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Stephanie Chun March 5, 2010 2213-020

Internal Assessment – Investigating the relationship between concentration of reactants and rate of reaction using hydrochloric acid and magnesium strip

Research Question To study the effect of manipulating the concentration of hydrochloric acid on the rate of hydrogen gas production during the reaction with magnesium, using the pressure buildup by hydrogen gas. Introduction Concentration and surface area of reactant, temperature, and catalyst are the factors that affect the rate of a reaction. In this experiment the reaction between hydrochloric acid solution and magnesium ribbon is used to investigate the effect of reactant concentration on the rate of reaction. The concentration of hydrochloric acid is manipulated using serial dilution.

Rate of reaction can be calculated by measuring the speed at which reactants are consumed or the speed at which products are formed. Since gas is produced in this reaction, rate of reaction can be determined by the change in pressure as reaction proceeds. The pressure inside the test tube in which the reaction occurs is measured every second over 70 seconds using a gas pressure sensor. The rate of reaction at differing concentration is obtained by calculating the slope of time vs. pressure graph, since rate of reaction is change in pressure over time. If the change of pressure is significant the rate of reaction is high and if the change is small the rate is low. Hypothesis The collision theory must be used to study the relationship between concentration of a reactant and the rate of reaction. The collision theory states that in order for a reaction to occur, two particles involved must  collide with each other  the collision must be energetic enough to overcome the activation energy of the reaction  the collision must occur with the correct geometrical alignment, bringing the reactive parts of the molecules in contact Based on the collision theory, as the concentration of hydrochloric acid solution increases (which means that the number of hydrogen and chloride ions increase), the chances of magnesium particle colliding with the hydrochloric acid particles at a given time period also increase. Increase in the overall number of collision would increase the number of effective collisions, although the ratio between effective and noneffective collisions would stay constant. As such, the hypothesis for this experiment is the higher the concentration of hydrochloric acid, the higher the rate of hydrogen gas production.


Stephanie Chun March 5, 2010 2213-020 Variables Method of measuring/ controlling variable

Variable Measured

The pressure is measured using gas pressure sensor. The pressure of the test tube only during initial 70 seconds is measured to find the change in pressure. Pressure inside the test tube To reduce random error three measurements are made for each concentration of hydrochloric acid solution.

Dependent variable

Independent variable

Rate of reaction

The rate of reaction is calculated by dividing the change in pressure over a certain period of time. Thus in the pressure vs. time graph the slope is the rate of reaction.

Concentration of hydrochloric acid solution

Using 1M hydrochloric acid solution, a serial dilution is performed to obtain solutions of 08M, 0.4M, 0.2M, 0.1M and 0.05M concentration, using a pipette ( Âą 0.040 ) and volumetric flask ( Âą 0.12 )

Length of magnesium ribbon

2cm of magnesium ribbon is used each trial. The length is accurately measured using a ruler.

Size of the test tube

The size of the test tube used should be constant since change in volume would also change the pressure inside the container for the same amount of gas. A small sized test tube is preferred since using a large sized container would require a longer time for the pressure to increase.

Volume of hydrochloric acid solution used

10 of hydrochloric solution is used every trial. The volume used is accurately measured using a pipette.

Temperature of reactants

Temperature of reactants is kept constant by conducting the experiment at room temperature for every trial.

Controlled variables

Table 1: List of variables


Stephanie Chun March 5, 2010 2213-020 Apparatus and Materials     

100 volumetric flask ( ± 0.12 10 pipette ( ± 0.040 ) Gas pressure sensor probe Test tubes Test tube rack

 Magnesium ribbon  1M hydrochloric acid solution

)

Procedure Preparation of hydrochloric acid of different concentrations 1. 2. 3.

4.

80 of 1M hydrochloric acid solution is transferred into a 100 volumetric flask using a 10 pipette and distilled water is added up to the marked line. The volumetric flask is capped and inverted several times until a homogenous solution is made. 50 of the obtained solution is transferred into another 100 volumetric flask using a pipette and distilled water is added up to the marked line. The volumetric flask is capped and inverted several times. 0.8M hydrochloric acid solution is obtained. Procedure 3. is repeated four times to obtain 0.4M, 0.2M, 0.1M, 0.05M solution. The pipette is rinsed with the 0.8M solution obtained since the pipette has 1M solution droplets attached inside.

50

distilled water

50

0.8M

0.4M

0.2M

0.1M

0.05M

Figure 1: Serial dilution Conducting the experiment 5. 6. 7. 8. 9.

Magnesium ribbon is cleaned with sandpaper and 2 cm of the cleaned magnesium ribbon is obtained using a ruler. 10 of 0.8M hydrochloric acid solution is transferred into a test tube. The test tube is closed with a stopper immediately after magnesium ribbon is immersed into the solution. Gas pressure sensor connected to the stopper measures the pressure inside the test tube. The sensor is removed after 70 seconds. Procedure 5. to 8. is repeated three times for different concentrations of hydrochloric acid.


Stephanie Chun March 5, 2010 2213-020 Data Collection and Processing Quantitative Data

Hydrochloric acid concentration / 0.05 0.1 0.2 0.4 0.8

Change in pressure over time at different trials / T1

T2

T3

0.03414 0.06250 0.1429 0.2701 0.8813

0.04731 0.06205 0.1313 0.2692 0.9008

0.04300 0.05996 0.1330 0.2787 0.8686

Table 2: Change in pressure over time at different concentrations of hydrochloric acid concentrations1 Qualitative Data  Hydrochloric acid solution was colorless.  Magnetic ribbon turned to shiny silver color after it was cleaned using sand paper.  Effervescence occurred when magnetic ribbon was immersed into hydrochloric acid. The reaction became less rigorous as the solution concentration decreased. Processed Data Hydrochloric acid concentration /

Calculation

Average rate of reaction/

0.05

0.04148

0.1

0.06150

0.2

0.1357

0.4

0.2727

0.8

0.8836 Table 3: Calculation of average rate of reaction

1

The change in pressure over time was calculated by finding the slope of pressure vs. time graph using Logger Pro 3 software.


Stephanie Chun 2213-020

March 5, 2010 Chemistry SL

Data Presentation

Figure 2: Graph of pressure inside test tube against time for every trial of five different concentrations of hydrochloric acid solution2 2

Slopes of lines that have their slope value closest to the average slope value for each concentration are shown in boxes.


Stephanie Chun 2213-020

March 5, 2010 Chemistry SL

Concentration of hydrochloric acid, mol dm¯³ against rate of reaction, kPa s¯¹ 1 0.9 y = 1.126x - 0.0701 R² = 0.9629

Average rate of reaction/ ∆kPa s¯¹

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Concentration of hydrochloric acid solution/ mol dm¯³

Figure 3: Graph of average rate of reaction against concentration of hydrochloric acid solution

0.8

0.9


Stephanie Chun March 5, 2010 2213-020 Uncertainties Standard deviation Standard deviation was calculated and represented in the rate of reaction vs. concentration graph as error bars. Hydrochloric acid Change in pressure over time / concentration Average Standard deviation T1 T2 T3 / 0.05 0.1 0.2 0.4 0.8

0.03414 0.0625 0.1429 0.2701 0.8813

0.04731 0.06205 0.1313 0.2692 0.9008

0.04300 0.05996 0.1330 0.2787 0.8686

0.04148 0.06150 0.1357 0.2727 0.8836

0.006715 0.001356 0.006264 0.005244 0.01622

Table 4: Standard deviation at different concentrations Standard deviation calculation: [HCl] = 0.05

Same calculation was done for 0.1

, 0.2

, 0.4

, and 0.8

.

Uncertainty due to dilution of hydrochloric acid solution Hydrochloric acid concentration /

Uncertainties3 Volume of hydrochloric acid solution used/

0.05

Volume of distilled water used/

Total percentage error/ % 0.32% 0.32%

0.10

0.32%

0.20

0.32%

0.40

0.65%

0.80

Table 5: Uncertainties of solution concentration due to pipette and volumetric flask 3

Uncertainty due to 10 Uncertainty due to 100

pipette = Âą 0.040 volumetric flask = Âą 0.12


Stephanie Chun March 5, 2010 2213-020 Uncertainty due to gas pressure sensor The resolution of gas pressure sensor is 0.05 kPa. Thus the pressure value in pressure vs. time graph has an uncertainty of . The rate of reaction is change in pressure over time which can be expressed as below

when P represents pressure and t represents time. Therefore the uncertainty for values of rate of reaction is 0.05 + 0.05 = 0.1 kPa since each pressure value has an uncertainty of 0.05 kPa. The uncertainty for the average value of the rate of reaction should be calculated.


Stephanie Chun March 5, 2010 2213-020 Conclusion The relationship between the rate of a reaction and the concentration of reactants can be seen in Figure 3. The data (average rate of reaction at different concentrations) creates a linear regression line, which means that the rate of reaction is directly proportional to the concentration of reactants. Thus, as concentration of hydrochloric acid increases, the rate of reaction increases in linear manner. As such, the hypothesis is valid. Evaluation The results, average rates of reaction, are precise as shown by the standard deviation values in Table 4. However, the accuracy of the results can be doubted due to the uncertainties of the pipette, volumetric flask, and gas pressure sensor used. The lack of accuracy of the data is shown in Figure 3, the graph of rate of reaction against concentration of hydrochloric acid solution. Although the general trend is linear, proving the hypothesis to be valid, it can be seen that the values are lower than they should be, because the linear regression line does not pass the origin. The following are parts of the experiment that may have made the data results less accurate. Error

Using 10 pipette to transfer 80 of solution

Impact Transferring a solution eight times using 10 pipette to obtain 80 of the solution would result in greater error than transferring the solution once, since error occurs every time a solution is transferred.

Improvements

50 pipette or graduated cylinder can be used.


Rate of reaction between magnesium with acid