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Determination of Reaction Order AP Chemistry Lab

Introduction: In this experiment you will measure the time required for magnesium to react with

hydrochloric acid of various concentrations. From this data you will calculate the rate of reaction for each

experiment and graphically determine the order of reaction with respect to the hydrogen ion concentration.

Magnesium ribbon will be cut into six equal pieces. Since the ribbon is fairly uniform in manufacture, thelength is proportional to the surface area, and the area for reaction is very nearly the same for each piece.

We can assume that the concentration of Mg is the same for each trial and that variations in rate are

attributes to variations in acid concentrations. Six, 24 mL aliquots (portions) of HCl, having varying

molarities, will be used. These aliquots will prepared by diluting portions of a 6.0 M stock solution.

Amounts of 6.0 M HCl used will be calculated according to the equation M1V1=M2V2.

For this reaction, the net ionic equitation is Mg + 2H+(aq)!H2(g) + Mg

2+. The general rate equation is

rate= k[H+]

nwhere n is the order of the reaction and the rate can be expressed in terms of reciprocal time,

i.e. rate=1/t. Taking the natural log of both sides of the equation gives ln(1/t) = n(ln[H+]) + ln(k). This has

the form of a linear equation y = mx+b. where the slope is the order (n) and the y-intercept is the natural log

of the rate constant, k.

Objectives:1. Separation of solutions of hydrochloric acid in concentrations of 0.5M, 1.0M, 2.0M, 3.0M, 4.0M,

5.0M.

2. Reaction of measured amounts of magnesium metal with each of the portions of HCl, withmeasurement of time necessary for reaction to be complete.

3. Graphing of data and calculation of rate constant, k.Safety precautions: HCl is a strong acid and present in this lab in high concentration. Be careful to avoid

skin contact as well as clothing contact. Wear safety goggles and an apron for this experiment.

Equipment: Test tubes or small beakers Ruler Stopwatch or watch

Graduated cylinder Stirring rod with second hand

Procedure:

1. Prepare the six aliquots of HCl from the 6.0M stock solution. For each reaction, prepare 24.0 mLof solution in a small beaker or test tube.

2. Clean a 12-cm length of magnesium ribbon by rubbing it lightly with paper towel to remove themagnesium oxide. Cut the ribbon into six 2.0-cm pieces.

3. Drop a piece of Mg ribbon into the 0.5M acid. Time the reaction carefully. Stir the solution as themetal reacts. Be careful not to allow the metal to stick to the sides of the container or to float

above the acid solution.

4. Stop the timer when the Mg is completely reacted and record the elapsed time.5. Repeat steps 3 and 4 for each of the other five acid concentrations.

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Data and Calculations:

Trial Amount of 6.0M

HCl diluted to

24 mL

[H+] ln[H

+] Time of

reaction (t)

(sec)

1/t 1/[H+]

1.

2.

3.

4.

5.

6.

Carefully graph ln[H+] on the y-axis and t on the x-axis, drawing a line of best fit. Use your calculator to

perform a l inear regression. Carefully graph 1/[H+] on the y-axis and t on the x-axis, drawing a line of best

fit. Use your calculator to perform a linear regression. Note: there are many other ways to represent this

graph with the data/calculations listed above. It is up to you to understand the math and the relationships of

these variables to the line equation.

Which graph is linear?

What is the slope of the graph?

What is the y-intercept?

What is the order of the reaction with respect to hydrogen ions?

Determine the value of the rate constant.

Post-Lab Questions:

1. Why does reaction rate usually change as the concentration changes?2. Why does the reaction rate usually change with temperature?3. What is the difference between reaction rate and specific rate constant?4. How could you improve your data?