kinetics thermodynamics

8/14/2019 Kinetics Thermodynamics

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Chemical Kinetics and Thermodynamics

Chemical Kinetics- concerned with:1. Rates of Chemical Reactions-

# of moles of reactant used up or product formedUnit time

Or

2. Reaction Mechanisms-

Rate of Reaction and the Collision TheoryReactions take place at different rates

Collision Theory

Used to explain why reactions take place at different rates

Effective Collision

Factors that Affect the Rate of a Chemical Reaction1. What bonds are being broken and formed?Fast = slight rearrangement of electrons

AgNO3 + NaCl AgCl ppct. + NaNO3 ---FAST!

In reactions in which ionic bonds are broken and formed occur quickly at roomtemperature

Slow= many covalent bonds broken2H2O2 2H2O + O2 ---Slow!Reactions that include covalent bonds being broken and formed occur very slowly atroom temperature

2.

An increase in concentration of any one of the reactants usually, but not alwaysincreases the rate of the reaction if the reaction is homogeneous

i. Homogeneous reactions- all the reactants are in the same phase.ii. Heterogeneous reactions- reactants are in more than one phase.

I.e.- rusting How can we increase the concentration of reactants in the following:

i. Gases-ii. Liquids-


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Collision Theory-

Heterogeneous systems- solid and liquid- increaseSurface Area

Grinding, pulverizing, smashing, stirring, pureeing, blending, etc, etc, etc

Reactions that incorporate two gasesPartial pressure increase in one of the gases results in an increase in reaction rate:

As a result of decreased volume or Increase in the number of molecules of gas

3. Temperature

For many reactions- 10oC increase in temperature = 2X rateCollision Theory-Temperature increase =

Energy Distribution Curve

4.

Catalyst-

Pt metal Enzymes

2H2O2 2H2O + O2 ---Slow!2H2O2 2H2O + O2 ---Fast of MnO2 present as catalyst

5.

Only for systems in which the reactants are in more than one phase

Solid and a liquid

Reaction Mechanisms

2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l)What is the likelihood that 2 acetylene molecules will come into contact with 5 oxygenmolecules with the correct orientation and energy?

Most reactions proceed through a series of simple steps Each step- collision of two particles


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Reaction Mechanism-

Consider:2A + B A2B

It is unlikely that this reaction occurs in one step. One proposed mechanism may be-

OR

Often, chemists dont know the mechanism. They may only know the end result.

Why might it be difficult?

It is difficult to determine the reaction mechanisms because the intermediateproducts have short lives

Intermediate products have structures unlike the structure of either the products orthe reactants

Activated Complex:

Reaction Mechanisms and Rates of Reactions

Rate determining step-We stated earlier:An increase in concentration not always increases the rate of a homogeneous reactionWHY?-

Potential Energy Diagrams A pictorial way to describe the energy involved in a reaction

Activation Energy and Temperature, Concentration, and Catalysts

Activated Complex:

Short lived, unstable particle that will temporarily exist when reacting molecules collideat the proper angle with the proper amount of energy

Rate Laws A rate law is an equation that can be used to calculate the reaction rate for any given

concentration of reactants.


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The rate law can be determined by keeping the concentrations of all but one reactantconstant while measuring the reaction rate for various concentrations of that reactant.The process is repeated for each reactant.

[A] and [B] represent the molar concentrations of reactants A and B in moles / liter (M) exponents x and y are the powers of the concentrations of the reactants (determined

experimentally)

k = proportionality constant, rate constant, has a fixed value for a reaction at aparticular temperature (determined experimentally)

The actual form of the rate law varies from reaction to reaction

NO2(g) + O3(g) NO3(g) + O2(g)

Rate = k[NO2][O3] In this reaction, the rate is directly proportional to the concentration of both NO2 and

O3 Suppose the initial concentration of each reactant is 1.00M. If the concentration of

either reactant is doubled to 2.00M, the rate increases by a factor of 2.

Also, if the concentration of either reactant is multiplied by 4 to 4.00M, the rate isincreased by a factor of 4.

2 NO(g) + O2(g) 2NO2(g)

Rate = k[NO]2[O2] Notice the squaring of the NO concentration. If the concentration of NO is multiplied

by 6, the rate increases by a factor of 36 (6)2. If the concentration of O2 increases by a factor of 6, the rate only increases by a

factor of 6. Question: By how much would the rate increase if the concentration of both reactants

is multiplied by 2?

Important: the exponent in the rate law has no relationship to the coefficient in thebalanced chemical equation. The coefficients are determined experimentally!

2N2O5 (g) 4 NO2 (g) + O2(g)

Rate = k[N2O5]

Important: not necessarily all reactants appear in the rate law. If changing the

concentration of a particular reactant does not change the rate, that reactant does notappear in the rate law.

NO2(g) + CO(g) NO (g) + CO2(g)

Rate = k[NO2]2

Because changes in the concentration of carbon monoxide do not affect the reactionrate, [CO] does not appear in the rate law for this reaction.


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Questions:

1. The rate law for the reaction in which nitrogen monoxide reacts with oxygen to producenitrogen dioxide was shown to be

Rate = k[NO]2[O2]Suppose you measure the rate of the reaction with the concentration of each reactant at

1.00M. What will happen to the reaction rate if the concentration of NO is doubled? Whatwill happen to the reaction rate if the concentration of O2 is doubled instead?

2. The chemical equation and the rate law for the decomposition of hydrogen iodide areshown. What will the effect on the reaction rate if the concentration of HI is raisedfrom 1.00M to 4.00M?

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

Rate = k[HI]2

Thermodynamics

Enthalpy =

1st Law of Thermodynamics heat energy of a system is constant as long as no energyenters or leaves the system

In all chemical reactions, there is a change in enthalpy = heat of reaction

Heat of Formation When one mole of a compound is formed from its elements

Dependent on:

Temperature of reaction Pressure of the reaction Phase of the product

Standard Heat of Formation -D

HfHeat of formation at 25oC and 760 mmHg Phases of the reactant and the products must be stated

H2(g) + 1/2O2(g) H2O(l) + 286 kJ

When one mole of hydrogen gas reacts with mole of oxygen gas, liquid water is formedand heat is given off.


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Exothermic reactionEnergy is regarded as a product

H2O(l) + 286 kJ H2(g) + O2(g)

Heat must be absorbed by the water in order for one mole of liquid water decomposesinto its elements

___________________ reaction

Energy is regarded as a ______________**Notice that the standard heat of formation is written so that one mole of product isformed

Stability of Compounds

Unstable compounds- tend to break down into simpler substances or elements

Which of the following compounds is most stable?

Compound Standard Heat of Formation DHfCarbon Monoxide -110 kJ/mol

Nitric Oxide +90.4 kJ/mol

Compounds that give off large amounts of heat during their formation are consideredstable

Compounds that give off low amounts of heat during their formation or require heat to beformed are unstable

Require little or no net input of energy to cause them to decompose

Explosives!!!

Hess Law of Constant Heat Summation

When a reaction can be expressed as the algebraic sum of two or more other reactions,then the heat of the reaction is the algebraic sum of the heats of these other reactions

CuO(s) + H2(g) Cu(s) + H2O(g) DH = ?kJExpress as the sum of a series of other reactionsCuO(s) Cu(s) + O2(g) DH=155kJH2(g) + O2(g) H2O(g) DH = -242kJ

CuO(s) + H2(g) Cu(s) + H2O(g) DH=-87kJ

Heat of Formation =

Spontaneous Chemical Reactions


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Consider:A waterfallSkiingCycling

The bond strength of the products is greater than the bond strength of thereactantsheat is give off in the reactions

Energetically favorable, should occur spontaneously

But

H2O(l) H2O(s) +6.03 kJThis reaction should be spontaneous, however, we know that this occurs only at 0oC

Entropy

A system has a large entropy if its is in a great state of disorder

DS = Sf -Si

Both the final and the initial entropies will be positive numbers, but the change betweenfinal and initial can be negative

+DS = ________________ in disorder

-DS = ________________ in disorder

Substances in solid phase have fixed particle = low entropy

As a solid changes to a liquid, entropy increases- particles that make up the liquid havegreater freedom of movement

Particles making up a gas have a great deal of random motion- highest entropy of all thephases

What changes in entropy occur when substances react? Formation of a compound- ________________________________ The bonding

of elements to form a compound creates a more orderly state for the atomsinvolved.

Compound decomposition- order and organization breaks down-_______________________

What effect does temperature have on Entropy?


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Entropy and Spontaneous Change An increase in entropy (+DS) favors spontaneous reactions Nature prefers a change from a more orderly system to a less orderly system When a system is observed to have a special order, you can assume that there is

some reason for it. Some restraint on the system must prevent it from assuming amore random arrangement.

In Summary:Two factors that determine if a reaction will occur spontaneously are

1.2.

Gibbs Free Energy Equation

DH = T = D

S =

DG =

Spontaneous reactions DG

*Notice temperature is a factor in this equationPb(s) + O2(g) PbO(s)

PbO(s) DHf = -215kJDSf = -0.092 kJ/K

Calculate DG at room temperature

But What about when T = 3000K?

N2(g) + O2(g) NO(g)DHf = + 90kJDHf = + 0.012kJ/K

At room temp, T = 298K, will this reaction be spontaneous?

kinetics thermodynamics

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