Chemical Kinetics

اميرالظلام: فريق الإدارة; عدد المساهمات : 58
نقاط : 120
تاريخ التسجيل : 19/11/2009

مساهمة رقم 1

Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 8:55 am

Rate of a Reaction

Chemical Kinetics - The study of the rates of chemical reactions.
Rate of a Reaction - The change in concentration of one of the reactants (DX), during a given period of time (Dt)

The reaction rate gradually decreases as reactants are consumed.
Instantaneous Rate of Reaction - The rate of a reaction at an instant in time.

where d(X)/dt is the derivative X/t in terms of t.

Rate Law - An equation that describes how the rate of a chemical reaction depends on the concentrations of the reactants consumed in that reaction.
rate = k[O3]

Rate Constant (k) - The proportionality constant in the rate law equation that describes the relationship between the rate of a step in a chemical reaction and the product of the concentrations of the reactants consumed in that step.
**The rate law for a reaction cannot be determined by the coefficients of the balanced reaction; it must be determined experimentally.

For gas equilibrium equations, the equilibrium constant (Kc) is equal to the rate constant for the forward reaction divided by the rate constant of the reverse reaction.

For the reaction:

where a, b, c, d are the coefficients for the reactants and products A, B, C, D

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اميرالظلام: فريق الإدارة; عدد المساهمات : 58
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مساهمة رقم 2

رد: Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 8:57 am

Order and Molecularity

Molecularity - The number of molecules consumed in a chemical reaction

Step 1: N2O5 NO2 + NO3
Step 2: NO2 + NO3 NO2 + NO + O2
Step 3: NO + NO3 2 NO2

Unimolecular - When a single molecules is consumed. e.g. Step 1 in the above reaction.
Bimolecular - When two molecules are consumed. e.g. Steps 2 and 3 in the above reaction.

Order - Describes the relationship between the rate of a step in a chemical reaction and the concentration of one of the reactants consumed in that step.

First-Order - The rate of the reaction depends on the concentration of a reactant raised to the first power.
The #1 for the first power is usually omitted because any number raised to the first power equals itself.
Rate = k[N2O5]

The above reaction is first order with respect to [N2O5] and first order overall.

The rate of the overall reaction is the sum of the orders of all the reactants
If there is only one reactant, then the order with respect to that reactant is equal to the order of the overall reaction.

Second-Order - The rate of the reaction depends on the concentration of a reactant raised to the second power.
Rate = k[HI]2

The above reaction is second order with respect to [HI] and second order overall.

Rate = [NO]2[O2]

The above reaction is first order with respect to [O2] and second order with respect to [NO].
The overall reaction is third order, or higher order.

Higher Order - The rate of the reaction depends on the concentration of a reactant raised to a power greater than two.
Rate = [NO]1/2[Cl2]3

The above reaction is mixed order with respect to [NO] and higher order with respect to [Cl2]3.

Mixed Order - The rate of the reaction depends on the concentration of a reactant raised to a fractional power.

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مساهمة رقم 3

رد: Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 8:59 am

Sample Rate Calculations

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Rate Laws
For each of the following, determine the rate law of the reaction and find the value of the rate constant (k) with proper units.

2 O3 3 O2

Simplify and solve for x:

The reaction is first order with respect to [O3] because the exponent was calculated to be 1.
Now we can write the rate law and solve for k by plugging in values for the rate and the [O3] from a single experiment:

The rate law includes the concentrations of all the reactants:
rate = k[Xe]X[F2]Y
Therefore, we must solve for both X and Y separately.
First choose two experiments in which the initial [F2] are the same and set up a ratio with these experiments:

The [F2] cancel each other out ,and the equation simplifies to find that X = 2. The reaction is second order with respect to [Xe].
Next, we do the same to solve for Y. Choose two experiments in which the initial [Xe] are the same and set up a ratio.

The [Xe] cancel each other out, and the equation simplifies to find that Y = 1. The reaction is first order with respect to [F2].
Now we can write the rate law:

* The reaction is first order with respect to F2 and second order with respect to Xe.
Now we have to solve for k:

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اميرالظلام: فريق الإدارة; عدد المساهمات : 58
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مساهمة رقم 4

رد: Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 9:01 am

Factors Affecting Reaction Rates

Concentration

As the concentration of the reactants increases, the reaction rate increases.
WHY?
According to the collision theory, the rate of a reaction is directly proportional to the number of effective collisions per second between the reactant molecules.
Effective Collisions - the fraction of total collisions that actually result in the formation of the product(s).
If the concentration of the reactants increases (i.e. particles per given volume) the greater the number of total collisions.
The greater the frequency of total collisions, the greater the frequency of effective collisions.
Surface Area

As the surface area of the reactants increases, the reaction rate increases.
WHY?
Increasing the surface area of the reactants results in a higher number of reaction sites.
Reaction sites - specific sites on molecules at which reactions occur.
Increasing the number of reaction sites increases the number of total collisions.
The greater the frequency of total collisions, the greater the frequency of effective collisions.
If the frequency of effective collisions increases, so does the reaction rate.

Temperature

As the temperature of a system increases, the reaction rate increases.
WHY?
Temperature (T) - A measure of the average kinetic energy (KEavg) of the particles of a substance.
Increasing T increases KEavg.
At higher T, the fraction of molecules with energies greater than the activation energy (Ea) increases.
Activation Energy (Ea) - the energy level that must be overcome for a reaction to occur.

Catalysts

The presence of a catalyst increases the reaction rate.
Catalyst - A substance that increases the rate of a reaction but is not consumed in the reaction. It does so by lowering the activation energy of a reaction.
Possible ways of lowering the Ea of a reaction:
Increases the frequency of collisions between the reactant molecules.
Changes the relative orientation of the reactant molecules.
Donates electron density to the reactant molecules.
Reduces the intramolecular bonding within the reactant molecules.
Provides an alternate pathway or mechanism for the reaction.

For equilibrium reactions, both the forward and reverse reaction rates are affected by the catalyst.
i.e. the Ea for both directions is decreased.
Therefore, the equilibrium constant is notchanged by the presence of a catalyst.
The relative concentrations of the reactants and products is not changed.

Examples of common catalysts:
Platinum
Nickel
Manganese Dioxide (MnO2)

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مساهمة رقم 5

رد: Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 9:02 am

Activation Energy

Activation Energy (Ea) - The energy level that the reactant molecules must overcome before a reaction can occur

Exothermic Reaction

In an exothermic reaction in which
the change in enthalpy between the
products and the reactants is positive,
there must be an extra input of energy
above the energy level of the products
in order for a reaction to occur.

Endothermic Reaction

Even in an endothermic reaction in which
the change in enthalpy between the products
and the reactants is negative, there must be
an input of energy to start the reaction.
Arrhenius Equation

By rearranging this equation, it can be used to determine the activation energy of a reaction.
This equation is in the form:

Arrhenius Equation

When the lnk (rate constant) is plotted versus the
inverse of the temperature (kelvin), the slope is a
straight line. The value of the slope (m) is equal
to -Ea/R where R is a constant equal to 8.314 J/mol-K.
When the equation of the slope is rearranged:

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اميرالظلام: فريق الإدارة; عدد المساهمات : 58
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مساهمة رقم 6

رد: Chemical Kinetics

مُساهمة اميرالظلام الخميس فبراير 18, 2010 9:03 am

[center]Integrated Form of the First-Order Rate Law

The original first-order rate law equation is:

The integrated form of the first-order rate law equation is:

Where X is the concentration of a reactant at any moment in time, (X)o is the initial concentration of this reactant, k is the constant for the reaction, and t is the time since the reaction started.
This equation is useful in calculating how much of a substance remains after a certain amount of time has passed, or to calculate how long it takes until the concentration is at a certain point.

First-Order Reaction

If the rate law of a reaction is first order
with respect to [A], then the graph of ln[A]
versus time (t) creates a straight line with a
negative slope. The value of the slope of the
line is equal to the negative value of the
rate constant (k).

The equation for the half-life of a substance is derived from this equation.
Half-life - The length of time it takes for exactly half of the nuclei of a radioactive sample to decay.

Integrated Form of the Second-Order Rate Law
The original equation for a second-order rate law with a single reactant is:

The integrated form of the second-order rate law equation is:

Where X is the concentration of a reactant at any moment in time, (X)o is the initial concentration of this reactant, k is the constant for the reaction, and t is the time since the reaction started.
This equation is useful in calculating how much of a substance remains after a certain amount of time has passed, or to calculate how long it takes until the concentration is at a certain point.

Second-Order Reaction

If the rate law for a reaction is second order
with respect to [A], a graph of 1/[A] versus
time (t) creates a straight line with a positive
slope. The value of the slope of the line is
equal to the value of the rate constant (k).[/center]

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Chemical Kinetics

Chemical Kinetics

رد: Chemical Kinetics

رد: Chemical Kinetics

رد: Chemical Kinetics

رد: Chemical Kinetics

رد: Chemical Kinetics

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