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Reversible Reactions
• Some reactions do not go to completion as we have assumed–They may be reversible – a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously
• Forward: 2SO2(g) + O2(g) → 2SO3(g)
• Reverse: 2SO2(g) + O2(g) ← 2SO3(g)
33
Reversible Reactions
• The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction:
2SO2(g) + O2(g) ↔ 2SO3(g)
A chemical equilibrium occurs, and no net change occurs in the actual amounts of the components of the system.
44Reversible Reactions
• Even though the rates of the forward and reverse are equal, the concentrations of components on both sides may not be equal–An equlibrium position may be shown:
A B or A B 1% 99% 99% 1%Note the emphasis of the arrows direction It depends on which side is favored; almost all reactions
are reversible to some extent
55
Le Chatelier’s Principle
• The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions
• Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress
66
Le Chatelier’s Principle
• What items did he consider to be stress on the equilibrium?
1) Concentration
2) Temperature
3) Pressure
• Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product
Each of these will now be discussed in detail
77
Le Chatelier’s Principle
• Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat• If heat is one of the products (just like a
chemical), it is part of the equilibrium• so cooling an exothermic reaction will
produce more product, and heating it would shift the reaction to the reactant side of the equilibrium: C + O2(g) → CO2(g) + 393.5 kJ
88
Le Chatelier’s Principle
• Pressure – changes in pressure will only effect gaseous equilibria• Increasing the pressure will usually
favor the direction that has fewer molecules
N2(g) + 3H2(g) ↔ 2NH3(g)
• For every two molecules of ammonia made, four molecules of reactant are used up – this equilibrium shifts to the right with an increase in pressure
99
Predicting shifts in equilibrium
• H2(g) + I2(s) ↔ 2HI(g)(colorless) (purple) (colorless)
∆H = + 52 kJ/mol
1. In which direction will the equilibrium shift if more H2 is added?
2.In which direction will the equilibrium shift if HI is removed?
3.In which direction will the equilibrium shift if heat is added to the reaction system?
1010
Predicting equilibrium shifts
• C(s) + H2O(g) ↔ CO(g) + H2(g)
• Which way will the equilibrium shift if pressure is increased?
1111
Equilibrium Constants: Keq• Chemists generally express the position of equilibrium in terms of numerical values, not just percent
–These values relate to the amounts (Molarity) of reactants and products at equilibrium
–This is called the equilibrium constant, and abbreviated Keq
1212Equilibrium Constants
• consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient):
aA + bB cC + dD
–The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power (which is the balancing coefficient).
1313
Equilibrium Constants• consider this reaction:
aA + bB cC + dD
–Thus, the “equilibrium constant expression” has this general form:
[C]c x [D]d
[A]a x [B]b
(brackets: [ ] = molarity concentration)
KKeqeq ==
Note that Keq has no units on the answer; it is only a number because it is a ratio
1414
Equilibrium Constants
• the equilibrium constants provide valuable information, such as whether products or reactants are favored:
if Keq > 1, products favored at equilibrium
if Keq < 1, reactants favored at equilibrium
1515
Expressing and Calculating Keq
• A colorless gas dinitrogen tetroxide (N2O4) and the dark brown gas nitrogen dioxide (NO2) exist in equilibrium with each other.
N2O4 ↔ 2 NO2
1.00 Liter of a gas mixture at equilibrium at 10oC contains 0.0045 mol of N2O4 and 0.030 mol of NO2. Write the expression for the equilibrium constant and calculate the equilibrium constant (Keq) for the reaction.
1616
Answer
• Keq = [NO2]2
[N2O4]
Keq = (0.030)2 = 0.20 (0.0045)
Does this reaction favor the reactants or the products?
1717The Chemistry The Chemistry of Acids and of Acids and BasesBases
The Chemistry The Chemistry of Acids and of Acids and BasesBases• `
2121Acids
Have a sour taste. Vinegar is a solution of acetic acid. CitrusHave a sour taste. Vinegar is a solution of acetic acid. Citrusfruits contain citric acid.fruits contain citric acid.
React with certain metals to produce hydrogen gasReact with certain metals to produce hydrogen gas..
React with carbonates and bicarbonates to produce carbon React with carbonates and bicarbonates to produce carbon dioxide gasdioxide gas
Have a bitter taste.Have a bitter taste.
Feel slippery. Many soaps contain bases.Feel slippery. Many soaps contain bases.
Bases
2222
Some Properties of Acids
Produce H+ (as H3O+) ions in water (the hydronium ion is a
hydrogen ion attached to a water molecule)
Taste sour
Corrode metals
Electrolytes
React with bases to form a salt and water
pH is less than 7
Turns blue litmus paper to red “Blue to Red A-CID”
2323
Anion Ending Acid Name
-ide hydro-(stem)-ic acid
-ate (stem)-ic acid
-ite (stem)-ous acid
Acid Nomenclature Review
No OxygenNo Oxygen
w/Oxygen w/Oxygen
An easy way to remember which goes with which…An easy way to remember which goes with which…
““In the cafeteria, you In the cafeteria, you ATEATE something something ICICky”ky”
2424Acid Nomenclature Flowchart
h yd ro - p re fix-ic en d in g
2 e lem en ts
-a te en d in gb ecom es-ic en d in g
-ite en d in gb ecom es
-o u s en d in g
n o h yd ro - p re fix
3 e lem en ts
AC ID Ss ta rt w ith 'H '
2525
• HBr HBr (aq)(aq)
• HH22COCO33
• HH22SOSO33
hydrohydrobromicbromic acidacid
carboncarbonicic acidacid
sulfursulfurousous acidacid
Acid Nomenclature Review
2727
Some Properties of Bases
Produce OHProduce OH-- ions in water ions in water
Taste bitter, chalkyTaste bitter, chalky
Are electrolytesAre electrolytes
Feel soapy, slipperyFeel soapy, slippery
React with acids to form salts and waterReact with acids to form salts and water
pH greater than 7pH greater than 7
Turns red litmus paper to blue “Turns red litmus paper to blue “BBasic asic BBlue”lue”
2828
Some Common Bases
NaOHNaOH sodium hydroxidesodium hydroxide lyelye
KOHKOH potassium hydroxidepotassium hydroxide liquid soapliquid soap
Ba(OH)Ba(OH)22 barium hydroxidebarium hydroxide stabilizer for plasticsstabilizer for plastics
Mg(OH)Mg(OH)22 magnesium hydroxidemagnesium hydroxide “MOM” Milk of “MOM” Milk of
magnesiamagnesia
Al(OH)Al(OH)33 aluminum hydroxidealuminum hydroxide Maalox (antacid)Maalox (antacid)
2929
Dissociation
• Strong acids and bases completely dissociate in water. Acids dissociate to form H+ ions. Bases dissociate to form OH-1 ions. The more ions in solution, the stronger the acid or base it will be. Weak acids and bases partially dissociate.
3030
Dissociation Reactions
• Write dissociation reactions for the following acids and bases in water.
– HCl (l) → H+(aq) + Cl-1 (aq)
– NaOH (s) → Na+ (aq) + OH- (aq)
– Ca(OH)2 (s)→ Ca+2 (aq) + 2OH-1 (aq)
– H2SO4 (l) → 2H+(aq) + SO4-2(aq)
3131
Acid/Base definitions
• Definition #1: Arrhenius (traditional)
Acids – produce H+ ions (or hydronium ions H3O+)
Bases – produce OH- ions
(problem: some bases don’t have hydroxide ions!)
3232Arrhenius acid is a substance that produces H+ (H3O+) in water
Arrhenius base is a substance that produces OH- in water
3333
Acid/Base Definitions
• Definition #2: Brønsted – Lowry
Acids – proton donor
Bases – proton acceptor
A “proton” is really just a hydrogen atom that has lost it’s electron!
3434
A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor
acidconjugate
basebase conjugate
acid
3535
ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES
The Brønsted definition means NHThe Brønsted definition means NH33 is is aa BASEBASE in water — and water is in water — and water is itself anitself an ACIDACID
BaseAcidAcidBaseNH4
+ + OH-NH3 + H2OBaseAcidAcidBase
NH4+ + OH-NH3 + H2O
3737
Learning Check!
Label the acid, base, conjugate acid, and Label the acid, base, conjugate acid, and conjugate base in each reaction:conjugate base in each reaction:
HCl + OHHCl + OH-- Cl Cl-- + H + H22OO HCl + OHHCl + OH-- Cl Cl-- + H + H22OO
HH22O + HO + H22SOSO44 HSO HSO44-- + H + H33OO
++ HH22O + HO + H22SOSO44 HSO HSO44-- + H + H33OO
++
3838Acids & Base Acids & Base DefinitionsDefinitions
Lewis acid - a Lewis acid - a substance that substance that accepts an electron accepts an electron pairpair
Lewis base - a Lewis base - a substance that substance that donates an electron donates an electron pairpair
Definition #3 – Lewis Definition #3 – Lewis
3939
Formation ofFormation of hydronium ion hydronium ion is also an is also an excellent example.excellent example.
Lewis Acids & BasesLewis Acids & Bases
•Electron pair of the new O-H bond Electron pair of the new O-H bond originates on the Lewis base.originates on the Lewis base.
HH
H
BASE
••••••
O—HO—H
H+
ACID
4040The The pH scalepH scale is a way of is a way of expressing the strength expressing the strength of acids and bases. of acids and bases. Instead of using very Instead of using very small numbers, we just small numbers, we just use the NEGATIVE use the NEGATIVE power of 10 on the power of 10 on the Molarity of the HMolarity of the H++ (or (or OHOH--) ion.) ion.
Under 7 = acidUnder 7 = acid 7 = neutral 7 = neutral
Over 7 = baseOver 7 = base
4242Calculating the pH
pH = - log [H+] or [H3O+](Remember that the [ ] mean Molarity)
Example: If [H+] = 1 X 10-10
pH = - log 1 X 10-10
pH = - (- 10)
pH = 10
Example: If [H+] = 1.8 X 10-5
pH = - log 1.8 X 10-5
pH = - (- 4.74)
pH = 4.74
4343
Try These!Try These!
Find the pH of Find the pH of these:these:
1) A 0.015 M 1) A 0.015 M solution of solution of Hydrochloric acidHydrochloric acid
2) A 3.00 X 102) A 3.00 X 10-7-7 M M solution of Nitric solution of Nitric acidacid
4545pH calculations – Solving for pH calculations – Solving for H+H+pH calculations – Solving for pH calculations – Solving for H+H+
If the pH of Coke is 3.12, [HIf the pH of Coke is 3.12, [H++] = ???] = ???
Because pH = - log [HBecause pH = - log [H++] then] then
- pH = log [H- pH = log [H++]]
Take antilog (10Take antilog (10xx) of both) of both sides and get sides and get
1010-pH -pH == [H[H++]][H[H++] = 10] = 10-3.12-3.12 = 7.6 x 10 = 7.6 x 10-4-4 M M *** to find antilog on your calculator, look for “Shift” or “2*** to find antilog on your calculator, look for “Shift” or “2nd nd
function” and then the log buttonfunction” and then the log button
4646pH calculations – Solving for pH calculations – Solving for H+H+
• A solution has a pH of 8.5. What is the A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the Molarity of hydrogen ions in the solution?solution?
pH = - log [HpH = - log [H++]]
8.5 = - log [H8.5 = - log [H++]]
-8.5 = log [H-8.5 = log [H++]]
Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])
1010-8.5-8.5 = [H = [H++]]
3.16 X 103.16 X 10-9-9 = [H = [H++]]
pH = - log [HpH = - log [H++]]
8.5 = - log [H8.5 = - log [H++]]
-8.5 = log [H-8.5 = log [H++]]
Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])
1010-8.5-8.5 = [H = [H++]]
3.16 X 103.16 X 10-9-9 = [H = [H++]]
4747
More About WaterMore About WaterHH22O can function as both an ACID and a BASE. For O can function as both an ACID and a BASE. For
this reason, water is said to be amphoteric.this reason, water is said to be amphoteric.
In pure water there can beIn pure water there can be AUTOIONIZATIONAUTOIONIZATION
In pure water, the concentration of hydronium In pure water, the concentration of hydronium
and hydroxide have been experimentally and hydroxide have been experimentally
measured as 1.00 x 10measured as 1.00 x 10-7-7 M M
4848
More About WaterMore About Water
In a In a neutral neutral solution [Hsolution [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M
KKw w is the ion product constant of wateris the ion product constant of water
KKww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC
OH-
H3O+
OH-
H3O+
AutoionizationAutoionization
4949
[H+] & [OH-] calculations
• Given a 6.25 x 10-2 M solution of HCl, find [OH-]
Since HCl is a strong acid, it dissociates completely to form H+ and Cl- ions.
» Remember 1 x 10-14 = [H+] [OH-] so,
• 1 x 10-14 = [6.25 x 10-2][OH-]
• [OH-] = 1.60 x 10-13M
5252pOH
• Since acids and bases are Since acids and bases are opposites, pH and pOH are opposites, pH and pOH are opposites!opposites!
• pOH does not really exist, but it is pOH does not really exist, but it is useful for changing bases to pH.useful for changing bases to pH.
• pOH looks at the perspective of a pOH looks at the perspective of a basebase
pOH = - log [OHpOH = - log [OH--]]Since pH and pOH are on opposite Since pH and pOH are on opposite
ends,ends,pH + pOH = 14pH + pOH = 14
5454
[H[H33OO++], [OH], [OH--] and pH] and pHWhat is the pH of the What is the pH of the
0.0010 M NaOH solution? 0.0010 M NaOH solution?
[OH-] = 0.0010 (or 1.0 X 10[OH-] = 0.0010 (or 1.0 X 10-3-3 M) M)
pOH = - log 0.0010pOH = - log 0.0010
pOH = 3pOH = 3
pH = 14 – 3 = 11pH = 14 – 3 = 11
OR KOR Kww = [H = [H33OO++] [OH] [OH--]]
[H[H3OO++] = 1.0 x 10] = 1.0 x 10-11-11 M M
pH = - log (1.0 x 10pH = - log (1.0 x 10-11-11) = 11.00) = 11.00
5555The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?
The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?
On your calculator, key in:
2nd then log then ( - ) 4.82 then press ENTER
On your calculator, key in:
2nd then log then ( - ) 4.82 then press ENTER
On your calculator, key in: ( - ) log then 2.5 EE - 7 then press ENTEROn your calculator, key in: ( - ) log then 2.5 EE - 7 then press ENTER
5757
[OH[OH--]]
[H[H++]] pOHpOH
pHpH
1010 -pOH
-pOH
1010 -pH-pH-Log[H
-Log[H++]]
-Log[OH
Log[OH
--]]
14 -
pOH
14 -
pOH
14 -
pH
14 -
pH
1.0
x 10
1.0
x 10-1
4-14
[OH[O
H-- ]]
1.0
x 10
1.0
x 10-1
4-14
[H[H
++ ]]
5858Calculating [H3O+], pH, [OH-], and pOH
Problem 1: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.
Problem 2: A solution has a pH of 3.67. Calculate IN ORDER: [H3O+], [OH-], and pOH. Is this an acid, base, or neutral?
Problem 3: Problem #2 with pH = 8.05?
6060
HNO3, HCl, H2SO4 HI, HBr, and HClO4 are strong acids. All other acids are WEAK.
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
The strength of an acid (or base) is determined by the amount of IONIZATION (dissociation).
The strength of an acid (or base) is determined by the amount of IONIZATION (dissociation).
6161
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
• Generally divide acids and bases into STRONG or Generally divide acids and bases into STRONG or WEAK ones. Strong acids are strong electrolytes WEAK ones. Strong acids are strong electrolytes meaning they dissociate completely in water.meaning they dissociate completely in water.
STRONG ACID:STRONG ACID: HNOHNO3 3 (aq) + H(aq) + H22O (l) --->O (l) --->
HH33OO+ + (aq) + NO(aq) + NO33- - (aq)(aq)
HNOHNO33 is about 100% dissociated in water. is about 100% dissociated in water.
6262
• Weak acidsWeak acids are much less than 100% ionized in are much less than 100% ionized in
water. Most molecules do NOT dissociate, but water. Most molecules do NOT dissociate, but
stay in tact.stay in tact.
One of the best known is acetic acid = HCOne of the best known is acetic acid = HC22HH33OO22
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
6363
• Strong Base:Strong Base: 100% dissociated in water. Group IA 100% dissociated in water. Group IA and most of IIA hydroxides are strong bases.and most of IIA hydroxides are strong bases.
NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Common strong bases Common strong bases include KOH andinclude KOH and Ca(OH)Ca(OH)22..
CaO (lime) + HCaO (lime) + H22O -->O -->
Ca(OH)Ca(OH)22 (slaked lime) (slaked lime)
CaOCaO
6464
• Weak base:Weak base: less than 100% ionized less than 100% ionized in waterin water
One of the best known weak bases is One of the best known weak bases is ammoniaammonia
NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
Strong and Weak Strong and Weak Acids/BasesAcids/Bases
6666pH testing
• There are several ways to test pHThere are several ways to test pH
–Blue litmus paper (red = acid)Blue litmus paper (red = acid)
–Red litmus paper (blue = basic)Red litmus paper (blue = basic)
–pH paper (multi-colored)pH paper (multi-colored)
–pH meter (7 is neutral, <7 acid, >7 pH meter (7 is neutral, <7 acid, >7 base)base)
–Universal indicator (multi-colored)Universal indicator (multi-colored)
– Indicators like phenolphthaleinIndicators like phenolphthalein
–Natural indicators like red cabbage, Natural indicators like red cabbage, radishesradishes
6767Paper testing
• Paper tests like litmus paper and pH Paper tests like litmus paper and pH paperpaper
– Put a stirring rod into the solution Put a stirring rod into the solution and stir.and stir.
– Take the stirring rod out, and Take the stirring rod out, and place a drop of the solution from place a drop of the solution from the end of the stirring rod onto a the end of the stirring rod onto a piece of the paperpiece of the paper
– Read and record the color change. Read and record the color change. Note what the color indicates. Note what the color indicates.
– You should only use a small You should only use a small portion of the paper. You can use portion of the paper. You can use one piece of paper for several one piece of paper for several tests.tests.
6868pH indicators
• Indicators are dyes that can be added that will change color in the presence of an acid or base.
• Some indicators only work in a specific range of pH
• Once the drops are added, the sample is ruined
• Some dyes are natural, like radish skin or red cabbage
6969
• A neutralization reaction is a reaction between an acid and a base.
• ACID + BASE → SALT + WATER
• SALT – is defined as any ionic compound.
• Ex. HBr + KOH → H2O + KBr
NEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONS
7070
ACID-BASE REACTIONSACID-BASE REACTIONSTitrationsTitrations
ACID-BASE REACTIONSACID-BASE REACTIONSTitrationsTitrations
• An acid-base titration is a procedure which is used to determine the concentration of an acid or base. A measured volume of an acid or base of known concentration is reacted with a sample to the equivalence point.
• Acid-base titration reactions are neutralization reactions.
7171
• Click here to watch titration movie (no sound)
Setup for titrating an acid with a Setup for titrating an acid with a basebase
7272
TitrationTitrationTitrationTitration
1.1. Add solution from the buret.Add solution from the buret.
2. Reagent (base) reacts with 2. Reagent (base) reacts with compound (acid) in solution compound (acid) in solution in the flask. This is called in the flask. This is called NEUTRALIZATION.NEUTRALIZATION.
3.3. Indicator shows when exact Indicator shows when exact stoichiometric reaction has stoichiometric reaction has occurred. (moles of Acid = occurred. (moles of Acid = moles of Base) This is called moles of Base) This is called the EQUIVALENCE POINT.the EQUIVALENCE POINT.
7373
35.62 mL of NaOH is 35.62 mL of NaOH is
neutralized with 25.2 mL of neutralized with 25.2 mL of
0.0998 M HCl by titration to 0.0998 M HCl by titration to
an equivalence point. What an equivalence point. What
is the concentration of the is the concentration of the
NaOH?NaOH?
Titration problem - accurately Titration problem - accurately determine its concentration.determine its concentration.Titration problem - accurately Titration problem - accurately determine its concentration.determine its concentration.
7474
• Step One: Write the balanced equationNaOH + HCl → H2O + NaCl
• Step Two: Determine the moles of substance using the Molarity and volume of that substance.
– Remember M = moles/Liters so moles = M x L– 0.0252 L x 0.0998 M = 0.00251 moles HCl
• Step Three: – Multiply the moles obtained in step 2 by the mole ratio:
» 0.00251 moles HCl x 1 mole NaOH = 0.00251 mol NaOH
• Step Four: Divide moles in Step 3 by volume of NaOH to find the concentration of NaOH
» 0.00251 mol NaOH = 0.0704 M
Titration solution.Titration solution.Titration solution.Titration solution.
1 mole HCl1 mole HCl
0.03562 L0.03562 L
7575
Example 2 - titration
• Determine the volume of 0.100M HNO3 needed to neutralize 50.00 mL of 0.842M Ca(OH)2.