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Benjamin Johnson Juadi

Ibu Evi Sinaga

Science 10D

14 February 2024

Rate of Chemical Reaction Through the Disappearing Cross Experiment

I. INTRODUCTION

The chosen investigation indulges in the rate of reaction using the Disappearing Cross

Experiment. The process of investigation involves HCL (hydrochloric acid), exploring its

concentration and its effects on the rate of reaction when combined with Na₂S₂O₃ (sodium

thiosulfate). S (sulfur) is the product yielded by the chemical reaction, measured by time

(seconds) as it turns opaque, eventually covering the 5cm cross. The concentration of HCl is

modified by a variety of increments used to commence the investigation, analyzing the rate of

reaction through a cross placed under the beaker. The concentration of hydrochloric acid is

modified by 5 increments measured by mol/dm ⁻³, varying at measurements 0.1, 0.2, 0.3, 0.4,

0.5. The concentration of each increment will be altered by the proportion of HCl to distilled

water while maintaining a volume of 10cm³. The result of each increment is averaged by three

trials to achieve the most accurate outcome, using the formula: (T1 + T2 + T3) / 3. The averaged

time (s) taken for the cross to disappear is recorded as a method of measurement, determining the

rate of reaction based on the different increments.

The balanced equation of the chemical reaction used for the experiment is:

Na₂S₂O₃ (aq) + 2HCl (aq) ® 2NaCl (aq) + SO2 (g) + S (S) + H2O (L)

Rate of reaction can be defined as the speed of product formation from reactants in a

reaction. When measured and recorded, the rate of reaction provides insight of the time frame
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required to complete a chemical reaction. Some reactions can be instant, while others take time

to reach final equilibrium. A major factor that affects the rate of reaction is the concentration of a

reactant. According to the Collision Theory, the higher the concentration of a reactant, the faster

the rate of reaction (BJYU’S, par. 2 and 8). Effective collision based on the Collision Theory is

the breaking and forming of bonds and atoms that result in the formation of new chemical

products. Additional factors that affect effective collision includes the physical state of reactants,

temperature, and the presence of a catalyst. The physical state of reactants affects the rate of

reaction through the surface area that are in contact with other reactants. For example, a larger

surface area of a physical object will increase the contact with reactants, increasing the rate of

reaction. Temperature plays a role in the rate of reaction through the increase of kinetic energy,

which then lowers the minimum energy required for an effective collision. A catalyst is also a

highly effective factor in increasing the rate of reaction. The presence of a catalyst accelerates a

reaction without being consumed as a reactant in the chemical reaction (Key, par. 4-7).

The formula used to find the rate of reaction is as follows:

Rate of Reaction = 1 / Time

Rate of reaction is often calculated using the included formula above. In the formula,

time represents the time required to achieve a certain point. The point this investigation aims to

achieve is the complete reaction where the solution turns opaque, and the cross is no longer

visible. With that said, time can also be expressed as the rate of reaction or the time taken to

complete a chemical reaction (BBC, par. 3).

Knowing the rate of a reaction is highly applicable into daily activities. Understanding

the rate of reaction allows individuals to estimate the amount of time required for a reaction to

occur, like in cooking. Cooking is a science as it mixes different ingredients and utilizes heat to
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blend it all together. Controlling the temperature and number of materials/catalysts used to create

a dish involves understanding the rate of reaction. A good comprehension over the rate of

reaction provides a complete understanding over the ability to adjust and control conditions to

achieve the desired outcome (Helmenstine, par. 20).

II. RESEARCH QUESTION

How does the concentration of hydrochloric acid, varying between the increments of 0.1,

0.2, 0.3, 0.4, 0.5 mol/dm ⁻³ averaged by three trials, affect the rate of reaction measure by

time (seconds) to obfuscate the visibility of the 5cm cross placed under the beaker while

maintaining the concentration of sodium thiosulfate at 3.16 g/dm⁻³?

III. HYPOTHESIS

Null Hypothesis:

The alteration of hydrochloric acid and its concentration has no effect on the rate of

reaction. All increments (0.1, 0.2, 0.3, 0.4, 0.5 mol/dm ⁻³) of hydrochloric acid, per 3 trials each,

will have the same outcome, with the rates of reaction all being the same.

Alternate Hypothesis:

With the addition of each increment, through the alteration of the proportion of HCl to

distilled water, to the concentration of hydrochloric acid, the rate of reaction will increase. The

first increment of 0.1 mol/dm ⁻³ will show the slowest timed chemical reaction, slowly turning

the mixture opaque and covering the cross placed under the beaker. On the other hand, the last

increment of 0.5 mol/dm ⁻³ will show the fastest times chemical reaction, quickly turning the
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mixture opaque and covering the cross placed under the beaker. With the increase of the

concentration, the rate of reaction will gradually grow due to the higher frequency of molecule

collisions. The concentration which a higher frequency of collisions will result in a faster rate of

reaction as the molecules collide at a speed faster compared to the solution with lower

concentration (Dillon, par. 7).

IV. VARIABLES

Independent Variable:

The independent variable used in this investigation is the concentration of hydrochloric

acid (HCl). This variable will be investigated using 5 different increments, 0.1, 0.2, 0.3, 0.4, 0.5

mol/dm ⁻³ each one ranging 0.1 mol/dm ⁻³. The concentration of the independent variable will be

altered by the proportion of HCl to distilled water used. The volume of the HCl solution should

remain consistent throughout all 15 trials at the measurement of 10cm³. A limited range given by

the constraints of the teacher limit the increments from ranging any larger. With that being said,

it can be assumed that the range of the data points are chosen to allow consistent data analysis,

ensuring that the reaction is sufficient enough be analyzed and investigated but not too large in

order to ensure safety while conducting the experiment. Calculating the concentration of HCl is

as follows:

Calculation of Concentration (amount of HCl required per increment) = C1V1 = C2V2

In the formula above, C1 is the initial concentration while V1 is the initial volume. On the

other hand, C2 is the final concentration while V2 is the final volume. The formula calculates the

amount of HCl required to achieve a certain increment in order to balance it with the distilled

water.
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Example: The target increment is 0.1. Using the formula above, it can be calculated that

the required amount of HCl to achieve the concentration of 0.1 is 2ml. The calculation is as

follows:

0.5 x V1 = 0.1M x 10mL (1)

V1 = (0.1 x 10)/0.5 = 2 (2)

2ml (3)

Dependent Variable:

The direct dependent variable being investigated in this experiment is the rate of reaction.

Rate of reaction, or the time taken for the solution to turn opaque, will be measured by time,

specifically in the unit of seconds. A stopwatch will be used during this part of the investigation

to get close-to-accurate raw data points from the experiment. The stopwatch will begin as soon

as the two solutions have been combined and will end as soon as the 5cm cross placed under the

beaker is no longer visible. To ensure the viability of the raw data being processed from the

investigation, three trials will be conducted per increments, totaling to 15 trials. The three total

trials per increment will then be averaged out to get an accurate outcome using the formula:

Average Time per Increment = (T1 + T2 + T3) / 3

To process the information and obtain the dependent variable of the rate of reaction, the

following formula will be used:

Rate of Reaction = 1 / Time taken for cross to disappear

By calculating and processing the data obtained from the investigation, the effects of the

concentration of hydrochloric acid when in contact with sodium thiosulfate will be revealed.
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Controlled Variable:

- Size of Cross: By maintaining the size of the cross at 5cm, the experiment is preventing

accidental, additional variability that has to be considered when determining the effects of

concentration in the rate of reaction. A larger cross would require more time to be

obscured while a smaller cross would take less time, affecting the observer’s ability to

determine a constant and accurate time frame. To maintain this variable, the same 5cm

cross will be used across all 15 trials to ensure that consistency remains throughout the

investigation.

- Concentration of Sodium Thiosulfate: The concentration of sodium thiosulfate is meant

to remain the same throughout the investigation, in each trial. Maintaining the same

concentration avoids the possibility of additional factors that have to be considered. By

maintaining the same concentration of sodium thiosulfate, an accurate outcome of the

rate of reaction and how it is affected by the various concentrations of hydrochloric acid

can be achieved. To maintain this variable, the same solution will be used for all 15 trials

from the same bottle to ensure stable molarity, assuring that the rate of reaction will only

be affected by the concentration of hydrochloric acid.

- Temperature: A change in temperature will affect the rate of reaction and the overall

outcome of the investigation heavily. Temperature has a major influence in the rate of

reaction as more heat increases kinetic energy, thus the higher frequency of collisions

(Murillo, par. 1). To maintain the temperature of the room where the experiment is being

conducted, the same room will be used throughout all 15 trials and all heat sources will

be avoided. The area of experimentation should ideally remain at room temperature and a

thermometer may be used if required.

- Volume of Solution used in Experiment: There are two solutions in this investigation that

require attention as their volume will affect the outcome of the experiment. These two

solutions are the HCl and distilled water solution with the requirement of maintaining the

volume at 10ml and the sodium thiosulfate and distilled water solution that will be

decided by the teacher. Maintaining the volume of the solution, specifically the ratio of

solution x to solution y, when identifying the rate of reaction is crucial as different

volumes will have different rates of reaction. To maintain this variable, a measuring

cylinder and pipette will be used to achieve an accurate measurement of volume.

- Observer Consistency: Having various observers who conduct the experiment and collect

data can affect the experiment as each individual possesses a different ability in

identifying time points accurately and their perspective towards the outcome. Different

strands of opinions will cause inconsistency in the results of the investigation, adding

further factors that require consideration when calculating the rate of reaction. To control

this variable, the same person/people will be used to conduct the experiment throughout

all 15 trials, ensuring that all data analysis and collection remains consistent.

V. APPARATUS

Materials Size Amount

Sodium Thiosulfate 3.16g N/A

Hydrochloric Acid (2ml x 3) + (4ml x 3) + (6ml N/A

x 3) + (8ml x 3) + (10ml x 3)

= 90ml

Distilled Water 1500ml N/A

Paper A4 2

Pen N/A 1

Conical Flask 250ml 1

Stopwatch N/A 1

Measuring Cylinder 30ml & 50ml 2

Lab Coat N/A 1

Safety Goggles N/A 1

Rubber Gloves N/A 1

Ruler 30cm 1

Camera N/A 1

Pipette N/A 1

VI. METHOD

1. Gather all material required to commence with the experiment. Ensure that preparation is

complete before beginning the investigation – beakers, conical flasks, paper, pen, ruler,

stopwatch, pipette, & liquid solutions.

2. Use protective measures – gloves, goggles, lab coat.

3. Create experimental setup/arrangement – arrange all material in an orderly fashion to

avoid any spills or accidents.

4. Create a 5cm cross using the paper, pen, and ruler. Ensure that the paper does not get wet

as it will be used for all 15 trials to maintain consistency.

5. Prepare a second sheet of A4 paper, a pen, and the stopwatch nearby to take note as soon

as the experiment begins.

6. Begin by measuring out 50ml of the sodium thiosulfate solution prepared by the teacher

and place it into the conical flask.

7. The investigation will begin with the first increment (concentration of 0.1). Pour 2ml of

hydrochloric acid into the measuring cylinder.

8. Pour exactly 8ml of distilled water (use a pipette if required) into the measuring cylinder

with the hydrochloric acid to form a new solution. Lift the flask and stir anti clockwise

exactly three times in a constant manner.

9. Combine the solution in the measuring cylinder (HCl and distilled water) with the

solution in the conical flask. Begin the timer as soon as the two solutions make contact.

10. Place the conical flask directly on top of the paper cross, try to keep it as centered as

possible.

11. Observe the chemical reaction from a safe distance and wait for the solution to turn

opaque, preferably at a 180 degree angle (directly above) to have an accurate vision over

the visibility of the cross).

12. Once the cross is no longer visible, stop the timer and take note of the time point.

13. Rinse any material before reusing it to prevent additional factors to be considered and any

effect on the rate of reaction.

14. Repeat steps 6 to 13 with the remaining increments (0.2, 0.3, 0.4, and 0.5).

Fig.1.Diagram of experiment (Juadi, 2025)

VII. SAFETY GUIDLINE FINISH THIS & USE A SOURCE

1. Use a lab coat to prevent direct contact of skin and chemicals. Also prevents the damage

of clothing.

2. Wear safety goggles to prevent contact of eyes and chemicals.

3. Wear gloves to prevent direct contact of skin and chemicals, preventing burns or

irritation.

4. Handle all solutions with care, ensuring the safety of the fragile vessels used to conduct

the experiment (beakers, measuring cylinders) and to prevent chemical spills. Neutralize

any chemical spills using baking soda (Hunter, par. 3).

5. Avoid direct skin contact with sodium thiosulfate and wash hands after handling. Skin

contact with hydrochloric acid can cause eye damage, severe burns, and stomach

problems. Ensure that all safety equipment is used at all times when handling the solution

(ATSDR, par. 98).

6. Conduct experiment in an open space with ventilation to prevent any damage to the lungs

as it can cause irritation, coughs, and shortness of breath (ATSDR, par. 98). Conducting

the experiment in an open space can also prevent accidental spills.

7. Discard of all chemical waste properly. Follow teacher’s instructions and the lab layout to

find proper disposal areas after completing the investigation.

8. Throw non-reusable safety equipment, like rubber gloves, away and ensure that it is not

misplaced. Wash all reusable equipment such as goggles and lab coats as well.

9. Wash hands and clean up all material to prevent post-investigation accidents to conclude

the experiment.
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Works Cited

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Feb. 2019, byjus.com/chemistry/rate-of-reaction/. Accessed 11 Feb. 2025.

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Key, Jessie A. “Factors That Affect the Rate of Reactions.” Opentextbc.ca, BCcampus, 16 Sept.

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