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Collision Theory Part 4. Investigating the Effect of Surface Area on Reaction Rates

Soomin Bae




Introduction


Zinc reacting with Hydrochloric acid is one of most frequently used chemistry class demonstrations. Due to its fastly-occuring nature with noticeable changes – such as bubble formation and gas evolution – it is perfect to introduce students to the reaction between an acid and a metal. In Chemistry, I learned about kinetics and the components that can accelerate chemical reactions. Thinking back at this reaction, I began to wonder about how the surface area of Zinc could affect the rate of reaction with HCl. This is mainly because during lab demonstrations, I often noticed how Zinc metals pieces that were cut into smaller pieces reacted much faster with HCl. This leads to my research question: How does the surface area of Zinc affect the initial rate of volume of H2 gas formed over time measured by a gas syringe when reacted with HCl?




Background


Zinc is often misunderstood as a transition metal due to its position in the periodic table. However, because Zinc has a completely filled d-orbital, it is not a transition metal. As a result, unlike transition metals that have multiple oxidation states, Zinc only forms 2+ ions. The process of Zinc losing its 2 electrons to form Zn2+ ion when reacted with HCl is shown by this chemical equation.

Zn (s) → Zn2+ (aq) + 2e–

In an aqueous hydrochloric acid solution, the hydrochloric acid dissociates into hydrogen ions (H+) and chloride ions (Cl-) as shown by the chemical equation.

HCl (aq) → H+ (aq) + Cl –(aq)

During the reaction, when Zn2+ cations meet Cl– anions, they react by transferring electrons in order to become more stabilized. Since Zn2+ ions become more stable with 2 more electrons in its outermost shell due to the octet rule, 2 Cl– ions are needed. As a result, ZnCl2 (aq) is formed. On the other hand, the electrons lost by Zinc atoms are gained by H+ cations. Since each H+ cation gains only 1e– to fill its valence electron shell, 2 H+ cation is needed to react with 2 e– from Zinc. When the hydrogen ion reacts with the electrons from Zinc, hydrogen gas (H2) is formed. This is shown by the chemical equation

2H+ (aq) + 2e– → H2(g)

Therefore, the overall chemical reaction between Zinc and Hydrochloric Acid is as shown below:

Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g)


Rate of reaction is defined as the change in reactant or product concentration per unit time. One of the indirect ways to measure rate of reaction is measuring the rate of formation of gas. Since Zinc and HCl reaction forms H2 gas as one of its products, the rate of reaction can be indirectly calculated through the gas syringe method. By recording the volume of H2 gas formed during a set time interval, the initial rate of formation of H2 gas can be calculated.


According to collision theory, for a reaction to occur, particles must collide with each other in an appropriate orientation and kinetic energy greater than or equal to activation energy. Thus, a greater rate of reaction means a greater frequency of the collisions happening with the 2 required factors aforementioned. Another factor that affects the rate of reaction, which is going to be the main focus of this experiment, is surface area. It is because with a greater surface area of Zinc, there are more areas where Zinc and HCl can collide. With an increased collision frequency, the frequency of successful collisions would increase, leading to an overall faster rate of reaction, and thus a greater volume of H2 gas formed per unit time. As a result, theoretically, when the Zinc pieces are cut into smaller pieces, the surface area would increase and lead to more volume of H2 gas formed per unit time. In this experiment, I will be using a Zinc metal strip that is 20mm long. Then, for each trial, I would cut the whole piece into 2 pieces, 3 pieces, 4 pieces, and 8 pieces. The resulting length for one Zinc metal would be: 1) 20mm, 2) 10mm, 3) 6.7mm, 4) 5mm, and 5) 2.5mm. (In this experiment, I will be assuming that the metal Zinc Strip has a width of 0.5mm)




Materials and Methods


Hypothesis: Zinc metals with a larger surface area (cut into more pieces) would have a faster rate of reaction and thus form more Hydrogen gas per unit time.


Variables:

  • Independent variable: Surface area of Zinc metal (1 whole piece, cut into 2 pieces, cut into 3 pieces, cut into 4 pieces, cut into 8 pieces)

  • Dependent variable: Volume of H2 gas formed per unit time

  • Control variable:

    Concentration of HCl (aq)

The concentration of HCl should be constant throughout the entire experiment – as 1.0 mol dm-3. This is because the concentration of the HCl solution can directly affect the rate of reaction with Zinc and HCl by either accelerating or decelerating it. A higher concentration means there are more moles of HCl molecules in the 10 cm3 HCl aqueous solution. Consequently, with more moles available for collision, the frequency of the successful collision between HCl and Zinc would increase leading to a faster rate of reaction. Thus, in order to minimize the change of rate of reactant due to the concentration of HCl, the concentration will be kept constant throughout the entire experiment. This will be done by using the same 1.0 mol dm-3 HCl solution from the stock solution.

Total mass of Zinc used

A greater mass of Zinc would lead to an increase in rate of reaction with HCl since there are more moles available for collision. As a result, if it were not to be controlled, the rate of reaction between different surface areas of Zinc would not be reasonable to be compared. Thus, I will be controlling the mass of Zinc by cutting the original Zinc pieces by the same size before cutting them into smaller pieces– and weighing them on an electronic balance to ensure they are constant.

Temperature of the HCl solution

In order to keep the temperature constant, the experiment will be conducted in a water bath tub set to the room temperature. This is because temperature is an important variable that needs to be kept constant as it can significantly impact the rate of reaction. An increase in temperature means an increase in the average kinetic energy of all molecules in the given container. With HCl molecules moving faster, the frequency of their collisions with Zinc metal molecules will increase. This would in turn lead to a faster rate of reaction since the number of successful collisions would increase as well.

Time intervals for recording the volume of gas

Using the gas syringe method requires recording the volume of H2 gas produced per unit time. Therefore, to effectively compare the initial rate of the formation of H2 gas, it is necessary to record the volume of H2 gas in a set time interval. For this experiment, the volume of H2 gas in the gas syringe would be measured every 5 seconds for 2 minutes.


Materials & Apparatus:

700 mm Zinc metal strip (0.1mm)

300 cm3 Hydrochloric Acid (1.0 mol dm-3)

10 cm3 Graduated Cylinder (0.1cm3)

50 cm3 Conical Flask (5 cm3)

10 cm3 Gas syringe ( 0.5 cm3)

Electric balance ( 0.01 g)

Rubber stopper with a hole for the syringe

A timer (1s)

A scissor

A 100 mm ruler (0.1mm)

A thermometer (0.1℃)

Stand and Clamp

Safety goggles and gloves


Safety:

Hydrochloric acid in contact with the skin and eye can cause severe damage such as chemical burns and blindness. (“Hydrochloric Acid Poisoning: MedlinePlus Medical Encyclopedia”) Although diluted Hydrochloric acid with concentration less than 2.7 mol dm-3 is considered relatively non-hazardous, it can still cause harm as mentioned above. (CLEAPSS) Thus, during the entire duration of using Hydrochloric acid, safety goggles and gloves should be worn. In the case of contact with the skin, rinse it thoroughly under running water for 10 minutes, and if contacted with the eye, make sure to use the eyewash immediately for 10 minutes. Zinc, in the metal form, is relatively non-toxic. (Teck) However, when reacted with strong acids such as Hydrochloric acid, Zinc produces hydrogen gas that is highly flammable. As a result, it is important to conduct the experiment away from sources of ignition or heat, in a hume hood.


Procedure:

Preparation of the Zinc metals:

Cut a piece of Zinc metal that is 20 mm long

For the first manipulation, use the Zinc metal as a whole

For the second to fifth manipulation, using a scissor and a 100 mm ruler, cut the Zinc metal into

2 equal pieces, each length being 10 mm

3 equal pieces, each length being 6.7mm

4 equal pieces, each length being 5mm

5 equal pieces, each length being 4mm


Set up:

  1. Measure 10.0 cm3 of 1.0 mol dm-3 HCl solution from the stock solution with a graduated cylinder into a conical flask

  2. Record the room temperature, and set up the water bathtub making sure the temperature matches the room temperature

  3. Record the temperature of the solution with a thermometer and make sure the temperature matches the water bathtub and room temperature

  4. Using an electric balance, record the mass of the total Zinc metals used, making sure they are constant for all trials

  5. Set up the gas collection apparatus

  6. Using a stand and clamp, clamp the gas syringe 30 cm above the ground and make sure it is perfectly horizontal

  7. Connect the rubber stopper with the gas syringe and its hose




Results


The experiment demonstrated that as the surface area of Zinc increased (achieved by cutting the Zinc metal into smaller pieces), the initial rate of reaction, as indicated by the volume of hydrogen gas produced over time, also increased. For example, with the Zinc piece cut into eight smaller sections, a greater initial volume of H₂ gas was recorded in the gas syringe compared to the uncut Zinc strip. This trend was consistent across all trials, showing a clear relationship between increased Zinc surface area and faster hydrogen gas production. The highest gas volumes were observed within the first 30 seconds across trials, aligning with the theory that increased surface area promotes higher collision frequency between Zinc and hydrochloric acid molecules.


Conclusion


The hypothesis was supported by the findings, confirming that Zinc with a larger surface area reacts more rapidly with hydrochloric acid, leading to a greater initial rate of hydrogen gas formation. This outcome is consistent with the collision theory, as the increased surface area provides more opportunities for Zinc atoms to collide with HCl molecules. This finding illustrates the impact of surface area on reaction rate, with potential applications in processes that benefit from faster reaction rates through surface manipulation of reactants. Further research could investigate additional variables, such as the impact of different acid concentrations or temperatures, to explore broader kinetic relationships.

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