Acids & Bases slides

High School Chemistry Rapid Learning Series - 21

© Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 1

Rapid Learning CenterChemistry :: Biology :: Physics :: Math

Rapid Learning Center Presents …p g

Teach Yourself High School Chemistry in 24 Hours

1/42 http://www.RapidLearningCenter.com

A id d BAcids and Bases

HS Ch i t R id L i S i

Rapid Learning Centerwww.RapidLearningCenter.com/© Rapid Learning Inc. All rights reserved.

HS Chemistry Rapid Learning Series

Wayne Huang, PhDKelly Deters, PhDRussell Dahl, PhD

Elizabeth James, PhD



Learning Objectives

Several definitions of “acid” and “base”.

By completing this tutorial you will learn about…

Strong versus weak acids and bases.

Conjugate acids and bases.

How to find pH of strong acids and bases.

H lt b idi

3/42

How salts can be acidic or basic.

How buffers work.

Titrations.

Concept MapChemistry

Studies

Previous content

New contentand Bases

Acids and Bases Can be

Matter

pHConjugates

Form

Form

Scale to measure

Can be

4/42

Strong or WeakStrong or Weak

Buffer



Acid and Base Definitions

5/42

Definition - Arrhenius

Arrhenius Acid – Produces the hydronium ion in waterhydronium ion in water.

Arrhenius Base – Produces the h d id i i

H3O+

6/42

hydroxide ion in water.

OH-

Note: Neutralization is the combination of H3O+ and OH- to form H2O, i.e. H3O+(aq) + OH-(aq) 2H2O(l)



OHH Cl

Arrhenius Acids and BasesArrhenius Acid: HCl + H2O H3O+ + Cl-

OHH Cl+1

-1

HHO

HHHHO

HH Acid

WaterHydronium ion

Arrhenius Base: NH + H O NH + + OH-

7/42

Arrhenius Base: NH3 + H2O NH4+ + OH-

HH N HHHH

OHHHH

HH N HHHHHH

OHH

+1

-1

Definition - Brønsted-Lowry

Brønsted-Lowry Acid – Donates a proton (H+).

Brønsted-Lowry Base – Accepts a proton (H+).y p p ( )

Example: NH3 + HCl NH4+ + Cl-

H+ donor – Bronsted-Lowry AcidH+ acceptor – Bronsted-Lowry Base

8/42



Definition – Lewis

Lewis Acid – Electron (lone pair) Acceptor.Lewis Base – Electron (lone pair) Donor.

Example: BCl3 + :NH3 Cl3B-NH3

Electron pair acceptor – Lewis Acid Electron pair donor – Lewis Base

9/42

Comparing the DefinitionsHow do the 3 definitions relate?

Acid Base Note

Produces H3O+ in water.

Donates H+.

Accepts electrons.

Produces OH- in water.

Accepts H+.

Donates electrons.

Requires water.

Does not need to be in water.

Does not need to use “H+”.

Arrhenius

Brønsted-Lowry

Lewis

10/42

Many Arrhenius acids/bases are also Brønsted-Lowry and Lewis acids/bases.



Properties of Acids and BasesWhat are some common properties?

Acids Bases

Taste sour (e.g. citrus). Taste bitter (e.g. soap).

React with metals to form H2 gas.

Have pH levels below 7.

Turn Litmus red.

Feel slippery.

Have pH levels above 7.

Turn Litmus blue.

11/42

StrongStrong versus Weak

12/42



Definition - Strong and Weak

Strong – Most of the molecules perform their “duty”.e.g. Strong Acid: Most of the molecules donate their H+ and completely ionized in solution.(i.e. HCl H+ + Cl-, Ka = [H+][Cl-]/[HCl] … large).

Weak – Very few of the molecules perform their “duty”.

13/42

p y

e.g. Weak Acid: Only a very few of the molecules donate their H+ and partially ionized in solution. (i.e. HC2H3O2 C2H3O2

- + H+, Ka = [H+][C2H3O2

-]/[HC2H3O2] … small).

Acetic Acid can be written as:HC2H3O2 or CH3COOH or simply HAc.

Definition - Concentrated and Dilute

Concentrated – Many acid or base molecules have beenbase molecules have been added to the system with high concentration, i.e. 16M HCl.

Dilute – Only a few acid or base molecules have been

14/42

added to the system with low concentration, i.e. 0.16M HCl.



Possible CombinationsThere are 4 combinations of strong, weak, concentrated and dilute:

Concentrated Dilute

A lot added & almost all do their “duty”.

A lot added, but very f d th i “d t ”

Not many added, but of what is there, most will do their “duty”.

Only a few added and of those, only a

ll % d th i

Strong

Weak

15/42

few do their “duty”. very small % do their “duty”.

Weak

Common Strong Acids and BasesThere are only a few common strong acids and bases to remember—the rest will most likely be weak!

HX(HCl, HBr, HI), HNO3, H2SO4, HClO4, HClO3

Sr(OH)2 , Ca(OH)2, Ba(OH)2, NaOH, KOH

Strong Acids

Strong Bases

16/42

Strong Acid Mnemonic: HX, HNO3, H2SO4, HClO4(3) = “Strong acids are eXtra Nasty, Sour and Clear.”Strong Base Mnemonic: Sr(OH)2, Ca(OH)2, Ba(OH)2, NaOH, KOH = “Strong bases (for beach fun) are Surf, CaBaNa & Kayak.



Definition - Polyprotic Acids

Polyprotic Acid – Acid with ypmore than one proton to donate.

They ionize to give more than one H+ ions per molecule.

17/42

Examples:Diprotic: H2CO3 (HCO3

-, CO32-)

Triprotic: H3PO4 (H2PO4- ,HPO4

2- , PO43-)

Strength of Polyprotic acidsEach proton is “weaker” than the one before.

A negatively charged ion is

An acid donates a proton.

e.g: H2CO3

The results is a negatively

charged ion.

Becomes HCO3-

charged ion is less likely to give

up another proton and become -2 charged.

Would be CO32-

18/42

H2CO3 is a stronger acid than HCO3- .

Sulfuric acid (H2SO4) is the only common strong polyprotic acid.

The 1st hydrogen is “strong” and the 2nd one is “weak”.



Conjugates of Acids and Bases

19/42

Definition - Conjugate Base

Conjugate Base – What’s left after an acid has donated its hydrogen.

HCl + H2O H3O+ + Cl-

Conjugate base

20/42

Acid

After the proton is donated, it can now act as a base… it could accept a proton.



Definition - Conjugate Acid

Conjugate Acid – What’s left after a base has accepted a proton.


Base Conjugate acid

21/42

After the proton is accepted, it can now act as an acid… it could donate the extra proton.

Labeling SpeciesExample: For each of the following, label the acid (A), the base

(B), the conjugate acid (CA) and conjugate base (CB).

NH3 + H2O NH4+ + OH-

3 2 4

H2SO4 + H2O H3O+ + HSO4-

AB CA CB

A B CA CB

22/42

NH3 + H2SO4 NH4+ + HSO4

-

AB CA CB



Strength and ConjugatesHow does the strength of a species relate to the strength of its conjugate?

If it donated the

A strong acid donates it’s

proton easily.

After donating the proton, it

then becomes a (conjugate) base.

If it donated the proton easily, it will not easily grab and hold onto another

one.

It will be a weak base.

23/42

Strong Acid (e.g. HCl) Weak Conjugate Base (e.g. Cl-)Weak Acid (HCO3

-) Strong Base (OH-)Weak Base (H2O)

Strong Conjugate Base (CO32-)

Weak Conjugate Acid (H2O)Strong Conjugate Acid (H+)

Definition - Amphoteric

Amphoteric – A molecule that can act as pan acid or base.


Base

24/42

NH3 + H2O NH4+ + OH-

Acid



pH

25/42

Definition - Logarithms

Logarithm – The number of times a base must be multiplied by itself to reach amultiplied by itself to reach a given number.

# of multiples

x = logb(y) y = bx

26/42

Base

# you’re trying to reach



Definition - pH

pH scale – Logarithmic scale of the acidity of a solution.

The pH scale uses base “10”, i.e. b = 10.

pH = - log[H3O+] …. x = logb(y)

log[H3O+] = 10-pH …. y = bx

27/42

pH has no units.

Note: pOH provides another way to express the basicity, which is defined as pOH = -log[OH-] or [OH-] = 10-pOH. pOH + pH = 14.00 or the ionic product of water Kw = [H3O+][OH-] = 10-14.

pH Scale

0 7 14

Some common items and their pH values:

2.0Stomach Contents

Acidic Neutral

6.5~8.0

Tap Water

Basic

28/42

7.4Blood

Contents

3.0 Pop

4.5Beer

5.5Bread

MilkTap Water



pH of Strong AcidsFor strong acids, assume there is a complete dissociation.

Example: Find the pH of a 0.25 M solution of HCl.

[H3O+] = 0.25 MpH = ?

HCl is a strong acid (complete dissociation):HCl + H2O H3O+ + Cl-If [HCl] = 0.25 M, then [H3O+] = 0.25 M

pH = -log[H3O+]

29/42

pH ?

pH = 0.60

pH = -log(0.25M)

pH of Strong BasesWhen working base problems, you can find [OH-] by the base concentration…but you must find [H3O+] to find pH.

Example: Find the pH of a 0.15 M solution of NaOH.

NaOH is a strong base:NaOH Na+ + OH-

If [NaOH] = 0.15 M, then [OH-] = 0.15 M

Kw = [H3O+][OH-]

30/42

[H3O+] = 6.7×10-14 MpH = ?

pH = 13.18

pH = -log[H3O+]pH = -log(6.7x10-14M)

1.0x10-14 = [H3O+](0.15M)



Acid-BaseAcid Base Properties of Salts

31/42

How Salts Have Acid/Base PropertiesUnderstanding conjugates is a key to understanding acid/base properties of salts.

A weak acid has a strong

conjugate base that can form

salts.

That strong conjugate base will produce a basic solution.

Salts made from the conjugate of a weak acid will have a basic pH.

32/42

The opposite is also true: Salts made from the conjugate of a weak base will be acidic.



Combinations Forming Salts

Cation from… Strong Acid Weak Acid

Anion from…

Strong Base

Weak Base

Neutral Salt

Acidic Salt

Basic Salt

Neutral Salt

Think of it as the “strong on wins” (e.g. strong acid & weak base = acidic).

33/42

Example: Determine if each salt will be acidic, basic or neutral:

NH4Cl

NaH2PO4

Weak base & strong acidCame from NH3 and HCl.Acidic salt

Came from NaOH and H3PO4. Strong base & weak acidBasic salt

Buffers

34/42



Definition - Buffer Buffer – Solution containing a weak acid and its conjugate or weak base and its conjugate that resists change in pH levels (upon addition of small amount of acid or base).)

e.g. H3PO4 (weak acid) and PO43-(conjugate base).

35/42

Conjugates are added in the form of a salt (soluble ionic compound).

e.g. To add PO43- to a buffer, Na3PO4 salt is added.

How Does a Buffer Work?

Buffer: Weak Acid and conjugate base.

How does a buffer resist pH changes?

Conjugate base “absorbs” acid and produces more

weak acid already in buffer.

Strong Acid added

Weak acid “absorbs” base and produces more

conjugate base already in buffer.

Strong Base added

Eventually, so much strong acid or base could be added and all of

36/42

the buffer material would be used up.

Buffer capacity – Amount of strong acid or base that can be absorbed without large pH change.



Titrations

37/42

Definition - Titrations

Tit ti Additi fTitration – Addition of a known volume of a known concentration solution to a known volume of unknown concentration solution to determine the concentration

38/42

determine the concentration.



Definition - End Point

End Point (or Stoichiometric Point or Equivalent Point) –When there is no reactant left over they have all been reacted

End Point (indicator changes color)

over - they have all been reacted and the solution contains only products (color change).

Indicators – Paper or liquid that h l b d h H

The end point must be reached in order to use stoichiometry to calculate the unknown solution concentration.

39/42

changes color based on the pH level (range).

If the pH of the products is known, the indicator (can be chosen to indicate the end point (or stoichiometric point if titrated correctly).

Before the endpoint

At the endpoint

There are several t d fi id

There are several t d fi id

Learning Summary

pH can bepH can be

ways to define acids and bases.

ways to define acids and bases.

40/42

pH can be determined from the

concentration of hydronium ions in a

solution.

pH can be determined from the

concentration of hydronium ions in a

solution.

Some acids and bases are strong,

while others are weak.

Some acids and bases are strong,

while others are weak.



Congratulations

You have successfully completed the core tutorial

Acids and Bases

Rapid Learning Center

Rapid Learning Center

Wh t’ N t

Chemistry :: Biology :: Physics :: Math

What’s Next …

Step 1: Concepts – Core Tutorial (Just Completed)

Step 2: Practice – Interactive Problem Drill

Step 3: Recap – Super Review Cheat Sheet

42/42

Go for it!

http://www.RapidLearningCenter.com

Date post:	15-Jul-2015
Category:	Education
Upload:	timothy-welsh
View:	78 times
Download:	0 times

Acids & Bases slides

Education