The Effect of Concentration on the Rate of a Reaction.

Introduction: (Initial Observation)

The rate of a chemical reaction has always been a subject of interest for chemists and manufacturers of chemical products. Sometimes, a higher rate of reaction results a higher rate of productivity and profit. In other cases a very high rate of reaction can be an explosive or out of control reaction and cause a disaster. Concentration of reactants are among the factors that may affect the rate of reactions.

Since chemical reactions are different in their outcomes, we cannot use any single method to study the rate of all chemical reactions. A sample reaction that is selected for this project is the

reaction of sodium thiosulfate solution and hydrochloric acid solution. This reaction is selected because of the availability of sodium thiosulfate from photography stores as well as scientific suppliers.

Information Gathering:

Find out about chemical reactions and factors affecting the rate of reaction. Read books, magazines, or ask professionals who might know in order to learn about the methods of measuring the rate of reactions. Also gather information about each of the chemicals that you want to use in your experiments while studying the effect of concentration on the rate of reaction. Keep track of where you got your information from.

Following are samples of information that you may find.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to determine the effect of changing the concentration of a reactant (sodium thiosulfate solution) upon the rate of its reaction with hydrochloric acid.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable is the concentration of sodium thiosulfate solution.

Dependent variable is the rate of reaction.

Controlled variable is temperature.

Constants are the concentration of hydrochloric acid, method, procedures and instruments.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

My hypothesis is that the rate of reaction increases by increasing the concentration of sodium thiosulfate.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1:

Rate of reaction vs. concentration of sodium thiosulfate

Introduction: In this investigation the effect of changing the concentration of thiosulfate ion is studied by observing the time required for a fixed amount of product to form. As the reaction proceeds, the solution becomes cloudy due to the formation of a colloidally dispersed precipitate of sulfur. In order to determine the point at which a specific amount of product has formed, an “x” is observed through the solution. The reaction is timed until the “x” is no longer visible.

Procedure:

1. Obtain five 250-mL beakers, about 30 mL of hydrochloric acid solution, and about 80 mL of sodium thiosulfate solution. Label the beakers from 1 to 5. Add the amounts of sodium thiosulfate solution and distilled or deionized water to each cup indicated in the following table:
   

   Beaker Number	Volume of Sodium Thiosulfate (mL)	Volume of distilled or deionized water (mL)
   1	25	0
   2	20	5
   3	15	10
   4	10	15
   5	5	20

   Note that the total volume in each beaker is 25 mL.

2. Make a table that shows the information in the table above and also includes a column for time (sec) and relative rate (sec-1).
3. Make a small “x” on a sheet of white paper with a pencil. Place a beaker containing the sodium thiosulfate solution over this “x.” Add 5 mL HCl solution and begin timing the reaction as soon as the acid touches the sodium thiosulfate solution. Stir the reaction mixture at a constant rate throughout the reaction.
4. Stop timing when the “x” under the beaker is no longer visible through the solution. Record this time in your data table.
5. Repeat steps 2-4 for the remaining samples.
6. In step 1, why was it necessary to keep the total volume constant at 25 mL? As the volume of sodium thiosulfate solution used was decreased, how did the concentration of Na2S2O3 change?
7. Make a graph of your data by plotting the time (in sec) for each reaction on the y-axis against the volume (in mL) of sodium thiosulfate stock solution on the x-axis.
8. What relationship exists between the volume of sodium thiosulfate solution used and the time it takes for the reaction? What relationship exists between concentration and time?
9. Calculate the relative rate of each reaction by taking the reciprocal of each time measurement. Record these results in your data table.
10. Make a second graph by plotting the relative rate of each reaction on the y-axis against the volume of sodium thiosulfate solution on the x-axis.
11. How does the relative rate of the reaction vary with concentration of sodium thiosulfate solution? If the concentration of the sodium thiosulfate solution doubled, what would happen to the relative rate of the reaction?

Small amounts of colloidal suspensions of sulfur can be washed down the drain.

Experiment 2:

Rate of reaction vs. concentration of Sulfuric acid

Introduction: Dilute sulfuric acid reacts with Iron to produce Iron Sulfate and hydrogen gas. In this investigation the effect of changing the concentration of sulfuric acid is studied by observing the amount of Iron consumed during reaction.

Material:

• Distilled or diaionized water
• Sulfuric acid 50% or 35% sulfuric acid (Battery acid)
• Bolts, nuts or similar iron pieces

Procedure:

1. Obtain seven 250-mL beakers, about 250 mL of Sulfuric acid 98%, and about one liter of distilled or deionized water. Label the beakers from 1 to 5. Add the amounts of water to each cup indicated in the following table:

    

   Beaker Number	Volume of Sulfuric Acid 50%	Volume of distilled or deionized water (mL)	Acid Concentration	Iron loss
   1	100	0
   2	80	20
   3	60	40
   4	40	60
   5	20	80
   6	10	90
   7	5	95

   Note that the total volume in each beaker is 100mL.

2. To the water already in the beakers, add the amounts of sulfuric acid 50% indicated in the above table.
3. Stir each solution for about 1 minute.
4. Get five identical black iron bolts (not galvanized), weight them, wash them and drop one bolt in each beaker.
5. After about 6 hours, remove the bolts from the beakers, wash them, dry them and weight them. Calculate the weight loss for each bolt and record that in the above table.
6. For each of the above solutions, calculate the concentration of acid and write it in the above table. To do that multiply the volume of sulfuric acid in each beaker by the concentration of acid and divide it by 100.
7. Make a graph of your data by plotting the acid concentration for each reaction on the y-axis against the iron loss on the x-axis. Iron loss represents the relative rate of reaction.

Materials and Equipment:

List of material for experiment number 1

Chemicals:

0.15 M sodium thiosulfate solution (23.7 g Na2S2O3 dissolved in deionized or distilled water to make one liter of solution)*
6 M HCl solution (dilute 500 mL concentrated HCl solution to one liter with distilled or deionized water)*
distilled or deionized water

Equipment:

250-mL beakers
stirring rods
25-mL graduated cylinder
clock which can measure seconds
white paper

Modification/Substitution:

1. Sodium thiosulfate is available from photographic supply stores as “hypo.” “Fixer”, also available from photographic supply stores, contains thiosulfate (usually in the form of Na2S2O3) and other chemicals used as hardening agents which do not seem to interfere with the reaction. Satisfactory results were obtained by using 47.4 g “fixer”/liter of solution.
2. HC1 solution is available from a hardware store as muriatic acid, 28% HCl; dilute this solution with 1 part distilled or deionized water to 1 part of muriatic acid to make a solution that is about 4.5 M and can be used in this experiment.
3. Plastic cups may be used in place of beakers.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Calculations:

Described in the experiment section.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you purchase solution of Sodium thiosulfate known as fixer, you should know that “fixer” contains additional hardening agents and the exact molar concentrations of sodium thiosulfate cannot be prepared. For this reason, you are asked to plot time vs. volume of sodium thiosulfate solution rather than molar concentration. If “hypo” or pure sodium thiosulfate are used and you are able, you could calculate exact molar concentrations and use this information in making your graphs. In either case, you will reach the same conclusion about the relationship between concentration and rate of the reaction.

References:

Dreyfus Curriculum Module, Chemical Kinetics and Catalysis, 1982. (No longer in print.) The use of the disappearing “x” to time the reaction is described.

Masterton, W., Slowinski, E., and Walford, E., Chemistry in the Lab, Rinehart and Winston, New York, 1980, p. 139. The same reaction is used to investigate the effects of concentration and temperature by timing until the first hint of cloudiness is observed.

Other inorganic chemistry books can also be used as a reference.