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REACTION RATES AND EQUILIBRIUM. Spontaneous process is the time-evolution of a system in which it releases free energy (most often as heat) and moves to a lower, more thermodynamically stable, energy state.

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REACTION RATES AND EQUILIBRIUM

Spontaneous processis the time-evolution of a system in which it releases free energy (most often as heat) and moves to a lower, more thermodynamically stable, energy state.

A spontaneous process is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy.

Nonspontaneous process takes place only

as the result of some cause

or stimulus.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

Endergonicmeans absorbing energy in the form of work.

Endergonic processis one wherein the system absorbs energy from the surroundings.

Exergonic means to release energy in the form of work.

Exergonic processis one that releases energy

from the system, of which it is a part, to the surroundings.

Entropyis a measure of the disorder or

mixed-up character (randomnes) of a system.

Entropy, symbolized by S, is a measure of

the unavailability of a system’s energy to do work.

It is a measure of the disorder of molecules in a system

Ice melting is a classic example

of entropy increasing

Melting of ice absorbs heat.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

(wood burning)

If the energy decreases

and entropy increases

Always spontaneous process

1

If spontaneous process is accompanied

By the energy increase

then there is also

large entropy increase

2

(ice melting)

If spontaneous process accompanied

By an entropy decreases

Then energy also must decrease.

(example – water freezing)

3

STABLE SUBSTANCE

does not undergo spontaneous changes

under surrounding conditions.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

REACTION RATE

is the speed of a reaction.

A + B → C

∆C Ct - C0

REACTION RATE ---------- = --------------

∆t∆t

∆C change in concentration

Ctconcentration at the end of reaction

C0 concentration at the beginning of reaction

∆ttime change

Reaction rate tends to increase with concentration - a phenomenon explained by collision theory

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

REACTION MECHANISM

is a detailed explanation of how

a reaction actually takes place.

  • Reaction particles must collide with one another in order for reaction to occur. (exception: e.g. decomposition)

  • Particles must collide with at least a certain amount of energy if the collision is to result in a reaction.

  • In some cases, colliding reactants must be oriented in a specific way if a reaction is to occur.

Kinetic Energy–

The energy of motion.

Reactions involving solids usually take place only on the solid surface.

Ek = kinetic energy

m = mass

v = velocity (of the particle)

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

Particles withunpaired electrons

react spontaneously and rapidly with each other.

(FREE RADICALS)

2O → O2

The energy level of products

is much lower than the

energy of reactants.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

INTERNAL ENERGY

The energy associated with

vibrations within molecule.

Energy =

Stretching Energy + Bending Energy + Torsion Energy + Non-Bonded Interaction Energy

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

INTERNAL ENERGY

The energy associated with

vibrations within molecule.

When INTERNAL ENERGY

Is high enough

the bond between atoms breaks.

A chemical reaction is the breaking of bonds and/or the formation of new bonds between atoms.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

ACTIVATION ENERGY

Energy needed to start some

spontaneousprocess.

Once started, the process

continues without further

stimulus or energy from the

outside source.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

H2 + Cl2 -> 2HCl

When two molecules colide (like hydrogen and chlorine molecules) they can

either bounce back unchanged, or, if there is enough energy, the “original” bonds will break and new bonds (new molecules) will be formed.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

NO + NO3 → 2NO2

If one or both reacting molecules are unsymmetrical, orientation effects play extremely important role.

Molecules have to be properly oriented

in order for reaction to take place.

importance of molecular orientations

during collisions

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

High activation energy.

Low activation energy.

4P(s) + 5O2(g) → P4O10(s)

S(s) + O2(g) → SO2(g)

EXOTHERMIC REACTIONS

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

ENDOTHERMIC REACTION

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

While ionic reactions in solutions

are instantaneous, such as e.g.:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

Reactions involving covalent bonds

take time – for bonds of reactants to

be broken and for the new bonds in

products to be formed.

  • REACTION RATE

  • depends on:

  • The nature of reactants

  • The concentration of reactants

  • The temperature of reactants

  • The presence of catalyst

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

While ionic reactions in solutions

are instantaneous, such as e.g.:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

Reactions involving covalent bonds

take time – for bonds of reactants to

be broken and for the new bonds in

products to be formed.

NO + NO3 → 2NO2

  • REACTION RATE

  • depends on:

  • The nature of reactants

  • The concentration of reactants

  • The temperature of reactants

  • The presence of catalyst

importance of molecular orientations

during collisions

Effective collision

A collision that causes a reaction to

occur between the colliding molecules.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

NO + NO3 → 2NO2

  • REACTION RATE

  • depends on:

  • The nature of reactants

  • The concentration of reactants

  • The temperature of reactants

  • The presence of catalyst

The reaction rate

generally doubles

for every 10oC

increase in temperature.

importance of molecular orientations

during collisions

Effective collision

A collision that causes a reaction to

occur between the colliding molecules.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

REACTION RATE

depends on:

The presence of catalyst

Solid heterogeneous catalysts such as in automobile catalytic converters are plated on structures designed to maximize their surface area

Generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction X + Y to give Z. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

HOMOGENEOUS CATALYST

a catalytic substance that is distributed

uniformly throughout the reaction mixture.

HETEROGENEOUS CATALYST

a.k.a. SURFACE CATALYST

A catalytic substance normally used in a form of solid

with a large surface area on which reaction takes place.

CATALYST

is a substance that

changes (usually

increases) reaction

rates without being

used up in the reaction.

INHIBITOR

is a substance

that decreases

reaction rates.

A catalyst works by providing an alternative reaction pathway to the reaction product. The rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst.

  • Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst, X and Y are reactants, and Z is the product of the reaction of X and Y:

    • X + C → XC (1)

    • Y + XC → XYC (2)

    • XYC → CZ (3)

    • CZ → C + Z (4)

  • Although the catalyst is consumed by reaction 1, it is subsequently produced by reaction 4,

  • so for the overall reaction:

    • X + Y → Z

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

the creation of products is called the forward reaction

the creation of reactants is called the reverse reaction

The equilibrium state is independent of the direction from which it is approached. Whether we start with an equimolar mixture of H2 and I2 (left) or a pure sample of hydrogen iodide (shown on the right, using twice the initial concentration of HI to keep the number of atoms the same), the composition after equilibrium is attained (shaded regions on the right) will be the same.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction.

A reversible reaction is one where the reactants and products exist in a state of equilibrium.

Although we learned in stoichiometry that the amount of product can be predicted by the reactants,

a reaction that goes to completion is very rare.  Most reactions exist in a state of equilibrium.

in equilibrium the rates are equal (forward = reverse)

NO2

N2O4

2 NO2 D N2O4 + ↓Pressure  + ↑ temperature

(nitrogen dioxide becomes dinitrogen tetroxide)

When the reaction is in equilibrium, a ratio is established between the products and the reactants.  According to Le Chatlier we can manipulate the reactions by manipulating the conditions.

D

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

aA + bB  → cC + dD

D

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

  • The principle is named after Henry Louis Le Chatelier

  • Le Chatelier’s principle

    • If a chemical system at equilibrium experiences a change in

    • concentration, temperature, volume, or total pressure,

    • then the equilibrium shifts to counter-act the imposed change.

N2 + 3 H2 ⇌ 2 NH3    ΔH = −92kJ mol-1

Henry Louis Le Chatelier,

an influential French/Italian

chemist and engineer

This is an exothermic reaction when producing ammonia. If we were to lower the temperature, the equilibrium would shift in such a way as to produce heat.

An increase in pressure due to decreasing volume causes the reaction to shift to the side with the fewer moles of gas.

As the concentration of N2 is increased, the frequency of collisions

(that are successful) of that reactant would increase also, allowing for

an increase in forward reaction, and generation of the product.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

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

D

spontaneous

spontaneous

equilibrium

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

CHEMICAL EQUILIBRIUM

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

D

When a catalyst is used activation energy for

both forward and reverse reactions is lowered.

Chemistry 21A Dr. Dragan Marinkovic


REACTION RATES AND EQUILIBRIUM

Chemistry 21A Dr. Dragan Marinkovic


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