CHEMICAL EQUILIBRIUM afety_mode=1&safe=active.

CHEMICAL EQUILIBRIUM

http://www.youtube.com/watch?v=f9aTOSpkUng&safety_mode=true&persist_safety_mode=1&safe=active

Rates of ReactionAs you know, some reactions happen very quickly (for

example: __________________________), and others happen very slowly (ex: ____________________________).

How quickly a reaction happens is called the rate of reaction, and is usually expressed as the amount of reactant changing per unit time.

Rates of ReactionAccording to collision theory, atoms, ions, and

molecules can react to form products when they collide with one another, provided the colliding particles have enough kinetic energy.

If you think about two balls of soft modeling clay, you can throw them together gently and they won’t stick – but if you throw them with enough force (energy), they will stick together (react).

Rates of ReactionThe energy required for a reaction to happen is called

the activation energy. In a sense, it is like a barrier the reactant particles

have to get past to create products.

Rates of ReactionActivation energy is often represented in a graph like

this:

Rates of ReactionEvery chemical reaction proceeds at its own rate.

Some reactions are naturally fast and some are naturally slow under the same conditions. However, by varying the conditions of the reaction, the rate of almost any reaction can be modified.

The rate of a chemical reaction depends upon:1. Temperature2. Concentration3. Particle Size4. Use of a Catalyst

Rates of Reaction – effect of temperature Usually, raising the temperature speeds up reactions,

while lowering the temperature slows down reactions.

At higher temperatures, the motion of the reactant particles are faster and more chaotic – increasing the temperature increases both the frequency of collisions and the number of collisions that have enough energy to react.

Rates of Reaction – effect of particle size Surface area plays an important role in determining

the rate of reaction.The smaller the particle size, the larger the surface

area is for a given mass of substance.An increased surface area increases the amount of the

reactant exposed for reaction, which further increases the collision frequency and the reaction rate.

One way to increase the surface area of solid reactants is to dissolve them – in a solution particles are separated and more accessible to other reactants.

Rates of Reaction – effect of concentration The number of particles in a given volume affects the

rate at which reactions occur.Cramming more particles into a fixed volume

increases the concentration of reactants and consequently the frequency of collision. Increased collision frequency leads to a higher reaction rate.

Rates of Reaction – effect of a catalyst Recall that a catalyst is a substance that increases the rate of

reaction without being used up during the reaction.Catalysts lower the activation energy barrier for a reaction.This means more reactants have enough energy to form

products in a given amount of time.

Chemical Equilibrium

http://phet.colorado.edu/simulations/sims.php?sim=Salts_and_Solubility

Chemical EquilibriumIn each of theses cases, an equilibrium was reached Equilibrium is reached once the numbers of

molecules/particles/moles are NO LONGER CHANGING

And, that means the rate of the forward reaction is equal to the rate of the reverse reaction.

NOTE: Chemical equilibrium does NOT necessarily mean that there are equal numbers of molecules/moles of reactants and products.

Reversible ReactionsMost chemical reactions can occur in both directionsChemical reactions that are reversible can operate in

both directions, forming both product and reactant at the same time

Reversible ReactionsSmog is given its characteristic brown color by nitrogen

dioxide. Nitrogen dioxide reacts to form dinitrogen tetroxide:

2NO2 (g) N2O4 (g)The reverse reaction also occurs:

N2O4 (g) 2NO2 (g)To show that both reactions occur at the same time, use

two half arrows or a double arrow ()2NO2 (g) N2O4 (g)

Reversible ReactionsChemical equilibrium does NOT necessarily mean

that there are equal numbers of molecules/moles of reactants and products…

…rather, equilibrium is reached once the numbers of molecules/particles/moles, in the reversible reaction, are NO LONGER CHANGING

And, it means that the rate of the forward reaction is equal to the rate of the reverse reaction.

Reversible Reactions—ConcentrationConsider the reversible

reaction: A BFirst, the reaction

contains only AAs time goes on, and A is

converted to BThe [A] decreases as the

[B] increases.Equilibrium has been

reached when the [ ] no longer changes.

Reversible Reactions—Reaction Rate

Reversible reaction: A BFirst, the forward

rate of reaction, A B is high (fast)

The reverse rate of reaction B A is low (slow)

As the [A] drops, the forward rate

A B slows down

Reversible Reactions—ReactionAnd the reverse rate of B

A increases Soon the rates of both

the forward and reverse reactions become equal

Equilibrium has been reached once the rate stops changing

Reversible ReactionsChemical Equilibrium: The forward and reverse

reactions are taking place at the same rate in a reaction system.

The concentrations of products and reactants are no longer changing.

Le Chatelier’s PrincipleIf a chemical system at equilibrium experiences a

change in conditions, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

change in conditions for a system at equilibrium is often called a stress

stresses include:1. change in concentration of a reactant or product2. change in pressure of the entire system3. change in temperature of the entire system

Changes in Concentration2NO2 (g) N2O4 (g)

adding a substance drives the system to consume that substance

removing a substance drives the system to produce more of the substance

Changes in PressureEquilibrium will shift to relieve the stress so that…

if pressure is increased, the system will shift in the direction that produces fewer gas molecules

if pressure is decreased, the system will shift in the direction that produces more gas molecules

Changes in Pressure

2NO2 (g) N2O4 (g)

if the volume is decreased, which way will the equilibrium shift?

if the volume is increased, which way will the equilibrium shift?

Changes in Pressure

NH4NO3 (s) N2O (g) + 2H2O (g)

moles of gas on reactant side =moles of gas on product side =

If the volume is decreased, which way will the equilibrium shift?

If the pressure is decreased, which way will the equilibrium shift?

Changes in TemperatureIf heat is produced (exothermic reaction), adding

heat by raising the temperature causes the system to shift to the left

If heat is consumed (endothermic reaction), adding heat by raising the temperature causes the system to shift to the right

Let’s Reviewhttps://www.youtube.com/watch?v=g5wNg_dKsYY(crash course video for equilibrium – 11 minutes)

Changes in Temperature

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

This means that the reaction is less complete (smaller amounts of product) at higher temperature.

Why?

Concentration—”the 30 sec. lecture!”

The symbol for “concentration of” is two square brackets: [ ]

The principle unit of concentration is MM = molarityM = mol/L (mol of solute in a L of solvent.An 8.5 M solution is said “eight point five

molar” solution

The Law of Chemical EquilibriumaA + bB cC + dDa, b, c, and d represent coefficientsA, B, C, and D are products and reactants

Keq = ba

dc

BA

DC

The Law of Chemical Equilibrium

Law of Chemical Equilibrium: In a reaction mixture at equilibrium, there is a condition, given by the equilibrium constant, Keq, that relates the concentrations of the reactants and products.

2NO2(g) N2O4(g)

A: Keq = [NO2]

[N2O4]

B: Keq = [N2O4]

[NO2]

C: Keq = [N2O4]

[NO2]2

D: Keq = [NO2]2

[N2O4]

Choose the correct equilibrium expression for each reaction

2CO(g) + O2(g) 2CO2(g)

A: Keq = [CO2]2

[CO]2 [O2]

B: Keq = [CO]2 [O2]

[CO2]2

C: Keq = [CO]2 [O2]

[CO2]2

D: Keq = [CO2]2

[CO] [O2]


2SO2(g) + O2(g) 2SO3(g)

A: Keq = [SO2] [O2]

[SO3]2

B: Keq = [SO3]2

[SO2] [O2]

C: Keq = [SO2]2 [O2]

[SO3]2

D: Keq = [SO3]2

[SO2]2 [O2]


CO(g) + 3H2(g) CH4(g) + H2O(g)

A: Keq = [CH4][H2O]

[CO][H2]

B: Keq = [CH4][H2O]

[CO][H2]3

C: Keq = [CO][H2]3

[CH4][H2O]

D: Keq = [CO][H2]

[CH4][H2O]


H2O(g) + CO(g) H2(g) + CO2(g)

A: Keq = [H2O][CO]

[H2][CO2]

B: Keq = [H2][CO2]

[H2O][CO]

C: Keq = [H2O][CO]

[H]2[CO2]

D: Keq = [H]2[CO2]

[H2O][CO]


Homogeneous and Heterogeneous Equilibria

Homogeneous Equilibria– an equilibrium system in which all products and reactants are in the same state

Heterogeneous Equilibria– an equilibrium system in which more than one state of matter is present

Example…gas and solid in the same equation

How can you measure the concentration of a pure solid or pure liquid??

Homogeneous and Heterogeneous Equilibria

The only way you can measure it is by using the density…

…example: Lead (Pb) has a density of…13.5 g/cm3

The density of a pure liquid or solid is essentially constant, so they are NOT in the equilibrium expression

(it is actually included in the value of Keq)

C(s) + H2O(g) CO(g) + H2(g)

A: Keq = [C] [H2O]

[CO] [H2]

B: Keq = [CO] [H2]

[H2O]

C: Keq = [H2O]

[CO2] [H2]

D: Keq = [CO] [H2]

[C] [H2O]


NH4NO3(s) N2O(g) + 2H2O(g)

A: Keq = [N2O][H2O]

[NH4NO3]

B: Keq = [NH4NO3]

[N2O][H2O]

C: Keq = [N2O][H2O]2

1D: Keq = ____1_____

[N2O][H2O]


ZnCO3(s) ZnO(s) + CO2(g)

A: Keq = [CO2]

1B: Keq = ___1__

[CO2]

C: Keq = [ZnO] [CO2]

1D: Keq = ____1_____

[ZnO] [CO2]


SnO2(s) + 2CO(g) Zn(s) + 2CO2(g)

A: Keq = [CO2]2

[CO]2

B: Keq = [CO]

[CO2]2

C: Keq = [Zn][CO2]2

[CO]D: Keq = [CO]

[Zn][CO2]2


C(s) + CO2(g) 2CO(g)

A: Keq = [CO]2

[C][CO2]

B: Keq = [CO]2

[CO2]

C: Keq = [CO2]

[CO]D: Keq = [C][CO2]

[CO]


Calculating Keq

Expt Initial [NO2] (M)

Initial [N2O4]

(M)

Equilibrium [NO2] (M)

Equilibrium [N2O4] (M)

Keq

1 0.0200 0.0 0.0172 0.00140

2 0.0300 0.0 0.0243 0.00280

3 0.0 0.0200 0.0310 0.00452

2NO2(g) N2O4(g)

The Reaction QuotientThe reaction quotient, Q, is calculated like

Keq is calculated, but instead of inserting equilibrium concentrations, we insert measured concentrations

If Q = Keq, then the system is at equilibrium

If Q < Keq, then the system is not at equilibrium

If Q > Keq, then the system is not at equilibrium

Solubility EquilibriaDissolving Process

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/thermochem/solutionSalt.html

Precipitation Processhttp://phet.colorado.edu/simulations/

sims.php?sim=Salts_and_Solubility

Solubility EquilibriaWhen the process of dissolution and

precipitation are occurring at the same time and at the same rate, equilibrium is established.

The equilibrium can be shown by an equation with two arrows or a double arrow:

CaCl2 (s) Ca2+ (aq) + 2Cl- (aq)Note that the equation must be balanced just

like other equations so equal numbers of ions (and charges) appear on both sides of the equation.

Solubility EquilibriaWrite the solubility equilibrium equation

for each of the following ionic compounds:

K2S(s) 2K+(aq) + S2-(aq)

NH4NO3(s) NH4+(aq) + NO3

-(aq)

Solubility ProductZnS(s) Zn2+(aq) + S2-(aq)Ksp =

Cu3(PO4)2(s) 3Cu2+ (aq) + 2PO43- (aq)

Ksp =

SrSO4(s) Sr2+(aq) + SO4

2- (aq)

Ksp =

Solubility ProductDetermining the value of Ksp must be done

at equilibrium

AgCl(s) Ag+ (aq) + Cl- (aq)1.3 x 10-5 mol/L of silver ion1.3 x 10-5 mol/L of chloride ion

Ksp =

As with Keq, there are no units for Ksp.a small value of Ksp indicates low solubilitya large value of Ksp indicates high solubility

Solubility Product

Calcium sulfate has a Ksp value of 9.1 x 10-6 at 25ºC. Determine the equilibrium concentrations of the calcium and sulfate ions in a saturated solution at this temperature

Using Ksp and Q to Predict the Formation of a Precipitate

lead (II) nitrate + potassium iodide lead (II) iodide + potassium nitrate

The reactants are both clear solutions; when mixed, a bright yellow solid forms in a clear solution

Which product is which?


unsaturated solution – a solution in which additional solute can be dissolved

saturated solution – a solution in which the maximum amount of solute is dissolved for the existing conditions

supersaturated solution – a solution in which more than the normal amount of solute is dissolved; this is an unstable, non-equilibrium state


Ksp (solubility product) describes the solute ion concentrations in solution for a saturated, equilibrium state

Q (reaction quotient) describes the solute ion concentrations actually in solution


Determine if a precipitate will form when 0.010 mol of PbCl2 is dissolved in 150 mL of hot water and then cooled slowly to 25ºC where the Ksp value is 1.6 x 10-5

What must the reactants be in order for a double replacement reaction to occur?A: IonsB: SolubleC: SolidD: Insoluble

Double replacement reactions review

What must the products be in order for a reaction to happen?A: GasesB: AqueousC: Covalent CompoundsD: SolidsE: all of the aboveF: A, C & D only


Will a double replacement reaction occur between solutions of copper (II) nitrate and sodium hydroxide?A: YesB: No


Precipitation ReactionsIf the final solution is too dilute, no

precipitate can form and the double replacement reaction doesn’t occur

Only if the ion product (Q) exceeds the solubility product (Ksp) will a precipitate form

Precipitation Reactions Will AgBr(s) precipitate if 20.0 mL of 0.010

M AgNO3 and 20.0 mL of 3.0 x 10-4 M KBr are mixed. (Ksp for AgBr = 5.0 x 10-13)

AgNO3 (aq) + KBr(aq) AgBr(s) + KNO3(aq)

Precipitation ReactionsComplete ionic reaction

Ag+ + NO3- + K+ + Br- AgBr(s) + K+ +

NO3-

Net Ionic equation

Ag+ + Br- AgBr(s)

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