TOPIC 11: Acids and Bases

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TOPIC 11: Acids and Bases

ELECTROLYTES

•

________________________ are substances that when dissolves in water conduct electricity.

•

They conduct electricity because they will break apart into ________________________

•

Ex. NaCl(s) ! Na

⁺

(aq) + Cl

^-

(aq)

•

, and _______________ are electrolytes

DEFINING ACIDS AND BASES 1. Arrhenius acids and bases

•

An arrhenius acid: substance that when dissolved in water releases ____________________

_______________________________________________________________________________

o Ex.

o Remember: an acid must start with H or end with –COOH (Table K)

" KH not an acid!

•

The H

⁺

ions combine with water molecules to make hydronium ions (H

3

O

⁺

) o H

⁺

+ H

2

O ! H

3

O

⁺

•

A monoprotic acid releases _________________________ in solution. Ex. HCl, HNO

3

•

A diprotic acid yields _______________________________ in solution. Ex. H

2

SO

4

•

An arrhenius base: is a substance that when dissolved in water ______________________

____________________________________________________________________________

o Ex.

o Remember: An arrhenius base is a metal + OH or a polyatomic + OH (Table L)

o Alcohols are not bases! Ex. C

2

H

5

OH

o According to this definition, ammonia is not a base 2. Bronsted-Lowry Acids and bases (Alternate Acid-base definition)

•

A Bronsted-Lowry acid is any atom or ion that ____________________________________

-

B-L acids lose H

⁺

•

It is broader than the arrhenius definition. It includes all arrhenius acids plus other atoms/ions—like NH

4+

•

A Bronsted-Lowry base is any atom or ion that ____________________________________

-

B-L bases gain H

⁺

(2)

•

Ex NH

3

+ H

2

O #! NH

4+

+ OH

^-

-

In the forward reaction, the water molecule starts with___ H’s and ends up with_.

•

It lost an H and therefore is the ____________

-

The ammonia starts with H’s and ends up with .

•

It gained an H and therefore is the _______________

-

In the reverse reaction, ____ is the acid and _ is the base

•

Ex. HCl + H

2

0 #! Cl

^-

+ H

3

0

⁺

-

Forward reaction: ___ is the acid,_ is the base

-

Reverse reaction, _______ is the acid and _ is the base

• Conjugate acid: the ion or molecule that formed from the original base

• Conjugate base: the ion or molecule that formed from the original acid Ex. NH

3

+ H

2

O ↔ NH

4+

+ OH

^-

Acid: _ Conjugate base: __

Base: Conjugate acid: __

+ : are a conjugate acid-base pair + : are a conjugate acid-base pair Ex. HCl + H

2

O ↔ H

3

O

⁺

+ Cl

^-

Acid: Conjugate base: _

Base: Conjugate acid:

+ : are a conjugate acid-base pair + : are a conjugate acid-base pair

Properties of Acids and Bases

Acids Bases

•

Found on ______________________

•

Begin with _ or end with __________

•

Have a sour taste (lemon, vinegar) and a slippery feel

•

React with metals to produce H

2

(g) (single replacement)

•

__________________________________________

____________________________________________

•

ex. Na + HCl ! NaCl + H

2

(g)

•

Reacts with a base to produce ____________________

•

Ex. HCl + NaOH ! H

2

O + NaCl

•

Electrolytes: conduct electricity in water

•

pH is less than 7

•

Found on ___________________________

•

___________________________________

•

bitter taste and soapy feel

•

Reacts with an acid to produce salt and water

•

Electrolytes : conduct electricity in water

•

pH is greater than 7

(3)

Amphoteric substances

•

Also called Amphiprotic substances

•

can act as either an acid or a base

•

ex. HSO

4-

•

ex. Water

o H

2

O + H

2

SO

4

! H

3

O

⁺

+ HSO

4-

o H

2

O + NH

3

! OH

^-

+ NH

4+

Salts

•

_______________________________________________________________

•

Contain:

1.

Metal + nonmetal

2.

Metal + a polyatomic ion (except OH)

3.

nonmetal + polyatomic ion

4.

poly + poly

•

Ex. LiCl, NaCl, K

2

SO

4

, MgCl

2

•

Salts are neutral, they have a pH of 7

•

electrolytes

IDENTIFY THE FOLLOWING AS AN ACID (H- OR –COOH, Table k), BASE (-OH, table L)), SALT (IONIC) OR OTHER

1. NaOH ________________ 2. C

2

H

5

OH ___ 3. HF _

4. KF ________________ 5. C

2

H

3

COOH _______________ 6. NH

3

____________

7. C

6

H

12

O

6

________________ 8. H

2

SO

4

_______________ 9. K

2

SO

4

__________

Practice Regents Problems

1) Which formula represents an electrolyte?

A) CH

3

OH B) CH

3

COOH C) CH

3

OCH

3

D) C

2

H

5

CHO 2) In the reaction NO

2-

(aq) + H

2

O(l) ! HNO

2

(aq) + OH

^-

(aq), the NO

2-

(aq) acts as

A) a Bronsted base B) an Arrhenius base C) a Bronsted acid D) an Arrhenius acid 3) Which substance is an electrolyte?

A) H

2

O B) CH

3

OH C) KOH D) C

6

H

12

O

6

4) Which compound is an electrolyte?

A) C

6

H

12

O

6

B) CCl

4

C) CH

3

OH D) CaCl

2

(4)

5) When an Arrhenius acid dissolves in water, the only positive ion in the solution is

A) K

⁺

B) Na

⁺

C) Li

⁺

D) H

⁺

6) A hydrogen ion, H

⁺

, in aqueous solution may also be written as

A) H

3

O

⁺

B) H

2

O C) OH

^-

D) H

2

O

2

7) Which substance yields hydroxide ion as the only negative ion in aqueous solution?

A) C

2

H

4

(OH)

2

B) MgCl

2

C) CH

3

Cl D) Mg(OH)

2

NEUTRALIZATION REACTIONS

•

Occur when: an arrhenius acid reacts with an arrhenius base to produce salt and water

•

Ex. H

2

SO

4

+ 2KOH ! K

2

SO

4

+ 2H

2

O Ex. H

⁺

+ OH

^-

! H

2

O (net equation)

•

When a solution is neutral, the moles of H

⁺

= moles OH

^-

(concentrations are equal)

•

Writing neutralization reactions:

HNO

3

+ KOH !

Acid + base ! water + salt

•

take the H+ from the acid and the OH- from the base to form water

•

then combine the other ions to form the salt.

o Criss-cross the oxidation numbers to get the subscripts.

•

make sure the equation is balanced

Ex. NaOH + HCl ! Ex. __H

3

PO

4

+ __NaOH !

TITRATIONS

•

The purpose of a titration is to determine the concentration of an acid or base by performing a neutralization reaction

•

A titration is the process of adding an acid/base OF KNOWN concentration to an acid/base that you don’t know the concentration of, until you have a neutral solution.

•

The indicator ______________________________ is used to indicate when the solution is neutral

•

Uses the formula:

Ex. What is the concentration of a HCl solution if 50mL of a .25M KOH solution are needed to neutralize 20 mL of the HCl solution?

(5)

Ex. If 50 mL of 3M HNO

3

completely neutralized 150 mL of KOH, what was the molarity of the KOH solution?

Solving titration problems using lab data:

3 We calculate the volume of acid used by:

2 We know the concentration (M) of the substance in the buret. In this case, it is the ______________________ The acid will be added to the base until the phenolphthalein goes

from to ___________

1 The substance in the flask is the one we don’t know the concentration (M) of. In this case, we are looking for the concentration

of_____________________________________.

We do know its _________________________ .

Ex. A student recorded the following buret readings during a titration of a base with an acid:

Calculate the molarity of the KOH.

•

For diprotic acids use:

Ex. What is the concentration of H

2

SO

4

if 50 mL of a .25M KOH solution are needed to neutralize 20mL of the H

2

SO

4

?

•

For bases that produce more than 1 mole of OH (ex. Ca(OH)

2

) use:

pH SCALE AND TABLE M

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•

Indicators are used to determine the pH of a substance

•

Table M: shows the color an indicator changes in varying pH values Ex. methyl orange 3.2-4.4 red to yellow

This means that:

1. methyl orange will be red in a pH of less than 3.2 (strong acid) 2. yellow in a pH greater than 4.4

3. orange in a pH between 3.2-4.4

•

2 indicators can be used to narrow down the pH range

Ex. Methyl orange turns yellow. This means that: _____________________________

Bromthymol blue turns yellow. This means that: _____________________________

Therefore, the pH is between ______________________________________________________

•

The easiest way to read table M: the first color listed in the one the indicator would be in an acid.

The second color listed is the one it would be in a base.

Practice Regents Problems:

1) Which of these pH numbers indicates the highest level of acidity?

A) 5 B) 12 C) 8 D) 10

2) Which indicator is yellow in a solution with a pH of 9.8?

A) methyl orange B) bromcresol green C) bromthymol blue D) thymol blue 3) Which aqueous solution would turn blue litmus red?

A) NaCl(aq) B) HCl(aq) C) NaOH(aq) D) K2CO3(aq)

Comparing pH and pOH

pH pOH

Indicates how many H

⁺

(H

3

O

⁺

) ions are in solution

Indicates how many OH

^-

ions are in solution

Calculated using pH = -log[H

⁺

] Steps:

• Put the molarity (concentration) in scientific notation

• The exponent is the pH

• Ex. what is the pH of a .001M HCl solution?

• Ex. If the pH is 6 what is the concentration of the solution?

Calculated using pOH = -log[OH

^-

] Same steps but using [OH

^-

]

pH + pOH = 14

[H

⁺

][OH

^-

] = 1 x 10

^-14

(7)

0 7 14

What a change in pH indicates pH change More acidic or

basic? Effect on [H

⁺

] Effect on [OH

^-

]

•

When the [H

⁺

] = [OH

^-

] the solution is _______________________

•

When the [H

⁺

] > [OH

^-

] the solution is _______________________

•

When the [H

⁺

] < [OH

^-

] the solution is _______________________

For each one-unit change in pH, the concentration of H+ (H

3

O

⁺

) ions changes by x10

0 7 14

Ex. if the pH of a solution DECREASES from 4 ! 3, the resulting solution is now 10x MORE acidic Ex. if the pH of a solution DECREASES from 6! 3 (3 units),

the solution is __________________________________(the amount of H+ ions increases by 1000x) Ex. if the pH of a solution INCREASES from 4 !7,

the amount of H+ ions __________________________________________________. It becomes 1000x more basic.

Ex. If the pH started at 8 and there was a 100-fold DECREASE (1/100) in hydronium ion

concentration, what is the new pH? __________________

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Practice Problems

Questions 1 through 3 refer to the following:

A student was studying the pH differences in samples from two Adirondack streams. The student measured a pH of 4 in

stream A and a pH of 6 in stream B.

1) For the situation described, compare the hydronium ion concentration in stream A to the hydronium ion concentration in stream B.

_________________________________________________________________________________

2) What is the color of bromthymol blue in the sample from stream A in the situation described?

___________________

3) Identify one compound that could be used to neutralize the sample from stream A in the situation described.

___________________________________

4) When the pH of a solution changes from a pH of 5 to a pH of 3, the hydronium ion concentration is

A) 0.01 of the original content B) 100 times the original content C) 0.1 of the original content D) 10 times the original content

KEY THINGS TO REMEMBER WHEN TAKING THIS EXAM:

1. Identify if the substance is an acid, base, or salt. Then you will know:

a. its properties (conducts electricity, pH range, reacts with metals to form H

2

) b. what color its turns an indicator (TABLE M)

i. first column is the color it would be in an acid ii. second column is the color it would be in a base c. use tables K and L to help you

2. Know definition of Arrhenius acids and bases

3. Be able to identify B-L acids and bases and know definitions 4. Know the names of the 3 ions we are working with

a. H

3

O

⁺

is a hydronium ion

b. H

⁺

is a hydrogen ion table E can help with this c. OH

^-

is a hydroxide ion

5. Realize that the [H

3

O

⁺

] and [H

⁺

] mean the same thing (both come from acids) 6. Understand how movement along the pH scale affects the [H

3

O

⁺

] / [H

⁺

]

a. pH ↓ the [H

3

O

⁺

] / [H

⁺

] ↑ b. pH ↑the [H

3

O

⁺

] / [H

⁺

TOPIC 11: Acids and Bases