01:160:161 Lecture Notes - Lecture 4: Rate Equation, Ammonium, Molar Concentration

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harlequinsnake190

19 Feb 2016

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A reaction with a constant change in concentration is a reaction of zero-order. However, most reactions have a curve when graphed concentration as a function of time. Where n and m are constants that must be found through experimentation. If you have a direct proportion between concentration and rate. (half the concentration, half the rate) - it is a reaction of first order and n/m=1. Differential rate law: where n and m are the order which can be determined, for example, by the method of initial rates: We see that as we double the [no2], the rate doubles. Also, as we double the [nh4], the rate doubles. Which means that both n and m are 1. To find the value of k, just plug in one of the pieces of sample data and solve for k. The units for k in the above reaction is. For a reaction of the nth order, the units for k are:

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Learning Goal:

To understand reaction order and rate constants.

For the general equation aA+bBâcC+dD, the rate law is expressed as follows:

rate=k[A]m[B]n

where m and n indicate the order of the reaction with respect to each reactant and must be determined experimentally and k is the rate constant, which is specific to each reaction.

Order

For a particular reaction, aA+bB+cCâdD, the rate law was experimentally determined to be

rate=k[A]0[B]1[C]2=k[B][C]2

This equation is zero order with respect to A. Therefore, changing the concentration of A has no effect on the rate because [A]0 will always equal 1.(Figure 1)

This equation is first order with respect to B. This means that if the concentration of B is doubled, the rate will double. If [B] is reduced by half, the rate will be halved. If [B] is tripled, the rate will triple, and so on.(Figure 2)

This equation is second order with respect to C. This means that if the concentration of C is doubled, the rate will quadruple. If [C] is tripled, the rate will increase by a factor of 9, and so on.

Overall reaction order and rate-constant units

The sum of the individual orders gives the overall reaction order. The example equation above is third order overall because 0+1+2=3.

For the units of rate to come out to be M/s, the units of the rate constant for third-order reactions must be Mâ2â sâ1 since

M/s=(Mâ2â sâ1)(M3)

For a second-order reaction, the rate constant has units of Mâ1â sâ1 because M/s=(Mâ1â sâ1)(M2). In a first-order reaction, the rate constant has the units sâ1because M/s=(sâ1)(M1).

Analyzing a specific reaction

Consider the following reaction:

2H2+2NOâN2+2H2O,rate=k[H2][NO]2

Part A

What is the overall reaction order?

Express your answer as an integer.

What are the units of the rate constant k for this reaction?

choose one

What are the units of the rate constant for this reaction?

Mâ sâ1

sâ1

Mâ1â sâ1