How do you prepare diluted solutions of sodium thiosulfate for the concentration dependence experiment?

Use a volumetric pipette to measure the required volume of 0.2 M sodium thiosulfate stock solution and dispense it into a volumetric flask. Fill the flask with deionized water to the mark, seal with plastic paraffin film, and invert several times to thoroughly mix. This dilution procedure ensures accurate target concentrations ranging from 0.05 M to 0.2 M for each trial.

Why is measuring the time to solution opacity used to track reaction progress?

The reaction produces solid sulfur that creates visible cloudiness in the initially clear, colorless solution. Since each trial uses the same test tube and total volume, it takes the same amount of sulfur to make the solution completely opaque. By timing when the X mark disappears, you measure how fast sulfur is produced, which directly reflects the reaction rate.

What does the relationship between sodium thiosulfate concentration and reaction time reveal about reaction order?

When sodium thiosulfate concentration doubles, the time to opacity is halved, indicating the reaction rate doubles proportionally. When concentration increases by a factor of four, the reaction rate also increases by a factor of four. This one-to-one relationship demonstrates that the reaction is first order with respect to sodium thiosulfate.

How does HCl concentration affect the rate of the sulfur precipitation reaction?

Reaction times remain nearly identical across trials with different HCl concentrations, from 1.5 M to 6.0 M. This lack of variation indicates that HCl concentration has no effect on the reaction rate, meaning the reaction is zeroth order with respect to HCl. The rate depends only on sodium thiosulfate concentration.

What precautions are necessary when handling the reactants and products in this experiment?

HCl is toxic and highly corrosive, requiring a lab coat, splash-proof safety glasses, and nitrile gloves. Sulfur dioxide, a gaseous product, is also toxic and must be allowed to escape harmlessly by leaving reaction waste in the fume hood overnight. Always work in a fume hood and neutralize waste with baking soda before disposal.

Why are benchmark trials performed before varying reactant concentrations?

Benchmark trials ensure that reactions occur at consistent room temperature and establish reproducible baseline measurements. Two benchmark trials must be within 3-5 seconds of each other to confirm experimental reliability. This consistency validates that subsequent variations in concentration, not temperature fluctuations, cause observed changes in reaction time.

How do you calculate the overall reaction order from individual reactant orders?

Add the reaction orders of all reactants to determine the overall reaction order. Since sodium thiosulfate is first order and HCl is zeroth order, the overall reaction order is one. This sum represents the total dependence of the reaction rate on all reactant concentrations combined.

Concentration Dependence

Concepts Instructor Prep Student Protocol

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The measure of how fast a reaction proceeds is called the reaction rate. For a single-step reaction, the rate is equal to the change in concentration of each reactant or product over time multiplied by the inverse of the corresponding stoichiometric coefficient. You can think of the change in concentration over time as the concentration at time t minus the starting concentration divided by t. Reaction rates are always positive, so the reactant expressions have negative signs. These rates may not be equal in multi-step reactions, but we can still use this relationship as an estimate for the overall reaction.

The rate law, or rate equation, describes the relationship between the speed of the reaction and the reactant concentrations. In this equation, k is the rate constant, A and B are the two reactants, and m and n are their respective reaction orders. Reaction order describes the relationship between the concentration of a reactant and the rate and is not related to stoichiometry. It is vital to remember that the reaction order is not the same as the coefficient of the reactant in the balanced equation.

When a reaction involves two or more reactants, we must consider the reaction order for each reactant. The overall reaction order is equal to the sum of the reactants' reaction orders. For example, if reactant A is first-order and reactant B is zero-order, the overall reaction order is one. The most common reaction orders in simple reactions are zero-order, first-order, and second-order. Let's go through them using a unimolecular reaction as an example.

If the reaction is zero order, the reactant concentration has no effect on the reaction rate. Thus, the reaction rate is equal to k, the rate constant. A graph of the reaction rate with respect to concentration is a horizontal line.

In first order relationships, the reactant concentration is linearly related to the reaction rate. Thus, the rate is equal to the rate constant times the reactant concentration. A graph of the reaction rate with respect to concentration will be linear, with k as the slope.

If the reaction is second order, there is a quadratic relationship between the concentration and the rate. Thus, the rate is equal to the rate constant times the concentration squared. A graph of the rate with respect to concentration will be parabolic, with k as the slope.

The rate constant, k, is a temperature-dependent value that relates the activation energy of the reaction to the reaction rate. You will explore this in the next lab experiment.

Reaction rates are typically given in moles per liter per second. The rate has the same units, regardless of the reaction order. Thus, the rate constant has different units, depending on the reaction order. For example, if A is first order and B is zero order, then we have one instance of moles per liter in the rate equation. Therefore, the rate constant must be in inverse seconds.

So how do we determine reaction order? The reaction order must be determined experimentally using a series of tests. If you have two reactants, one method is to hold the concentration of one reactant constant, vary the other, and time how long it takes to make a certain amount of product. The same process is repeated for the second reactant. You can then estimate the order for each reactant by plotting rate versus the reactants' varying concentration, and seeing whether it looks like a zero, first, or second-order graph. Matching your data to the corresponding rate equation will also let you calculate k.

In this lab, you will determine the reaction orders of two reactants by varying their concentrations and timing how long it takes the reaction to progress to turning the solution opaque.

Print Procedure Steps

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Source: Smaa Koraym at Johns Hopkins University, MD, USA

Concentration Dependence
In this experiment, you will combine varying concentrations of aqueous HCl and sodium thiosulfate to make solid sulfur, which rapidly gathers into opaque yellow particles at a certain sulfur concentration. Since the reaction solution starts clear and colorless, you can easily tell when it reaches that concentration.

You'll use the same test tube and total volume each time, so it will take the same amount of sulfur to make each solution completely opaque. Thus, you'll measure the reaction progress by timing how long that takes. You'll then use that data to estimate the reaction orders of the individual reactants and of the overall reaction.

Before starting the lab, make a table listing the reactant concentrations, the time to solution opacity, and the solution temperature for the trials.

Table 1: Reaction progress

Trial [Na₂S₂O₃] (M) [HCl] (M) Time (s) Temp (°C)
1 0.1 M 3.0 M
2 0.1 M 3.0 M
3 0.2 M 3.0 M
4 0.15 M 3.0 M
5 0.05 M 3.0 M
6 0.1 M 6.0 M
7 0.1 M 4.5 M
8 0.1 M 1.5 M

Click Here to download Table 1

We want to make sure that the reactions are all taking place at room temperature. You'll perform two benchmark trials, three trials with different sodium thiosulfate concentrations, and three trials with different HCl concentrations. You will vary the concentrations by diluting stock solutions of sodium thiosulfate and HCl, as shown in the following tables.

Table 2: Sodium thiosulfate dilutions

Target Concentration Volume of 0.2 M sodium thiosulfate Volume of DI water
0.05 M 5 mL 15 mL
0.10 M 10 mL 10 mL
0.15 M 15 mL 5 mL
0.20 M 20 mL 0 mL

Click Here to download Table 2

Table 3: HCl Dilutions

Target Concentration Volume of 6.0 M HCl Volume of DI water
1.5 M 2.5 mL 7.5 mL
3.0 M 5.0 mL 5.0 mL
4.5 M 7.5 mL 2.5 mL
6.0 M 10 mL 0 mL

Click Here to download Table 3

Remember to use caution when handling HCl, which is toxic and highly corrosive. Sulfur dioxide, which is a gaseous product of this reaction, is also toxic. You will leave the reaction waste in the fume hood overnight to let the sulfur dioxide escape harmlessly.
- First, put on a lab coat, splash-proof safety glasses, and nitrile gloves.
- Fill a plastic wash bottle with deionized water and get a test tube marked with an X.
- Clamp the test tube over the center of a small stir plate. The label must be on the opposite side from you. Make sure that you can clearly see the X. Place a small magnetic stir bar in the test tube.
- Use a thermometer clamp to secure a glass thermometer in the lower third of the tube, keeping it clear of the X and the stir bar.
- Cut a sheet of about 20 squares of plastic paraffin film from a roll and bring the sheet back to your hood.
- Lay paper towels in the fume hood as a clean surface for labware to be reused.
- Label a 600-mL beaker as ‘waste’ for the reaction, a 100-mL beaker as ‘0.2 M sodium thiosulfate’, and a 50-mL beaker as ‘6 M HCl’.
- Fill the 100-mL beaker with 0.2 M sodium thiosulfate from the stock bottle in the dispensing hood.
- After that, bring the 50-mL beaker and a watch glass to the acid-dispensing hood to obtain 50 mL of 6 M HCl. Cover the filled beaker with the watch glass and bring your portion of HCl back to your hood.
- Prepare 20 mL of 0.1 M sodium thiosulfate for the first benchmark trial. Use a 10-mL volumetric pipette to measure 10 mL of 0.2 M sodium thiosulfate and dispense it into a 20-mL volumetric flask. Fill the 20-mL volumetric flask with deionized water to the mark.
- Then, cut a square of plastic paraffin film, seal the flask, and invert it several times to thoroughly mix the solution.
- Remove the film and pour the solution into the test tube.
- Prepare 10 mL of 3 M HCl. Pipette 5 mL of 6 M HCl into a 10-mL graduated cylinder. Add deionized water to fill the graduated cylinder to the 10-mL line.
- Seal the top of the graduated cylinder with plastic paraffin film and thoroughly mix the HCl solution by inverting it several times.
- Now, start the stir motor and get a reset stopwatch. Confirm that you can see the X through the solution in the test tube.
- Simultaneously start the stopwatch and pour the 3 M HCl into the test tube.
- Watch the X through the solution, which will start becoming cloudy when enough sulfur has been produced to saturate it. Stop the stopwatch exactly when the solution becomes so opaque that the X disappears.
- Record the time in your lab notebook for benchmark trial 1 and then reset the stopwatch. Also, record the temperature of the solution.
- Now, turn off the stir motor, remove the thermometer, and rinse it with deionized water into the waste beaker.
- Then, empty the test tube into the waste beaker, retrieve the stir bar with long forceps, and rinse the test tube and stir bar with deionized water.
- Thoroughly clean the test tube with a test tube brush. Briefly rinse the volumetric flask and graduated cylinder with deionized water as well. Dry everything with paper towels.
- Repeat the trial in the exact same way for the second benchmark time. If the time is not within 3 - 5 s of the first trial, repeat the benchmark trial until you have two consistent trials.
- Follow the same overall procedure for each subsequent trial, changing the dilutions as indicated in your table and cleaning your glassware between each trial. Use a 5-mL volumetric pipette as needed when measuring the sodium thiosulfate stock solution in trials four and five.
- Once you have performed all eight trials, rinse your glassware and tools one last time and neutralize the reaction waste with baking soda. Use caution because sulfur dioxide is one of the reaction products.
- Transfer the neutralized reaction waste to the designated waste container. Then, pour any remaining sodium thiosulfate solution into its labeled waste container and rinse the beaker with deionized water.
- Neutralize any remaining HCl with baking soda before flushing it down the drain with tap water.
- Finally, clean and put away your labware according to your standard practices and throw out any trash remaining in your hood. Your instructor will dispose of the reaction waste later.
Results
- Now, estimate the reaction order of sodium thiosulfate and HCl. Calculate the starting concentration of sodium thiosulfate in the reaction mixture for trials 1 through 5. Looking at the concentration of sodium thiosulfate and the reaction times, we see that as the concentration doubles, the time to opacity is halved.
  Table 4: Estimating reaction order sodium thiosulfate
  
  Trial [Na₂S₂O₃] added Na₂S₂O₃ volume (mL) Total volume (mL) [Na₂S₂O₃] in mixture Time (s) Inverse time (s^-1)
  1, 2 0.1
  3 0.2
  4 0.15
  5 0.05
  Click Here to download Table 4
- Remember that each trial used the same volume, so it took the same amount of sulfur to make each solution opaque. Thus, the reaction is twice as fast when the sodium thiosulfate concentration is doubled.
- When the concentration increases by a factor of 4, the reaction rate also increases by a factor of 4. This one-to-one relationship suggests that the reaction was first order for sodium thiosulfate. If so, we would expect a plot of rate versus sodium thiosulfate concentration to be linear.
- We didn't measure how much sulfur it took to turn the solution opaque, but we assume that it was the same for each trial. This lets us use the inverse of time to opacity as a stand-in for the rate. The slope won't be equal to k, but that's OK for this lab.
- Plot inverse reaction time as a function of sodium thiosulfate concentration. This confirms the linear relationship between sulfur production and thiosulfate concentration. Thus, the reaction seems to be first order with respect to thiosulfate.
- Use the same method to estimate the reaction order of HCl. First, calculate the starting concentration of HCl in the benchmark trials and in trials 6 through 8.
  Table 5: Estimating reaction order HCl
  
  Trial [HCl] added HCl volume (mL) Total volume (mL) [HCl] in mixture Time (s)
  1, 2 0.1
  3 0.2
  4 0.15
  5 0.05
  Click Here to download Table 5
- Then, look at the concentration of HCl and the reaction times for these trials. The reaction times are nearly identical, so the HCl concentration has no effect on the reaction rate. This means that HCl seems to be zeroth order for this reaction.
- Lastly, add up the reaction orders of the reactants to get the overall reaction order.

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Reaction Order

Sodium Thiosulfate

Hydrochloric Acid

Dilution Procedure

Trial	[Na₂S₂O₃] (M)	[HCl] (M)	Time (s)	Temp (°C)
1	0.1 M	3.0 M
2	0.1 M	3.0 M
3	0.2 M	3.0 M
4	0.15 M	3.0 M
5	0.05 M	3.0 M
6	0.1 M	6.0 M
7	0.1 M	4.5 M
8	0.1 M	1.5 M

Target Concentration	Volume of 0.2 M sodium thiosulfate	Volume of DI water
0.05 M	5 mL	15 mL
0.10 M	10 mL	10 mL
0.15 M	15 mL	5 mL
0.20 M	20 mL	0 mL

Target Concentration	Volume of 6.0 M HCl	Volume of DI water
1.5 M	2.5 mL	7.5 mL
3.0 M	5.0 mL	5.0 mL
4.5 M	7.5 mL	2.5 mL
6.0 M	10 mL	0 mL

Trial	[Na₂S₂O₃] added	Na₂S₂O₃ volume (mL)	Total volume (mL)	[Na₂S₂O₃] in mixture	Time (s)	Inverse time (s^-1)
1, 2	0.1
3	0.2
4	0.15
5	0.05

Trial	[HCl] added	HCl volume (mL)	Total volume (mL)	[HCl] in mixture	Time (s)
1, 2	0.1
3	0.2
4	0.15
5	0.05

Concentration Dependence

Table 1: Reaction progress

Table 2: Sodium thiosulfate dilutions

Table 3: HCl Dilutions

Table 4: Estimating reaction order sodium thiosulfate

Table 5: Estimating reaction order HCl

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