Class #101: Acids and Bases - Strength and Concentration

Chemistry Class #101

1

Class #101 Do Now

According to the Arrhenius definition of an acid, it is
a substance that dissociates in solution to produce
a H+ ion.

Examine this image of a hydrogen atom. How does this become an H+

ion? Explain your thinking, and draw an image of an H+ to illustrate

your answer.

2

​Do this on your Class #101 Notes!

AIM: Students will understand that acids and bases
are categorized based on their production of ions in
solution.

Class #101

​2

Arrhenius Acid: Substance whose water
solution contains the hydrogen ion as the
only positive ion.

Svante Arrhenius

** The H+ ion (which is just a
proton) often combines with a
water molecule to create H3O+.
This is called a hydronium ion.

H+ ion + water molecule hydronium ion

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Arrhenius Base:A substance that produces

hydroxide ions(OH- ) when dissolved in

water

4

Strong and Weak Acids and Bases

Strong acids and bases
completely dissociate in water
= close to 100% of the strong
acid molecules dissociate, so
there are many H+ or OH - ions

Weak acids and bases do not
completely dissociate in water
→ only some of the weak acid
molecules dissociate. There are
not so many H+ or OH - ions

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Multiple Choice

The three diagrams represent aqueous solutions of three acids. Which diagram represents the strongest acid?

• Acid A is the strongest

• Acid B is the strongest

• Acid C is the strongest

• All three acids are the same strength

Acids and Bases are Electrolytes

Electrolytes are substances that dissociate into ions in water
●

ions in water can conduct electricity

●

higher concentration of ions →greater conductivity of
electricity

Weak Acids and Bases are poor

conductors of electricity.

Strong Acids and Bases are good
conductors of electricity.

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More Ions in Water → More Electrical Conductivity

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What makes an acid or base solution dilute or concentrated?

It’s all about those H+ or OH- ions!

Dilute aqueous
solutions have a low
ion concentration.

Concentrated solutions
have a high ion
concentration.

8

Strength of Acid or Base is not the same as Concentrated
Solution!

A strong acid/base can create a dilute solution if
there isonly a little bit of it in solution

A weak acid or base can create a concentrated
solution if there is a lot of it!

9

Multiple Choice

Based on the four images, which one shows a concentrated weak acid?

• Acid A

• Acid B

• Acid C

• Acid D

The pH Scale

The pH scale is used to measure how acidic or how basic a substance is.
●

The scale goes from 0 to 14

A pH of 7 is NEUTRAL
not an acid or a base

0

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The pH Scale

Below 7: TheLOWERthe pH value, theSTRONGERthe ACID

0

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The pH Scale

Above 7: The HIGHER the pH value, the STRONGER the BASE

12

Every day acids and bases on the pH scale

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The pH scale measures the concentration of H+ ions

Notation for concentration of ions:

[H+]

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The pH Scale is Logarithmic

This means: each change of a

single pH unit signifies a tenfold

change (10x) in the

concentration of the hydrogen

ion

Strongest Base

Strongest Acid

Example:
pH 5 has a [H+] 10 times greater than pH of 6

15

Multiple Choice

Pure water has a pH of

• 0.001

• 1

• 7

• 14

Multiple Choice

Which change in pH level represents a 100-fold increase in the concentration of hydrogen ions?

• pH 5 to pH 7

• pH 13 to pH 14

• pH 3 to pH 1

• pH 4 to pH 3

Multiple Choice

Which statement describes characteristics of a 0.1 M KOH (aq) solution?

• The solution is acidic with a pH less than 7

• The solution is acidic with a pH greater than 7

• The solution is basic with a pH less than 7

• The solution is basic with a pH greater than 7

Multiple Choice

Which of these pH numbers indicates the highest level of alkalinity?

• 5

• 8

• 10

• 12

Acid-Base Indicators: Table M

An indicator is a substance that
changes its color when it gains
or loses a proton.

Table M shows the most common
acid- base indicators

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Information on Table M

The left column lists the names of
the different Acid-Base Indicators.

The middle column tells you when
the indicator will change color,
based on the pH of the solution.

The right column tells you how the
color will change when it is in the
indicated pH range.

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Using Table M

Example: Thymol Blue

Approximate pH Range for Color Change:

8.0 ------------ 9.6

If added to a
solution with
a pH below
8, Thymol

Blue will look

yellow

If added to a
solution with a
pH between
8.0 and 9.6,
Thymol Blue
starts to
change from
yellow to blue

If added to a
solution with a
pH above 9.6,
Thymol Blue
will look blue.

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Using Table M

Example: Bromcresol Green

Approximate pH Range for Color Change:

3.8 ------------ 5.4

If added to a
solution with
a pH below

3.8,

Bromcresol
Green will
look yellow

If added to a
solution with a
pH between
3.8 and 5.4,
Bromcresol
Green starts to
change from
yellow to blue

If added to a
solution with a
pH above 5.4
Bromcresol
Green will look
blue.

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Why do we need so many different indicators?

●You can use different indicators to get more exact information about pH level

Example:
A solution was yellow in bromthymol blue and blue in bromcresol green.
According to Table M, what could be the
pH of the solution?

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​

​You will answer this question on the next slide!

Multiple Choice

Use Table M to help with this question!

A solution was yellow in bromthymol blue and blue in bromcresol green. According to Table M, what could be the pH of the solution?

• 6.5

• 4.2

• 5.6

• 3.7

Why do we need so many different indicators?

●You can use different indicators to get more exact information about pH level

Example:

Acid was added to a solution containing an indicator until the solution turned from
blue to yellow. Which solution would be the most acidic?

(A)

A yellow solution containing bromthymol blue

(B)

A yellow solution containing bromcresol green

(C)

A yellow solution containing thymol blue?

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​​You will answer this question on the next slide!

Multiple Choice

Acid was added to a solution containing an indicator until the solution turned from blue to yellow.

Which solution would be the most acidic?

• A yellow solution containing bromthymol blue

• A yellow solution containing bromcresol green

• A yellow solution containing thymol blue

Open Ended

What part of today's lesson was easiest to understand?

What part was the hardest to understand?