1.02a Obtaining a Rate Law from Experimental Data Equations

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From the course by University of Kentucky

Advanced Chemistry

335 ratings

University of Kentucky

Advanced Chemistry

335 ratings

A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.

From the lesson

Kinetics

The study of chemical kinetics is the study of change over time. It answers questions like: How fast are reactants consumed? How fast are products formed? This unit is dedicated to the exploration of how these questions are answered. We will look at the experimental evidence of how concentration affects these rates. We will also examine what occurs on the molecular level, especially with respect to the motion of molecules, that affects rates of reactions.

1.01 The Rate of Chemical Reactions10:12

1.02 Comparing Rate of Change for Reactants and Products13:52

1.02a Obtaining a Rate Law from Experimental Data Equations4:23

1.03 The Rate Law9:01

1.04 Obtaining a Rate Law from Experimental Data16:38

1.04a Rate Law Calculations8:20

1.05 First-Order Kinetics and the Integrated Rate Law0:00

1.05a Graphic 1st Order7:10

1.06 First-Order Kinetics and the Half-Life9:32

1.07 Second-Order Reactions8:18

1.07a Graphics 2nd Order6:02

1.08 Collision Theory13:13

1.08a Activation Energy3:48

1.09 The Arrhenius Equation15:32

1.09a Graphic Activation Energy2:57

1.10 Reaction Mechanisms12:45

1.10a RDS Fast Equilibrium4:38

1.10b Rate Determining Step2:41

1.11 Catalysis4:23

Meet the Instructors

Dr. Allison Soult
Lecturer
Chemistry
Dr. Kim Woodrum
Sr. Lecturer
Chemistry

In this problem we're asked to come up with a rate expression.

And it says to do it in terms of the disappearance of reactants and

the appearance of products.

So let's look at what we're going to do in part A.

A rate expression is a way of determining a rate of reaction.

You can determine it by observing any one of the reactants and products.

Now the reactants are disappearing.

And if we monitor the changing concentration of ammonia for

example, that concentration is decreasing with time.

But once the rate of the reaction will be a positive number, so

we put a minus sign there and we will divide by delta t.

To make this equal to the rate of the reaction,

we also must divide by the coefficient of 4.

We can do this for each substance in the reaction, so since oxygen is

also a reactant, we will do a minus sign, change in concentration of O2,

with respect to time, without forgetting to put in the five on the denominator.

For the products, we do not need the negative sign.

But we still need the coefficient, so we will divide it by four delta T.

And for water, it will be the change in concentration of H2O divided by 6 delta T.

So I could monitor any one of those substances and

determine the rate of the reaction.

Now somebody went and observed the reaction and

noticed that oxygen was disappearing at a rate of 0.335 M/s.

So that would be the change in concentration of O2 with respect to time.

Now it's disappearing.

So its change of concentration is negative.

When we say it's disappearing at this rate,

they're actually giving me the positive of that.

So this is what we know.

What we want to know is the rate of formation of NO.

So we're trying to determine this value.

We can come up to a rate expression and

notice that we have those two pieces in the middle part of this rate expression.

We also see that the change in O2 is observed.

And I'll put kind of a dash circle around it.

We see it right here.

Okay we see it observed right there, and this is the value that they gave for us.

So I'm going to write down this expression, minus the change in

concentration of O2, divided by 5 delta

T is equal to the change in concentration of no divided by 4 delta t.

I will put in place of this part,

the numerical value that they gave me, which is 0.355 molarity per second.

Over 5 is equal to the changing concentration of no divided by 4 delta t.

We don't want the 4.

Look up here what we're trying to define.

Just how NO is changing with time.

So I'm want to multiply both sides by the 4.

And that will leave me with a value of, let's see,

0.284 molarity per second for

the change in concentration of NO with respect to time, so

the rate of change of NO, how fast is it appearing?

Now let's look at the numbers and do some comparison.

We see that NO is appearing at a slower rate than O2 is disappearing.

See that number there?

See this number here?

Let me say that again.

We see that NO is appearing more slowly than the O2 is disappearing.

If we look up here at the balanced equation and

we see these coefficients that should make sense to us.

For every 5 02 that we consume we are only going to make 4 NO,

so give will be disappearing more rapidly than NO will be appearing, so

our number makes sense to us.

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