UNIT 2: WATER

SECTION 3: ACIDS AND BASES

I. DEFINING ACIDS AND BASESAcids and bases are just a specific type of solution in water. There are two main theories that explain

acids and bases.

1. Arrhenius Theory

Acids

• An _____ is a substance which dissociates (breaks up) in water and

_____________________

*Note – an ion is just an atom with a charge (+ or -)

Example: HCl H+ + Cl –

or

HCl (g) + H2O (l) H3O + (aq) + Cl - (aq)

*Note - _____is called a ____________. It forms when the H+ that is released

from the acid bonds to the H2O molecules. So H+ and H3O + both mean the

solution is acidic.

• For ___________, see _______in your reference tables

Dissociation of acids in water animation

acid

releases hydrogen ions (H+)

H3O + hydronium ion

common acids Table K

Bases

• A _____ is a substance which dissociates in water and

_____________________

Example: NaOH Na+ + OH-

• For ___________, see ______ in your reference tables

Arrhenius’s theory was the first to define acids and bases and it is simple and useful;

however, there were some flaws. Look at Table L. Do you see a common base that does not

fit the Arrhenius definition?

Ammonia (NH3) was a substance that acted like a base but did not have any OH- molecules

to release . As a result, the next, more advanced acid-base theory was proposed.

base

releases hydroxide ions (OH-)

common bases Table L

NH3

2. Bronsted-Lowry Theory

• Acid = substance that ____________

Example: HCl (g) + H2O (l) H3O + (aq) + Cl - (aq)

(the HCl is “donating” its H+ to the water)

• Base = substance that _____________

Example: NH3 + H2O NH4+ + OH-

(the NH3 is “accepting” a H+ from the water)

• _______Theory – _____________________

• This theory doesn’t go against the Arrhenius theory, it just adds to it.

donates H+ ions

accepts H+ ions

“BA-AD” Bases accept, acids donate

PRACTICE

1. Hydrogen chloride, HCl, is classified as an Arrhenius acid because it releases

(1) H+ ions in aqueous solution

(2) Cl– ions in aqueous solution

(3) OH– ions in aqueous solution

(4) NH4+ ions in aqueous solution

2. Which ion is released in an aqueous solution of an Arrhenius base?

(1) H- (3) H+

(2) O2 (4) OH-

3. Which compound is an Arrhenius acid?

(1) CaO (3) K2O

(2) HCl (4) NH3

*Hint – look for one that has an ___ and use Table ___ to check!

4. Which compound when dissolved in water is an Arrhenius base?

(1) CH4 (3) NaCl

(2) HCl (4) NaOH

*Hint – look for one that has an ___ and use Table __ to check!

H K

OH L

II. PROPERTIES OF ACIDS AND BASESAcids

• Acids ________or eat away (oxidize) certain ______ to produce hydrogen gas

• Acids have a ___________

• Acids are ________, meaning they _____________________________

• Acids _________, like lemons (but DO NOT TASTE unknown substances)

• Acids “sting”

• Acids have a more “sharp” odor

react with metals

pH less than 7

electrolytes conduct electricity when dissolved in water

taste sour

Bases

• Bases ____________to __________(process called saponification)

• Bases have a ______________

• Bases are also ________, meaning they__________________________

• Bases _________ like ___________ (but DO NOT TASTE unknown substances)

• Bases are _______

• Bases are odorless

*Note – another way to say something is basic is ______

react with fats make soap

pH greater than 7

electrolytes conduct electricity when dissolved in water

taste bitter soap/medicine

slippery

alkaline

WATER IS NEUTRAL, IT HAS A ______;

HOWEVER, IT CAN ACT AS BOTH AN ACID OR

BASE, DEPENDING ON WHAT SUBSTANCE IS

DISSOLVED IN IT.

Example:

Substances that can act as an acid or base are called ___________amphoteric

pH of 7

pH

• The way to measure if a solution is acidic or basic is by ___________

• The pH scale is easiest method to use for comparing the strengths of acids

and bases. We test our fish tanks (fish pee out ammonia, which is a base,

and brings the pH up), our pools, and even food is pH tested as it is being

made to make sure that it falls within the right range. So just what is pH?

• pH stands for the _____________(remember, hydronium is H3O + ) or

the _______________

III. HOW TO KNOW IF A SOLUTION IS ACIDIC OR BASIC

testing its pH

power of hydronium

potential of hydrogen

A pH of 0-7 is __________

A pH of 7 is ____________

A pH of 7-14 is _____________

acidic

neutral

basic

• pH is measured on a scale from ______0 to 14

PRACTICE1. The pH of various aqueous solutions are shown in the table below.

List the substances from most acidic to most basic:

Most Acidic Most Basic

2. Which solution is neutral?

HCl HC2H3O2 NaCl NaOH

NaCl because it’s pH is 7

Testing pH

• A way to test pH is through the use of ________

• Indicator – a substance that __________if an acid or base is present

Examples:

o pH paper—dip the pH paper into the solution and match up the color to

see what the pH is

o Liquid Acid-Base Indicators (Found on ______)

Remember: An acid has a pH less ______and a base has a pH ______

indicators

changes color

than 7 above 7

Table M

Tips on Using Table M

1. Low pH numbers are on the left and high pH

numbers are on the right. They match up.

2. Look at Methyl Orange, red is on the left, 3.2

is on the left. To the left (or below) a pH of

3.2 the color is red. To the right (above) a pH

of 4.4 the color is yellow.

3. Between the 2 numbers will never be asked

on a test. This is where the indicator

changes color.

1. A solution has a pH of 5.5. What color changes will happen to following indicators:

a) Methyl orange __________ c) Bromscresol green____________

b) Bromthylmol blue ________ d) Thymol blue ______________

yellow

yellow

blue

yellow

PRACTICE

*The best way to get an accurate reading is to use multiple indicators*

2. You’re trying to decide if an unknown solution is acidic or basic. It

yields the following results when tested with various indicators:

Methyl Orange = yellow

Phenolphthalein = clear

Bromcresol Green = blue

Thymol Blue = yellow

What can the pH be?

a) 2.8 b) 6.5 c) 8.5 d) 4.8

pH above 4.4

XXXX

pH below 8.0

XXXX

pH above 5.4

XXXX

pH below 8

IV. NEUTRALIZATION

• Neutralization- The process of mixing an ____ and a _____ to form _____ and

____

Example: HCl + NaOH H2O + NaCl

• The H+ from the acid and the OH- from the base combine to make water. The remaining

ions (elements with charges) combine to make the salt. Both products, the water and

salt, are neutral.

• _____________are an everyday example of a neutralization reaction. When you

experience heartburn or indigestion it because of a buildup of acid in your stomach.

_______ are made up of _____ that ____________ in your stomach to

_______________.

Acid + Base Water + Salt

https://www.youtube.com/watch?v=8IRI5gPR5EY&feature=player_embedded&safety_mode=true&p

ersist_safety_mode=1&safe=active

acid base water

salt

Antacids like tums

Antacids bases release OH- ions neutralize the acids