SNS - General Chemistry - Chemical Kinetics and Equilibrium Flashcards Preview

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Flashcards in SNS - General Chemistry - Chemical Kinetics and Equilibrium Deck (25):
1

Chemical Equilibria

At equilibrium, the rate of forward reaction is equal to that of the reverse reaction Rate = k[A]^x [B]^y

2

Chemical Equilibria

Keq

For example:

2[A] + [B] 3[C] + 2[D]

Rate (forward) = k[A]^2 x [B]

Rate (reverse) = k[C]^3 x [D]^2

Keq = ([C]^3 x [D]^2) / ([A]^2 x [B])

3

Chemical Equilibria

Qc

The reaction quotient To determine the direction of a reaction (ie whether products or reactants are favoured), you must find the value of Qc, found using the same method as for Keq but instead of using the concentrations at equilibrium, Qc utilises that under the experimental conditions

By comparing Qc to Keq, can determine whether a reaction will favour products or reactants Qc = Keq - reaction is at equilibrium Qc > Keq - favours reactants Qc < Keq - favours products

4

Chemical Equilibria Changes in Reactant/Product Concentration

Increasing the concentration of a substance causes the reaction to use that substance, producing more of the substance on the other side of the equilibrium

5

Chemical Equilibria Changes in Pressure and Volume

Increasing the pressure of a system at equilibrium will cause a shift to the side that reduces pressure. An increase in volume has the same effect as a decrease in pressure

6

Chemical Equilibria Changes in Temperature

We treat heat as a product in a exothermic reaction, and as a reactant in an endothermic reaction.. Adding heat to an endothermic reaction will favour the formation of products. Adding heat to an exothermic reaction will favour the formation of reactants.

7

Chemical Kinetics

Rate Laws

The rate of a reaction is the change in concentration of the reactants divided by the time. The rate law for almost all reactions,

aA + bB -> cC + dD

has a rate proportional to [A]^x [B]^y

Rate = k [A]^x [B]^y where the exponents x and y are the orders of reaction

8

Chemical Kinetics Reaction Orders Zero Order

Have a constant rate and occur independently of the concentrations of reactants

9

Chemical Kinetics Reaction Orders First Order

Have a rate proportional to the concentraion of one reactant

10

Chemical Kinetics Reaction Orders Second Order

Have a rate proportional to the products of the concentrations of the two reactants or to the concentration of one of the reactants squared

11

Chemical Kinetics Reaction Orders Methods of Acceleration of a Reaction

1.Increase the concentrations of reactants 2. Increase the temperature of the reaction 3. Increase the surface area of reaction 4. Catalyst

12

Chemical Kinetics Reaction Orders Homogenous catalyst

Catalyst which has the same state as the reactants

13

Chemical Kinetics Reaction Orders Heterogenous catalyst

Catalyst which has a different state to the reactants

14

Chemical Kinetics Activation Energy and Enthalpy Ea

Activation energy of a reaction The difference in potential energy between the activated complex and the reactants Represents the energy of collision necessary to drive the reaction Can be reduced by the addition of a catalyst

15

Chemical Kinetics Activation Energy and Enthalpy Ea Catalysts

Can be used to reduce the activation energy of a reaction. Works by increasing the frequency of collisions in the forward and reverse reaction

16

Chemical Kinetics Activation Energy and Enthalpy Ea reverse

Activation energy of the reverse reaction. The difference between the potential energy of the transition state and the products

17

Chemical Kinetics Activation Energy and Enthalpy ∆H

Change in enthalpy. Enthalpy is equal to the potential energy of the products minus the potential energy of the reactants If this number is negative, the recation is exothermic and vice versa

18

Chemical Kinetics Activation Energy and Enthalpy Transition State

Activated complex The arrangement of reactant and product molecules in their maximal energy

19

Chemical Kinetics Activation Energy and Enthalpy Collision Theory

States that reactant molecules must possess sufficient energy to combine and form an activated complex

20

Chemical Kinetics Half Life

Length of time required for the concentration of a reactant to decrease to one half of the original amount For a first order reaction = 0.693/k For a second order reaction = 1/(k[X]) where k=rate constant of the reaction and X is initial concentration

21

Chemical Kinetics

Half Life

Remaining Quantity (g) after n half lives

=x / 2^n Where x is number of grams originally

22

Trial [A]initial (M) [B]initial (M) rinitial (M/s)

1      1.00        1.00             2.0

2      1.00        2.00             8.1

3     2.00        2.00            15.9

 

Find the rate law for the following reaction at 300K

A + B → C + D

Rate = k [A]x [B]y          k = Rate / (A]x [B]y)

  1. 2/(1x 1y) = 8.1/(1x 2y) = 15.9/(2x 2y)
  2. 2/1y = 8.1/2y; 8.1/2 = 2y = ~4, y = 2
  3. 8.1/1x = 15.9/2x = 15.9/8.1 = 2x = ~2, x=1

So r = k[A] [B]2

To find k, substitute values in: 2 = k x 11 x 12; k = 2/2 = 1 M-2 s-1

Therefore the rate law is r = 1 M-2 s-1 [A][B]2

23

Activation Energy

Minimum energy of collision required for a reaction to take place.

24

Collisions and rate of reaction

Rate of reaction can be expressed as

rate = fZ

Where Z is the total  number of collisions per second and f is the fraction of collisions that are effective

25


What is the expression for the equilibrium constant for the following reaction:

3H2 (g) + N2 (g) ⇔ 2NH3 (g)

3A + B ⇔ 2C

Kc = [C]2 / ([A]3 x [B])

Kc = [NH3]2 / ([H2]3 x [[N2])

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