16. Kinetics HL

Distinguish between the terms rate constant, order of reaction, and overall order of reaction with respect to a particular reactant

Rate constant- A constant that gives an indication as to how fast a reaction proceeds

Order of reaction is a way of determining the relationship between rate and concentration

Overall order of reaction is the sum of the individual orders of reaction (m + n)

Zero order- Doubling the concentration will have no effect on the rate (2 to the 0^th power)

First order- Doubling the concentration will 2x the rate as well (2 to the 1^st power)

Second order- Doubling the concentration will 4x the rate (2 to the 2^nd power)

Third order- Doubling the concentration will 8x the rate (2 to the 3^rd power)

Deduce the rate expression for a particular reaction from experimental data AND solve problems involving the rate expression.

Rate can also be expressed by the equation Rate = k [A]^m[B]ⁿ

k = rate constant (varies from reaction to reaction and with temperature)

[A] [B] = concentration of A and B

m n = Order of reaction

EXAMPLE!!!

 

Experiment	[A]	[B]	[C]	Initial rate
1	0.4000	1.6000	0.0600	4.86 x 10-3
2	0.8000	1.6000	0.0600	9.72 x 10-3
3	0.4000	0.8000	0.0600	4.86 x 10-3
4	0.8000	1.6000	0.1800	87.5 x 10-3

Order of Reaction [A]

1	0.4000	1.6000	0.0600	4.86 x 10-3
2	0.8000	1.6000	0.0600	9.72 x 10-3

As Rate Doubles (4.86… à 9.72…)

Concentration of A increases by 2¹ (In other words it doubles from 0.4000 à 0.8000)

So there is FIRST order reaction

Order of Reaction [B]

1	0.4000	1.6000	0.0600	4.86 x 10-3
3	0.4000	0.8000	0.0600	4.86 x 10-3

As Concentration B decreases by ½ (1.6 à 0.8)

Rate does not change. In other words, there is ZERO change

So there is a ZERO order reaction

Order of Reaction [C]

2	0.8000	1.6000	0.0600	9.72 x 10-3
4	0.8000	1.6000	0.1800	87.5 x 10-3

As Concentration of B triples (0.06 à 0.18)

Rate increases by 3² (9 times from 9.72…à87.5)

So there is a SECOND order reaction

Rate Equation is written as Rate = k[A]¹[C]²                [B] can be omitted since its 0 order

Total order of reaction is Third (1 + 0 + 2 = 3)

K can be found by using the equation       Initial Rate

                                                                   [A]^m[B]ⁿ[C]^p

To find K for the information above, plug in the info from one data set

K = 9.72 x 10^-3

   [0.8000][0.0600]²

.00972      =     3.375 mol^-2 dm⁶ s^-1

.00144

The units K is displayed in depends on the overall order of reaction

Zero	mol1 dm-3 s-1
First	mol0 dm0 s-1
Second	mol-1 dm3 s-1
Third	mol-2 dm6 s-1

For each successive order of reaction mol decreases -1 power, and dm increases 3 powers

***Half-Life is the time needed for ½ of a sample to decompose. Indication of rate

Sketch and analyze graphical representations for zero, first, and second order reactions.

First Order Concentration vs. Time-

As t à1, Concentration à ½. As tà2, Concentration à ¼

Second Order Concentration vs. Time-

As t à 1, Concentration à ¼. As t à2, Concentration à1/8

Explain that reactions can occur by more than one step and that the slowest step determines the rate of reaction

Reactions can occur by more than one step

Reagents will often form Intermediates, or chemicals that exist temporarily before being converted to the final product.

In Reaction 2A + B à C,  A + B (reagents) will often from X ( an intermediate), which reacts with the second molecule of A to form C

The rate of reaction is determined by the slowest step, called the Rate Determining Step

Only by speeding up the Rate Determining Step, will the reaction be sped up as well. Speeding up the faster steps will do nothing

Reaction Mechanism- the series of steps by which an overall reaction occurs

Unimolecular- A reaction consisting of ONE reagent that breaks down or rearranges

                        (ex. Cl₂ –^{(UV Light)}à 2 Cl*)

Bimolecular- A reaction consisting of TWO reagents rearranging to form new compound

                        (ex. CH₄ + Cl* à CH_­3Cl + H*)

Transition State- A split-second moment where the bonds of one chemical are forming and the bonds of the other are breaking, forming a substance with no real chemical identity that never exists by itself unless its going on to form a reaction, which makes it different from an intermediate

Describe, qualitatively, the relationship between rate constant and Temperature

Rate constant increases with temperature

Determine activation energy values from the Arrhenius equation by a graphical method

Arrhenius Equation- A way of finding the activation energy based off of the temperature

Represented by the equation lnk = InA – ( E_a ) ( 1 )

                                                                     R       T

Where k = Rate constant, A = Arrhenius constant, R = gas constant, and T = temp in K

(Don’t worry. You’re not going to need to do calculations. You just need to identify info from a graph)

 

Activation energy = -mR

(where m is the slope of the gradient)