molecules react in water to produce OH-(aq). In a 0.10 mol/L solution of ammonia, the concentration of OH-(aq) is only about 0.0010 mol/L.

Chemistry 20

Homework 31

Date: ______Name: ______

Exercises:

Classify the following as neutral ionic, molecular, strong acid, weak acid, strong base or weak base.

a)C6H5COOH: ______b) HF : ______

c) NH3 : ______d) Na2CO3 : ______

e) KOH : ______f) Mg(OH)2 : ______

g) NaCl : ______h) Na3PO4 : ______

i) HCl: ______j) NH4Cl(aq) : ______

k) NaHSO4(aq) : ______l) HNO3(aq) : ______

Chemistry 20

Day 32

Date: ______Name: ______

Communicating concentration: pH and pOH

In 1909 a chemist, Soren Sorenson, developed a simplified system for referring to the degree of acidity of a solution. He used the term pH for power of hydrogen. pH refers to the concentration of hydronium ions in solution.

[H3O+(aq)] ranges from about 10.0 mol/L down to 1.0 x 10-15 mol/L in common aqueous solutions. A range on the order of a quadrillion to one can only be expressed by using logarithm. Expressed as a numerical value without units, the pH can be calculated using the following formula:

pH = - ( log10 [H3O+(aq)] )

If you know the pH of a solution, you can calculate the [H3O+(aq)] using the following formula:

[H3O+(aq)] = 10-pH

Although pH is used in most applications, in some applications it may be convenient to describe hydroxide ion concentration in a similar way. The definition of pOH follows the same format and the same rule as for pH

pOH = - ( log10[OH-(aq)] )

[OH-(aq)] = 10-pOH

The mathematics of logarithms allows us to express a simple relationship between pH and pOH

pH + pOH = 14

Summary : Formulas to be memorized
pH = - ( log10 [H3O+(aq)] )
[H3O+(aq)] = 10-pH
pOH = - ( log10[OH-(aq)] )
[OH-(aq)] = 10-pOH
pH + pOH = 14

Examples :

1.Calculate the pH of a sample of acid rain in which [H3O+(aq)] = 1.2 x 10-4 mol/L

2.Find [H3O+(aq)] in a bottle of pop that has a pH of 2.1

3.Find the pH of a 0.0060 mol/L KOH solution

4.Find [OH-(aq)] in solution with a pH of 12.9

6. A student dissolves 1.55 g of NaOH in 500 mL of water. What is the pH of the solution?

7. A student want to prepare a 1.00 L of a solution with a pH =1.4 . What volume of concentrated (12.4 mol/L) hydrochloric acid must be used?

Chemistry 20

Homework 32

Name: ______Date: ______

Exercises

1. Find the pH of the following solutions

a)Solution with [H3O+(aq)] = 1.00 x 10-6 mol/L : pH = ______

b)Solution with [H3O+(aq)] = 1.50 x 10-2 mol/L : pH = ______

c)Solution with [OH-(aq)] = 1.00 x 10-6 mol/L : pH = ______

d)Solution with [H3O+(aq)] = 0.00150 mol/L : pH = ______

Find the pH of lime juice if [H3O+(aq)] = 0.0120 mol/L? ______

3.Find the pH of blood if [OH-(aq)] = 2.6 x 10-7 mol/L? ______

4.Find the pH of a solution made of 1.00 g of KOH in 1.00 L of water? Show your work.

5. Classify the following substances from the least acidic to the most acidic.

Solution A : pH = 6Solution B : [H3O+(aq) ] = 1.0 x 10-4 mol/L

Solution C : [OH-(aq)] = 1 x 10-13 mol/L Solution D : pH 9

______

Least Most

7.What mass of potassium hydroxide is contained in 500 mL of solution that has a pH of 11.5? Show your work.

8.A student make a solution in which [H3O+(aq)] = 1.0 x 10-3 mol/L. This solution would

A. be green in bromothymol blue B. be pink in phenolphtalein

C. turn litmus red D. have a pOH of 3

9.Hydrogen is produced when magnesium reacts with

A. CH3OH(aq)B. C2H5OH(aq)C. CH3COOH(aq) D. C6H12O6(aq)

10. A student wants to clean the spill of a car battery. Which household substance should be used?

A. Water B. Vinegar C. Lemon juice D. Baking soda

11.Which of the following substance is a base

A. HOOCCOOH(aq)B. Ba(OH)2(aq)C. CH3OH(aq)D. NH4+(aq)

12.What is the pH of a 0.0010 mol/L LiOH (aq) solution?

A. 3.0B. 7.0C. 10D. 11

13.An acidic solution

A. does not conduct electricity B. reacts with zinc

C. neutralizes CH3COOH(aq)D. taste bitter

14.A 0.0300 mol/L HCl(aq) has a pH of

A. 2.48B. 2.00C. 1.52D. 0.480

15.If the pH of a solution is 8.0 ,

A. [H3O+(aq)] > [OH-(aq)]B. [OH-(aq)] > 1.0 x 10-7 mol/L

C. [OH-(aq)] = 1.0 x 10-8 mol/LD. [H3O+(aq)] = 8,0 mol/L

16.A student find the pH of a fruit juice to be 2.4 . The [H3O+(aq)] of this juice is

A. 4.0 x 10-3 mol/LB. 2.5 x 10-8 mol/LC. 3.8 x 10-1 mol/LD. 2.5 x 10-18 mol/L

17. A sodium hydrogen carbonate solution has a pH of 8.0 , [OH-(aq)] is

A. 1.0 x 10-13 mol/L B. 1.0 x 10-8 mol/L C. 1.0 x 10-6 mol/L D. 1.0 x 10-5 mol/L

Chemistry 20

Day 33

Date: ______Name: ______

Acid-Base Indicators

Acid-Base Indicators
Acid-base indicators are substances, which have one color in acidic solutions and another color in basic solutions.
The color change for each indicator is related to specific pH values
List of indicators : Data booklet P. 10
Indicators can be use to find:
1.the approximate pH of a solution.
2. the equivalence point for the neutralization reaction.

Example #1: Complete the following table : Find the color of the following indicators .

pH of solutions / Bromothymol
Blue / Methylorange / Phenolphtalein / Cresol red / Thymol blue
1 / / /
2 /
3
4
5
6 /
7 /
8 / / /
9 / /
10 /
11
12
13

Example #2:

A solution is yellow with methylorange and red with methyl red. What is the approximate pH of this solution?

Example #3:

Colors of an acid with different indicators.
Test tubesIndicatorColor
1 Orange IVYellow
2Methyl orangeYellow
3Bromocresol greenBlue
4Methyl redOrange
5Bromothymol blueYellow

The approximate pH of the solution is between ______

Example #4:

Four unknown solutions labelled I, II, III and IV were tested and these data were obtained:

Indicator I II III IV

Thymol bluegreenyellowgreenorange

Methyl orangeyellow orange yellow red

Thymolphtaleincolorless colorless blue colorless

The acidic solution(s) is/are ______

Lab #6

pH of solutions

Problem:Find the pH of 4 solutions using only indicators.

Design: Find the approximate pH using the following indicators:

Acidic range: Bromophenol blue – Methyl red

Neutral range: Bromothymol blue

Basic range: Phenol red – Phenolphtalein.

Compare with the real pH given by your teacher.

Questions

Make a table of observation.

Lab #7

Prepare solutions with specific pH

Problem : To prepare an acidic and a basic solution with a specific pH from a potassium hydroxide and a 0.6 mol/L HCl solution.

Materials : - Scale- Volumetric flask- Solid KOH0.6 mol/L HCl

Design:

Calculate the mass of solid potassium hydroxide needed to make a solution with a specific pH given by your teacher. Add the solid to a known volume of water. Mix and record the pH with a pH-meter.

pH = ______Volume = ______

Calculate the volume of 0.6 mol/L HCl needed to make a solution with a specific pH given by your teacher. Add the acid to a known volume of water. Mix and record the pH with a pH-meter.

pH = ______Volume = ______

Questions

1. Calculate the mass of base needed.

2. Calculate the volume of 0.6 mol/L HCl needed.

3.Make a table of observations.

4. Calculate the percent of error for each solution.

Chemistry 20

Homework 33

Name: Date:

2.If we add a base to a solution,

A. [H3O+(aq)] decreases and pH increasesB. [H3O+(aq)] decreases and pH decreases

C. [H3O+(aq)] increases and pH increasesD. [H3O+(aq)] increases and pH decreases

3. Phenolptalein would be pink in which of the following solutions?

A. [OH-(aq)] = 1.0 x 10-3 mol/L B. [H3O+(aq)] = 1.0 x 10-4 mol/L

C. [H3O+(aq)] = 1.0 x 10-5 mol/LD. [OH-(aq)] = 1.0 x 10-11 mol/L

4. If a solution easily conducts electricity and turns blue litmus red, the solution is probably

A. HNO 3(aq)B. NaCl (aq)C. Ba(OH) 2(aq)D. CH3COOH (aq)

Colors of an acid with different indicators.
Test tubesIndicatorColor
1 Orange IVYellow
2Methyl orangeYellow
3Bromocresol greenBlue
4Methyl redOrange
5Bromothymol blueYellow

5. The approximate pH of the solution is between

A. 3.0 and 4.0B. 4.0 and 5.0C. 5.0 and 6.0D. 6.0 and 7.0

6.A student make a solution in which [H3O+(aq)] = 1.0 x 10-3 mol/L. This solution would

A. be green in bromothymol blue B. be pink in phenolphtalein

C. turn litmus red D. have a pH of 4

7.The indicator bromothymol blue would be green in a solution with

A. pH = 1B. pH = 7 C. pH = 10D. pH = 14

Use the following information to answer the following question

Colour of an unknown solution with different indicators

Methyl violet is Blue Phenol Red is Red
Indigo carmine is BluePhenolphtalein is colorless

8. The approximate pH of the solution is ______

9.Four unknown solutions were tested with different indicators.

SolutionObservations

1Methyl orange was orange

2 Thymolphtalein and indigo carmine were blue

3Litmus was blue and phenolphtalein was colourless

4Bromocresol green was blue and bromothymol blue was yellow

The solutions from the most basic to the most acidic are respectively:

, , , ______

Chemistry 20

Day 39

Date: ______Name: ______

Chemical analysis by titration

Titration is a common experimental design in chemistry used to determine the concentration of a substance in solution.

Titration involves the carefully measured and controlled addition of a solution from a buret into a measured volume of a sample solution until the reaction is judged to be completed.

Buret:A buret is a precisely marked glass cylinder with a stopcock at one end. It measures a volume of reacting solution.

The solution in the buret known as the titrant is added to the sample until the reaction is complete.

An indicator is used in all acid-base titration. The color change of the indicator indicates the end of the reaction.

Some Definitions

Titration: It’s a progressive transfer of a solution from a buret into a measured volume of a sample solution. It is used to determine the concentration of a solution.
Endpoint:The completion of the reaction is indicated by a sudden color change of an indicator. This is referred as the endpoint of the titration.
Equivalence point:The measured quantity of titrant added when the endpoint occurs is called the equivalencepoint.
Standard solution:Solution used in the buret. Standard solutions are solutions with precisely known concentrations.

A titration analysis should involve several trials, using different samples of the unknown solution to improve the reliability of the answer. A typical requirement is to repeat measurements until three trial results within 0.2 mL (and then take the average).