Chemical Reactions

General Reactions

• Combination Rxns: 2 or more substances react to form a single product

• 2 H2 + O2 2 H2O

• Decomposition Rxns: single substance decomposes into 2 or more products

• opposite of combination rxns• 2 H2O 2 H2 + O2

• Single Replacement/Displacement: one element reacts with a compound to form a new compound and release a new element

• 2 Na + 2 H2O 2 NaOH + H2

• Double Replacement/Displacement: an interchange of partners between two compounds

• Also called “Exchange Reactions”• Pb(NO3)2(aq) + KI (aq) PbI2 (s) + 2 KNO3(aq)

• Combustion Rxns: the burning of a compound, usually a hydrocarbon, in oxygen to form carbon dioxide and water

Redox Reaction

• A reaction in which there is a transfer of electrons from one reactant to another

+

Rules for Assigning Oxidation States1) Free elements have an oxidation state = 0

• Na = 0 and Cl2 = 0 in 2 Na(s) + Cl2(g)2) Monatomic ions have an oxidation state equal to their

charge• Na = +1 and Cl = -1 in NaCl

3) Fluorine is always –1 in compounds with other elements

4) Cl, Br, and I are always –1 in compounds except when combined with O or F.• Cl = -1 in NaCl• Cl = +1 in ClO-

Rules for Assigning Oxidation States

5) The oxidation # of H is +1 in most compounds except when forming a binary compound with a metal• H = +1 in HCl• H = -1 in NaH

6) The oxidation # of O is –2 in most compounds except when forming a peroxide• O = -2 in H2O• O = -1 in H2O2

7) The sum of the oxidation states of all the atoms in a compound is 0• Na = +1 and Cl = -1 in NaCl, (+1) + (-1) = 0

8) The sum of the oxidation states of all the atoms in a polyatomic ion equals the charge on the ion• N = +5 and O = -2 in NO3

–, (+5) + 3(-2) = -1

Practice – Assign an Oxidation State to Each Element in the following

• Br2

• MgBr2

• KBr• LiF• CO2

• CO• SO4

2-

• Na2O2

Oxidation and ReductionOIL RIG

• oxidation occurs when an atom’s oxidation state increases during a reaction

• reduction occurs when an atom’s oxidation state decreases during a reaction

CH4 + 2 O2 → CO2 + 2 H2O-4 +1 0 +4 –2 +1 -2

oxidationreduction

Oxidation–Reduction• oxidation and reduction must occur simultaneously – if an atom loses electrons another atom must take them

• the reactant that reduces an element in another reactant is called the reducing agent– the reducing agent contains the element that is oxidized

• the reactant that oxidizes an element in another reactant is called the oxidizing agent– the oxidizing agent contains the element that is reduced

2 Na(s) + Cl2(g) → 2 NaCl(s)

Identify the Oxidizing and Reducing Agents in Each of the Following

1) Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(s)

2) MnO2 + 4 HBr MnBr2 + Br2 + 2 H2O

3) 3 H2S + 2 NO3– + 2 H+ 3 S + 2 NO + 4 H2O

4) MnO4-(aq) + Fe2+(aq) Mn2+ + Fe3+(aq)

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Common Oxidizing Agents

Oxidizing Agent Product When Reduced

O2 O2-

O3 O2

F2, Cl2, Br2, I2 F-, Cl-, Br-, I-

ClO3-, BrO3

-, IO3- Cl-, Br-, I-

ClO- Cl-

HNO3 NO, NO2

Hexavalent ChromiumCrO3

CrO42-

Cr2O72-

Cr3+, H2O

MnO4-

MnO42-

Mn2+(acidic), MnO2(basic)

H2O2 H2O

H2SO4 SO2 or S or H2S

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Common Reducing AgentsReducing Agent Product When Oxidized

H2 H+

H2O2 O2

I- I2

NH3, N2H4 N2

S2-, H2S S

SO32- SO4

2-

NO2- NO3

-

C (as coke or charcoal) CO or CO2

Fe2+ Fe3+

Cr2+ Cr3+

Sn2+ Sn4+

Metals Metal Ions

Collision Theory

For a reaction to occur, there must be:1) Collision2) Orientation3) Energy

Reaction RatesRate: change in the quantity of something over a given period of time

Rate of rxn =

Rate of rxn =

time

distance Speed

time

[reactant]

time

[product] Rate

time

ionconcentrat Rate

time

[reactant]

time

[product] Rate

time

ionconcentrat Rate

Questions

1) What is your speed, in mph, if you travel 12 km in 10 minutes?

2) Which reaction, in the above diagram, is faster?

at t = 0[A] = 8[B] = 8[C] = 0

at t = 16[A] = 4[B] = 4[C] = 4

at t = 0[X] = 8[Y] = 8[Z] = 0

at t = 16[X] = 7[Y] = 7[Z] = 1

3) Calculate the rates of rxn from t=16 to t=32 and from t=32 to t=48

4) Using the rates of rxn from problems 2 and 3, what can you conclude about the rate of a rxn as the rxn proceeds? Why does this happen?

at t = 16[A] = 4[B] = 4[C] = 4

at t = 32[A] = 2[B] = 2[C] = 6

at t = 48[A] = 0[B] = 0[C] = 8

Factors Influencing the Speed of Reactions

• Nature of reactants• Temperature• Concentration• Catalysts

1) Nature of Reactants: what kind of reactant molecules and what physical condition they are in • small molecules tend to react faster

than large molecules• gases tend to react faster than liquids

which react faster than solids• powdered solids are more reactive than “blocks”

• ions tend react faster than molecules

2) Temperature• Increase temp = Increase rxn rate• Decrease temp = Decrease rxn rate

3) Reactant Concentration• Increase [reactants] = Increase rxn rate• Decrease [reactants] = Decrease rxn rate

4) Catalysts: substances that affect the speed of a reaction without being consumed themselves• Provide an alternative pathway that has a

lower activation energy (Ea)

Ozone Depletion over the Antarctic

mechanism without catalyst

O3(g) + O(g) 2 O2(g) V. Slow

mechanism with catalyst

Cl(g) + O3(g) O2(g) + ClO(g) Fast

ClO(g) + O(g) O2(g) + Cl(g) Slow

Energy Profile of Catalyzed Reaction

Polar stratospheric clouds contain ice crystals that catalyze reactions that release Cl from atmospheric chemicals

Enzymatic Hydrolysis of Sucrose

Chemical Equilibrium

• H2 (g) + I2(g) 2 HI (g)• H2 (g) + I2(g) 2 HI (g)• H2 (g) + I2(g) 2 HI (g)

• Equilibrium: a condition in which the rate of the forward reaction is equal to the rate of the reverse reaction– Concentrations of reactants and products are constant, but

not necessarily equal

The Equilibrium Constant Expression

• K = equilibrium constant• For aA + bB cC + dD @ equilib.

Questions

• Write the equilibrium constant expression for the following reactions:

1) H2 (g) + I2(g) 2 HI (g)

2) CH3OH (g) CO(g) + 2 H2(g)

3) N2(g) + H2(g) NH3(g)

The Meaning of K• When K >> 1– Rxn is product favored– [P]’s higher than [R]’s @ equilibrium

K = 1.9 x 1019 @ 25 °C

• When K << 1– Rxn is reactant favored– [R]’s is higher than [P]’s @ equilibrium

K = 4.1 x 10-31 @ 25 °C

• When K ≈ 1– Neither direction is favored– Rxn proceeds about ½ way– Calculations must be done to figure out whether

the [R]’s or the [P]’s is higher

Questions1) Is the following reaction reactant favored or product favored?

• Cd(NH3)42+(aq) Cd2+(aq) + 4 NH3(aq)

• K = 1.0 x 10-7

2) A rxn mixture that originally contains 11 mol A and 0 mol B in a 1.0 L container is allowed to reach equilibrium. What are the [A] and [B] @ equilibrium?• A(g) B(g) K = 10

3) Phosgene, COCl2, is a toxic substance that is produced by the reaction of carbon monoxide and chlorine. The Kc for the reaction is 5.0. If the equilibrium concentrations for the reactio are [Cl2] = 0.25 M and [COCl2] = 0.80 M, what is the equilibrium concentration of CO(g)?• CO(g) + Cl2(g) COCl2(g)

Le Châtelier’s Principle

When Country A’s citizens feel overcrowded, some will emigrate to Country B.

However, as time passes, emigration will occur inboth directions at the same rate, leading topopulations in Country A and Country B that areconstant, though not necessarily equal

Equilibrium is disturbed when the population of Country B grows.

The result will be people moving from Country B into Country A faster than people moving from Country A into Country B. This will continue until a new equilibrium between the populations is established, however the new populations will have different numbers of people than the old ones.

Le Châtelier’s Principle

• Le Châtelier's Principle: if a system at equilibrium is disturbed, the position of equilibrium will shift to minimize the disturbance– Concentration– Pressure/Volume– Temperature– Concentrations of all the chemicals will change until

equilibrium is re-established– The new concentrations will be different, but the

equilibrium constant (K) will be the same

The Effect of Concentration Changes on Equilibrium

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The Effect of Pressure/Volume Changes

The Effect of Temperature Change on Equilibrium

• If the temp of a system at equilibrium is changed, the system will shift in a direction to counter that change

• A new K is established at the new temp

Catalysts

• Catalysts provide an alternative, more efficient mechanism

• Speed up rxn by lowering the Ea

• Result in the same ratios of products and reactants at equilibrium

• Do not affect the position of equilibrium• K stays the same

Questions• The reaction 2 SO2(g) + O2(g) Û 2 SO3(g) is exothermic.

How will each of the following changes affect the equilibrium concentrations of each gas once equilibrium is re-established?

1) Adding more O2 to the container2) Removing SO3

3) Compressing the gases4) Cooling the container5) Doubling the volume of the container6) Warming the mixture7) Adding SO3 8) Adding a catalyst to the mixture