REACTION RATES AND EQUILIBRIUM

REACTION RATES AND EQUILIBRIUMSpontaneous 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.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 onlyas the result of some causeor stimulus.

Sisyphus (SIS-i-fus)

Sinner condemned in Tartarus to an eternity of rolling a boulder uphill then watching it roll back down again.

Chemistry 21A Dr. Dragan Marinkovic

REACTION RATES AND EQUILIBRIUM

Endergonic means absorbing energy in the form of work. Endergonic process is one wherein the system absorbs energy from the surroundings.

Exergonic means to release energy in the form of work.Exergonic process is one that releases energy from the system, of which it is a part, to the surroundings.

Entropy is 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.



If the energy decreasesand entropy increasesAlways spontaneous process

If spontaneous process is accompanied

By an entropy decreasesThen energy also must decrease.

By the energy increasethen there is alsolarge entropy increase

If spontaneous process accompanied(example – water freezing)

(ice melting)

(wood burning)

STABLE SUBSTANCE does not undergo spontaneous changes under surrounding conditions.


1

2

3


REACTION RATE is the speed of a reaction.

∆C Ct - C0REACTION RATE ---------- = --------------

∆t ∆t

∆C change in concentration Ct concentration at the end of reaction

C0 concentration at the beginning of reaction

∆t time change

A + B → C


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

http://en.wikipedia.org/wiki/File:Molecular-collisions.jpg



REACTION MECHANISMis a detailed explanation of how a reaction actually takes place.

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

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

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

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

Kinetic Energy – The energy of motion.Ek = kinetic energym = massv = velocity (of the particle)



2O → O2

Particles with unpaired electrons react spontaneously and rapidly with each other.

The energy level of products

is much lower than the

energy of reactants.

(FREE RADICALS)


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


INTERNAL ENERGY The energy associated with vibrations within molecule.



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

INTERNAL ENERGY The energy associated with vibrations within molecule.

When INTERNAL ENERGY Is high enough the bond between atoms breaks.



ACTIVATION ENERGYEnergy needed to start some spontaneous process.

Once started, the process continues without further

stimulus or energy from the outside source.



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.


NO + NO3 → 2NO2

importance of molecular orientations during collisions


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.



Low activation energy.

High activation energy.

4P(s) + 5O2(g) → P4O10(s) S(s) + O2(g) → SO2(g)

EXOTHERMIC REACTIONS



ENDOTHERMIC REACTION



REACTION RATE depends on:

1. The nature of reactants2. The concentration of reactants3. The temperature of reactants4. The presence of catalyst

While ionic reactions in solutions are instantaneous, such as e.g.:

Ba2+(aq) + SO4

2-(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.





While ionic reactions in solutions are instantaneous, such as e.g.:

Ba2+(aq) + SO4

2-(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.

Effective collisionA collision that causes a reaction to occur between the colliding molecules.

NO + NO3 → 2NO2






Effective collisionA collision that causes a reaction to occur between the colliding molecules.

NO + NO3 → 2NO2


The reaction rate generally doubles for every 10oC increase in temperature.



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.

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

REACTION RATE depends on:The presence of catalyst



CATALYSTis a substance that changes (usually increases) reaction rates without being used up in the reaction.

INHIBITORis a substance that decreases reaction rates.

HOMOGENEOUS CATALYSTa catalytic substance that is distributed uniformly throughout the reaction mixture.

HETEROGENEOUS CATALYSTa.k.a. SURFACE CATALYSTA catalytic substance normally used in a form of solidwith a large surface area on which reaction takes place.

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



CHEMICAL EQUILIBRIUM

the creation of products is called the forward reaction

the creation of reactants is called the reverse reaction

H2 + I2 → 2 HI "synthesis of hydrogen iodide"

2 HI → H2 + I2 "dissociation of hydrogen iodide"

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.




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.

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




aA + bB → cC + dD




The principle is named after Henry Louis Le Chatelier

Le Chatelier’s principleIf 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.

Henry Louis Le Chatelier, an influential French/Italian chemist and engineer

N2 + 3 H2 2 NH⇌ 3 ΔH = −92kJ mol-1

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.




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

H2 + I2 → 2 HI "synthesis of hydrogen iodide“ →

← 2 HI → H2 + I2 "dissociation of hydrogen iodide"

equilibrium

spontaneous spontaneous




When a catalyst is used activation energy for both forward and reverse reactions is lowered.

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



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REACTION RATES AND EQUILIBRIUM

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