acids and bases chapter 16. a special solution acids and bases are always in a water solution. your...
TRANSCRIPT
A special solution Acids and bases are ALWAYS in a
water solution. Your body has water in it so they are
always dangerous to living things. Bases are just as dangerous as acids. In low concentrations they are not
that dangerous and found all over your house.
Acids and Bases
Aci Re less than 7 sour metals and
bases
ase lue more than 7 bitter and feel
slippery oils and acids
• Although they can be dangerous, acids and base do not react with or “eat”everything.• Neither has an effect on glass for example.
turn litmus paperhave a pHtastereact with
D B
Common places to find acids and bases
Acids Vinegar- acetic
acid citrus fruits-
citric acid carbonated
drinks- carbonic acid
Your stomach- hydrochloric acid
Bases Antacid tablets
(calcium hydroxide)
Windex- ammonia
Oven cleaner- sodium hydroxide
Draino – sodium hydroxide
Homework Using the litmus paper provided in class
check to see if two common items found in your house are acidic or basic.
Report your findings on a piece of paper, and staple the litmus paper with it.
You need to report what the items are and if they are acidic or basic.
Please exercise caution and common sense. Do NOT test anything dangerous!
Definitions Acid- a proton (H+) donor [force
feeder] Acids produce H3O+ (hydronium) in
water Base- a proton (H+) acceptor [thief] Bases produce OH- (hydroxide) in
water
Heat of solution Normally dissolving a substance is
an exothermic process. You are normally increasing the
state of entropy (measure of disorder)
Which normally means you will release heat.
There are exceptions, dissolving ammonium nitrate is an endothermic process
Always do what you oughta … Always add acid to water Dissolving the acid in water
releases heat If you have a lot of acid and a little
water on top, the water typically boils quickly causing the hot acid to spray out.
A lot of water on the bottom typically doesn’t boil if the acid is added slowly enough.
Self dissociation of water. Some water will dissociate itself H2O +H2O H3O+ +OH-
in “pure” water you will find H3O+ has concentration of 1 x 10-7 M OH- has concentration of 1 x 10-7 M The product of the conc. of H3O+ and
OH- is always 1 x 10-14 [ ]-conc. [H3O+] [OH-] = 1 x 10-14
pH
In any solution the H3O+ and OH-
concentration is always very small. pH- method of representing the
H3O+ concentration in a solution. pH = -log [H3O+] So the pH of water is… pH = - log 1 x10-7
pH = 7
What is a log log stands for logarithm ~we can use them to solve for an
exponent. log xy = y log x For example log 1 x10-7 = -7 the log key on your calculator is log10
meaning it will cancel out a 10^. To reverse a log10 raise the whole thing
to the 10th power (10^), this is an antilog
The reversed pH equation is [H3O+] = 10^(-pH)
pH values
pH of 7 is neutral- equal [H3O+] and [OH-]
below 7 is acidic, higher [H3O+] than [OH-]
above 7 is basic or alkaline, higher [OH-] than [H3O+]
Sig Figs and pH
The number of decimal places in the log value, pH value, is equal to the number of significant figures in the number that we took the logarithm of, concentration.
So [H3O+] = 2.45 x10-4 M 3 sig figs pH = -log 2.45 x10-4 M = 3.611 3 decimal places
Reversing that
Having a pH of 4.32 (2 decimal places) gives you a hydronium concentration of…
[H3O+] = 10^(-4.32) = 4.8 x10-5 M (2 sig figs)
pH problems
What is the pH of a 2.4 x 10-4 M H3O+?
pH = - log 2.4 x 10-4 pH = 3.62 What is the OH- concentration? [H3O+] [OH-] = 1 x 10-14
2.4 x10-4 [OH-] = 1 x 10-14
[OH-] = 4.2 x10-11 M
Backwards problem
What is the [H3O+] and [OH-] of a solution with a pH of 8.75?
[H3O+] = 10^(-pH) [H3O+] = 10-8.75 [H3O+] = 1.8 x 10-9 M 1.78…x10-9 [OH-] = 1 x 10-14
[OH-] = 5.6 x 10-6 M
Mixing acids and bases ~creates water H3O+ + OH- 2 H2O this is called neutralizing the
solution a neutralized solution is no longer
dangerous. The point where neutralization is
complete is called the equivalence point
Salts ~the byproduct of an acid and a
base. NaOH + HCl H2O + NaCl (base) (acid) (water) (salt) there are several more than just
table salt. HNO3 + NH4OH H2O + NH4NO3
Acid Base water salt
Gases can be created this depends on the reactants (not
all will) sodium bicarbonate (baking soda)
will pretty much always release a gas
NaHCO3 + H2SO4 H2O + NaHSO4 +
CO2
GasSalt
Titration ~mixing an acid and base
perfectly to make a neutral solution.
You normally need some kind of indicator for this.
Phenolphthalein- when in solution turns red if basic and is clear if acidic.
You can also use a pH probe
Using math To neutralize a solution you will
need to add an equal amount of H3O+ / OH- to what was already present.
so that mol H3O+ = mol OH-
This is used if and only if you are at the equivalence point (completely neutral solution)!
Problem If 94 mL of 4.0 M NaOH neutralizes
6.0 L of an unknown strong acid, what was the H3O+concentration of the unknown?
4 M NaOH x .094 L = .376 mol NaOH
.376 mol H3O+/ 6.0 L = .063 M H3O+
Another problem If 127 mL of 2.0 M NaOH
neutralizes 4.1 L of an unknown acid, what is the initial concentration of the acid?
2.0M(.127 L) = .254 mol NaOH =.254 mol OH- = .254 mol H3O+
4.1 L =.062 M
Rates of reaction Different reactions happen at different
speeds. There are ways to speed up or slow
down a reaction. Changing the surface area of the
reactants Powders react more quickly than “chunks”
Changing the amount/concentration of reactants More reactant speeds up the reaction
Changing the temperature Warmer reactions tend to go faster
Amount of rate change depends on the reaction.
Catalysts and Inhibitors Catalyst- something that increases
the rate of a reaction without changing the products of the reaction.
Catalytic converter speeding the reaction of emissions of a car to less dangerous products
Inhibitor- something that slows or stops a reaction
-food preservatives
Catalyst example 2 O3 3 O2
Ozone will decompose into elemental oxygen, however this process is very slow.
Chlorine acts as a catalyst as shown in this two step reaction
2 O3 + 3 Cl2 6 ClO 6 ClO 3 O2 + 3 Cl2 ClO is an intermediate, something
formed in the middle of the reaction that is later consumed.
Chlorine is a catalyst because it is a reactant in the first step, but a product in the last step. So it isn’t used up during the reaction.
Forwards and backwards Most reactions can go forwards or
backwards Neutralization equation H3O+ + OH- 2 H2O Self ionization of water 2 H2O H3O+ + OH-
Equilibrium In water, both of those reactions are
occurring simultaneously. Equilibrium is when the forward and
backward reactions are occurring at the same rate.
This will cause a stable amount of product and reactant to be present. No net change is occurring when it is at equilibrium. (dynamic equilibrium)
The amount of product and reactant do NOT have to be equal!
Representing equilibrium It is normally represented with a
double arrow 2 H2O H3O+ + OH-
This reaction comes to equilibrium when [H3O+ ] = 1 x10-7 M and [OH- ] = 1 x10-7 M (assuming the solution is neutral)
you won’t have to calculate this.
Le Châtelier’s Principle ~whenever stress is applied to a
system at equilibrium, a new equilibrium will be obtained to relieve this stress.
stress is a change in temperature, pressure, or concentration of some component.
This will change the rate of reaction of either the forward or backward reaction
So you will see an increase in the concentration of the substances on one side of the equation, and a decrease on the other.
This will “shift” the equation to the right or left.
Examples Endothermic reactions absorb
heat, i.e. they need heat to react. If the solution is heated prior to the
reaction (stress)… It will react more quickly So the equation will be forced to
the right (product side) If the reaction is cooled, it will be
forced to the left (reactant side)
Equilibrium Systems at equilibrium are still
dynamic (changing). However, no NET CHANGE will be observed.
A system is at equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction.
Changing concentration 2 H2O H3O+ + OH-
If I add more water It will force the reaction to the right Which means more hydronium and
hydroxide will be produced This is dilution (making the ratio of
hydronium/hydroxide closer)
Equilibrium Add water 2 H2O H3O+ + OH-
Stress + X 0 0 Shift -2y +y +y Final +X - 2y + y +y Since the stress was added to the
left we must take from the left and give to the right to relieve the stress
*where X is the amount of H2O added and larger than 2y
That means… Add water 2 H2O H3O+ + OH-
Final + X - 2y + y +y We increased water because X is
always larger than y (with any coefficent).
We increased H3O+ because +y is an increase
We increased OH- because +y is an increase
The only equilibrium calculation That you will have to do with
numbers is:
[OH-] [H3O+ ] = Kw
Kw is the equilibrium constant for water, it equals 1 x 10-14 M
We have already used the equation
More Le Châtelier’s If I add an acid to the equilibrium… 2 H2O H3O+ + OH-
Stress 0 +X 0 Shift +2y -y -y Final + 2y +X- y -y *Where X is larger than 2y so adding acid will decrease the
[OH-], only slightly increase the[H3O+ ], and increase water.
2 H2O H3O+ + OH-
If I remove hydroxide from the solution…
2 H2O H3O+ + OH-
Stress 0 0 -X Shift -2y +y +y Final - 2y + y -X +y *Where X is larger than 2y So removing hydroxide increases
[H3O+], only slightly decreases [OH-], and decrease the water
Different equation Adding hydrogen to the equilibrium 2 NH3 3 H2 + N2 Stress 0 +X 0 Change +2y -3y -y Final +2y +X-3y -y *where X is larger than 3y Increases the amount of NH3
decreases the amount of N2 and only slightly increases H2
With heat If I cool the following equilibrium Heat+ Co2+ + 4 Cl- CoCl42-
stress -x 0 0 0 Shift +y +4y -y Final +y +4y -y So cooling the solution will cause
more Co2+ & Cl- and less CoCl42- to form
Different definitions of acids and bases
Acids are proton donors (Brønsted Lowry definition) they generate H3O+ in water (Arrhenius
definition) Bases are proton acceptors
they generate OH- in water which is an acid/base? HF + H2O H3O+ + F-
NaHCO3 + H2O Na+ +H2O + CO2 + OH-
Follow the proton
HF + H2O H3O+ + F-
NaHCO3 + H2O Na++ H2O+CO2 +OH-
Joining equilibrium to acid base…What about the reverse reaction?
H+
H+
H+
H+
Conjugate acids and bases When you run the reverse reaction you
find the products are also acids and bases. The acids and bases that are formed are called conjugate acids or bases
H2O + HF H3O+ + F-
base acid conjugate acid conjugate base
NaHCO3 + H2O Na+ +H2O + CO2 +OH-
base acid CA CB
Label Acid, Base, Conjugate Acid, Conjugate Base
HClO3 + H2O ClO3- +H3O+
A B CB CA ClO- + H2O HClO + OH-
B A CA CB HSO4
- + H2O SO42- +H3O+
A B CB CA LiOH + H2O Li+ + H2O + OH-
B A CA CB
Conjugate acids and bases … Conjugate acids and bases
determine if an acid or base is strong or weak.
If the conjugate acid/base readily reacts to run the reverse reaction it is a weak acid/base.
If it does not react in the reverse reaction the acid or base is strong.
Strong Bases
Name Formula Name Formula
Sodium Hydroxide
NaOH Calcium Hydroxide
Ca(OH)2
Potassium Hydroxide
KOH Strontium Hydroxide
Sr(OH)2
Barium Hydroxide
Ba(OH)2
these make a lightning bolton the periodic table!
More with conjugate acids/bases
H2SO4 + H2O H3O+ + HSO4-
Sulfuric acid is a strong acid so its conjugate base, HSO4
-, will not run the reverse reaction.
HSO4 - is actually an acid in water.
HSO4 - + H2O H3O+ + SO4
2-
SO42- will run the reverse reaction, so
it is a weak acid
Other weak acids and bases
Weak Acids Acetic Acid (vinegar) Citric Acid Ascorbic Acid (vitamin C) Boric Acid Carbonic Acid
Weak Bases Sodium Bicarbonate Ammonia Sodium Hypochlorite (bleach)
Danger!!! Strong and Weak acids and bases do
NOT necessarily tell you how dangerous they are.
Concentration is the most important factor for determining danger.
Ammonia is a weak base, if it is highly concentrated it can burn you.
Dilute hydrochloric acid (less than 1 M) is not particularly dangerous
What is water
Water is either an acid or base depending on the situation.
Anything that is either an acid or a base is called amphoteric.
Several things are amphoteric, like parts of you.
Donating Protons
Hydrochloric acid (HCl) can donate 1 proton, so it is called a monoprotic acid.
Sulfuric acid (H2SO4) can donate 2 protons, so it is called a diprotic acid.
Phosphoric acid (H3PO4) can donate 3 protons, so it is called a triprotic acid.
Weak acids and bases can act as an indicator
can be forced the other way So ammonia… NH3 + H2O NH4
++OH-
Ammonia is a gas with a distinct odor
Ammonium and hydroxide are both odorless.
If base is added to the solution you will smell ammonia, if hydroxide is removed you won’t.
Pet Stain Problem Urine has ammonia in it. Most cleansers are basic NH3 + H2O ⇌ NH4
++OH-
If I stress this equilibrium by adding a base…
It shifts to the left causing more ammonia to form. Animals sense of smell is better so they fine the same spot and mark it again.
That is why there are special cleansers (acidic) for pet stains
Indicators Indicators are a substance that
change color in the presence of (whatever they check for)
They do this because of Le Châtelier’s principle. All you need an equilibrium reaction with different colored products and reactants.
The pen used to check for counterfeit money is a starch indicator
How an acid base indicator works
A generic indicator will follow this reaction, HId is the reactant indicator, and Id- is its product
HId + H2O H3O+ + Id-
The color differences are important in an acidic solution (high H3O+) you
see reactant
HId + H2O H3O+ + Id-
in a basic solution (low H3O+) you see product
HId + H2O H3O+ + Id-
Acid Base indicators
Acid base indicators change color at certain pH levels
They don’t have to change at 7 (most don’t)
Universal indicator solution (phenolphthalein, bromthymol blue and methyl red dissolved in
ethanol and water) changes color at each integral pH value
Other pH indicators Litmus and
phenolphthalein are indicators
Red cabbage has a pigment that changes colors at different pH values
Buffers Buffers are solutions that don’t
change in pH when acids or bases are added.
They use weak acids/bases and Le Châtelier’s principle.
WA = weak acid HWA + H2O H3O+ + WA-
How? pH is determined by the
concentration of H3O+
Concentration is measured by mol /L
Moles of H3O+ / L (primarily of) H2O
What it does adding H3O+ should increase [H3O+] However, this forces the equation to the
left, decreasing H3O+ and increasing H2O so the [H3O+] remains constant Removing H3O+ (adding a base) should
decrease [H3O+] However, this forces the equation to the
right, increasing H3O+ and decreasing H2O
So again, there is no change to [H3O+] There is a breaking point where the pH
will change.
What does this have to do with my life? Your blood is a buffered solution The pH must remain between 7.35-
7.45 Outside of that range can kill you below this range is called acidosis above is called alkalosis