# AP Chemistry Chemical Kinetics Problem

• Hikari
In summary, the rate of the reaction was studied at a certain temperature. The rate was found to be -69.3 sec-1 with respect to [O].
Hikari

## Homework Statement

The rate of the reaction was studied at a certain temperature.
O(g) + NO2(g) --> NO(g) + O2(g)
In the first set of experiments, NO2 was in large excess, at a concentration of 1.0 * 10^ 13 molecules/cm3 with the following data collected.
time (s) [O] (atoms/cm3)
0 ......2400
0.01.....1200
0.02.....600
0.03......0

Part (a). Find the reaction order with respect to O. (Explain with graphs)

Part (b) Find the rate constant with respect to O.

## Homework Equations

Rate = K[A]
First Order Reaction Integrated Rate Law: ln[A] = -kt + ln[A]0

## The Attempt at a Solution

I want to know how to transform the graph/data set into a linear line to find the reaction order (I think it is first order). I think I ln[O] and plot it against time (s) to achieve a linear graph for a first order graph. How do I do that? Do I need to convert atoms/cm3 to mol/L? I tried doing that and it is not looking right. Help would be appreciated!

Hikari said:

## Homework Statement

The rate of the reaction was studied at a certain temperature.
O(g) + NO2(g) --> NO(g) + O2(g)
In the first set of experiments, NO2 was in large excess, at a concentration of 1.0 * 10^ 13 molecules/cm3 with the following data collected.
time (s) [O] (atoms/cm3)
0 ......2400
0.01.....1200
0.02.....600
0.03......0

Part (a). Find the reaction order with respect to O. (Explain with graphs)

Part (b) Find the rate constant with respect to O.

## Homework Equations

Rate = K[A]
First Order Reaction Integrated Rate Law: ln[A] = -kt + ln[A]0

## The Attempt at a Solution

I want to know how to transform the graph/data set into a linear line to find the reaction order (I think it is first order). I think I ln[O] and plot it against time (s) to achieve a linear graph for a first order graph. How do I do that? Do I need to convert atoms/cm3 to mol/L? I tried doing that and it is not looking right. Help would be appreciated!
You don't have to convert to mol/L. The first three data points are OK, but the 4th data point should be 300 atoms/cc (there must be a typo, or maybe you're supposed to think that the measurement is in the noise at 300 atoms/cc). You can plot the first three points, and leave out the 4th, or you can plot all four, with 300 as the last point. If you plot the first 3 points, they will lie on a straight line on a semi-log plot. If you have Excel, just change the vertical scale to a log scale (which Excel will automatically do for you). You can get the first order rate constant from the slope. Excel will even do a curve fit for you, and provide you with the slope, so that you can know the rate constant immediately.

Chet

Thanks you! I got the equation to be y = -69.315x + 7.7832 without plotting the last point. This means the rate constant is -69.3 respect to [O]. It is negative because the concentration is going down as time increases. Is that correct?

Hikari said:
Thanks you! I got the equation to be y = -69.315x + 7.7832 without plotting the last point. This means the rate constant is -69.3 respect to [O]. It is negative because the concentration is going down as time increases. Is that correct?
In your equation, there is a negative sign in front of k. So the rate constant should be +69.3/sec.

Chet

To transform the data into a linear line, you can plot ln[O] (y-axis) against time (x-axis). This will give you a straight line if the reaction is first order with respect to O. If the reaction is second order, the graph will be a curved line.

To convert atoms/cm3 to mol/L, you can use the ideal gas law: PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant (0.0821 L*atm/mol*K), and T is the temperature in Kelvin. Rearranging this equation to solve for n, you get n = (PV)/(RT). Since the volume is not given in the problem, you can assume it is constant and cancel it out. Then, n = (P)/(RT). To convert from atoms/cm3 to mol/L, you need to multiply by Avogadro's number (6.022*10^23 atoms/mol). So, the equation becomes n = (P)/(RT) * (6.022*10^23 atoms/mol). This will give you the concentration in mol/L.

For part (a), you can use the integrated rate law for a first order reaction: ln[A] = -kt + ln[A]0. In this case, [A] represents the concentration of O, k is the rate constant, t is time, and [A]0 is the initial concentration of O. You can plug in the values from the data set to solve for k. The slope of the line is -k, so the reaction order with respect to O is 1.

For part (b), once you have solved for k, you can use the rate equation: Rate = k[A]. Again, [A] represents the concentration of O. You can plug in the values from the data set to solve for the rate constant k.

## 1. What is chemical kinetics in AP Chemistry?

Chemical kinetics is the study of the rates of chemical reactions and the factors that affect them. It involves analyzing the speed at which reactants are consumed and products are formed, as well as the mechanisms that drive these reactions.

## 2. How are chemical kinetics problems typically presented in AP Chemistry?

Chemical kinetics problems in AP Chemistry are typically presented as reaction equations, with information about the initial concentrations of reactants and products, as well as the rate constants and reaction conditions.

## 3. What are the key factors that affect the rate of a chemical reaction?

The key factors that affect the rate of a chemical reaction include temperature, concentration of reactants, surface area of reactants, presence of a catalyst, and the nature of the reactants and products involved.

## 4. How can I determine the rate law for a chemical reaction?

The rate law for a chemical reaction can be determined by conducting experiments and analyzing the data to identify the relationship between the rate of the reaction and the concentrations of the reactants. This can be done using the method of initial rates, integrated rate laws, or graphical methods.

## 5. How can I use chemical kinetics to predict the progress of a reaction?

Chemical kinetics can be used to predict the progress of a reaction by analyzing the rate constant and the initial concentrations of the reactants. This information can be used to calculate the rate of the reaction and estimate the time it will take for the reactants to be consumed and products to be formed.

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