Effect of Changing Concentration on the Order of Reaction

1. Title : An Investigation into the Effect of Changing the Concentration by Continuous Method on the Order of Reaction

2. Planning

A. Hypotheses

I predict that the rate of reaction is influenced by the concentration of the reactants; and that the rate of reaction and the concentration of reactants decrease as time intervals increase. I predict that the order of reaction is affected or influenced by the concentration of the reactant. I also predict that the change in concentration of reactants affects the rate of reaction proportionately and hence, the order of reaction. Such that, if the concentration of the reactants is reduced by half, the rate of reaction will also decrease by a certain proportion.

The reaction that will be analyzed in this experiment is between a metal (magnesium ribbon) and an acid (sulfuric acid). The reaction is shown by the chemical equation below:

Mg (s) + H2S04 (aq) → MgS04 (aq) + H2 (g)

In this experiment, 0.15 grams of magnesium ribbon will be used in excess, together with 10 cubic centimeters of 0.3 M sulphuric acid (H2SO4). The continuous method will be used, where the metal is in excess and the reaction goes to completion. The volume of the hydrogen gas (H2) will be collected at a measured time interval of 30 seconds. These volume readings at the time intervals shall be recorded and analyzed.

III. Background

The rate of a reaction depends in part on the concentrations of the reactants. The rate at which a reactant or reactants is transformed into products is the change in concentration of the reactant or reactants with time, (Wilbraham, et.al., 1997). Mathematically, the rate can be expressed as:

Rate = Change in Concentration of Reactants / Change in time

 

The formula implies that the rate of disappearance of the reactants is proportional to its molar concentration. Hence:

Change in Concentration of Reactants / Change in time = proportional to the concentration of reactants

 

The reaction rate has to be experimentally determined. From the reaction rate equation, the order of reaction can be obtained. The order of reactions can be classified as zero order, first order, or second order, with respect to only one reactant.

IV. Risk Assessment

In this experiment, sulphuric acid (H2SO4), will be used as a reactant. Since it is a strong acid and is very corrosive, I will observe the following precautionary measures:

1. Wear protective goggles for the eyes.
2. Avoid pipetting the sulphuric acid by mouth.
3. Avoid inhalation of sulfuric acid.
4. Care in handling the acid should be observed. It should be prevented from being knocked over.

V. Fair Test

To ensure that the results that I will obtain in the experiment are reliable and accurate, I will observe the following:

1. The volume measurements of the sulphuric acid reactant will be made very accurately by reading from the lower meniscus of the 10 cubic centimeter mark
2. The bung will be securely and tightly placed to prevent the collected hydrogen gas (H2) from escaping.
3. The weight measurements of the magnesium ribbon will be made very accurately.
4. All materials will be thoroughly cleaned before each use in order to prevent contamination.
5. The experiment will have 3 runs or replicates in order to attain a high reliability of results.

VI. Procedure of the Experiment:

1. Materials needed:

Stopwatch for time interval measurements

100 cc gas syringe for the Hydrogen gas collection

100 cc conical flask for the sulphuric acid

100 ml graduated cylinder for measuring the sulphuric acid

Analytical balance for measuring 0.15 grams of magnesium ribbon

2. Procedure:

1. Set up the materials while making sure that they are thoroughly clean and dry.

2. Using a graduated cylinder, measure 10 cc of 0.3 Molar concentration of sulphuric acid.

3. Carefully weigh 0.15 grams of magnesium ribbon using an analytical balance to make sure that the weight measurement is accurate.

5. Put the 0.15 grams of magnesium ribbon into the conical flask with the sulphuric acid.

7. Measure the volume in cubic centimeters of hydrogen gas collected in the gas syringe at every time interval of 30 seconds.

8. Record the volume of collected hydrogen gas at each set time interval.

9. Make 2 more runs of this experiment by repeating steps 1-8 at every run.

VII. Results.

Below are the tabulated volume measurements at every time interval:

Table 1: Volume Measurements for Run 1

Time /s	Volume of gas evolved Vtotal/cm3	(Vfinal-Vt)/cm3
0	0	74.5
30	15	59.5
60	23	51.5
90	31.5	43
120	43	31.5
150	49.5	25
180	53.5	21
210	57	17.5
240	61	13.5
270	62.5	12
300	65	9.5
330	67	7.5
360	69	5.5
390	69.5	5
420	70	4.5
450	70.5	4
480	71	3.5
510	71.5	3
540	72	2.5
570	72	2.5
600	72.5	2
630	73	1.5
660	73	1.5
690	73	1.5
720	73.5	1
750	73.5	1
780	73.5	1
810	74	0.5
840	74	0.5
870	74	0.5
900	74	0.5
930	74.5	0
960	74.5	0
990	74.5	0
1020	74.5	0

The data in Table 1 were plotted in a graph below:

Graph 1: Volume vs. Time (Run1)

For Run 2, the data were obtained were tabulated below:

Table 2: Volume Measurements for Run 2

Time /s	Volume of gas evolved (Vtotal/cm3)	(Vfinal-Vt)/cm3
0	0	74
30	15	59
60	25.5	48.5
90	34.5	39.5
120	42.5	31.5
150	47	27
180	51.5	22.5
210	55.5	18.5
240	58.5	15.5
270	61.5	12.5
300	63.5	10.5
330	66	8
360	67.5	6.5
390	68.5	5.5
420	69.5	4.5
450	70.5	3.5
480	71	3
510	72	2
540	72	2
570	72.5	1.5
600	72.5	1.5
630	73	1
660	73	1
690	73	1
720	73.5	0.5
750	73.5	0.5
780	73.5	0.5
810	73.5	0.5
840	74	0
870	74	0
900	74	0
930	74	0

The data in Table 2 were plotted in a graph below:

Graph 2: Volume vs. Time (Run 2)

For Run 3, the data were obtained were tabulated below:

Table 3: Volume Measurements for Run 3

Time /s	Volume of gas evolved (Vtotal in cm3)	(Vfinal-Vt in cm3)
0	0	73.5
30	16.5	57
60	28	45.5
90	37	36.5
120	45	28.5
150	50	23.5
180	54	19.5
210	57.5	16
240	60	13.5
270	63	10.5
300	65	8.5
330	67	6.5
360	68.5	5
390	69.5	4
420	70	3.5
450	70.5	3
480	70.5	3
510	71	2.5
540	71	2.5
570	71	2.5
600	71.5	2
630	71.5	2
660	72	1.5
690	72	1.5
720	72	1.5
750	72.5	1
780	72.5	1
810	72.5	1
840	73	0.5
870	73	0.5
900	73.5	0
930	73.5	0
960	73.5	0
990	73.5	0

The data in Table 3 were plotted in a graph below:

Graph 3: Volume vs. Time (Run3)

All three graphs show the trend that as time increases, the volume of the reactant decreases, while the volume of the product increases.

B. Calculation of the Concentration of Reactant

From the reaction:

Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (aq)

Concentration is measured in terms of Molarity, where Molarity is equal to the number of moles of solute divided by liters of solution:

Molarity = No. of moles of solute / Liters of solution

For Magnesium (Mg), 0.15 grams were added to sulphuric acid. The number of moles is equal to: weight in grams divided by the formula weight. Hence, the number of moles = wt. in grams / FW. Since 0.15 grams were used, 0.15 grams should be divided by the formula weight of Magnesium (Mg), which is 24.

So: 0.15 grams / 24 = 0.00625 moles for Mg.

To get the molar concentration, the number of moles will be divided by the volume of the solution in liters, which is 0.01.

So, the molarity of Mg is : 0.00625 / 0.01 = 0.625 M

For the sulphuric acid (H2SO4) used, the molarity is 0.3 M. The number of moles of H2SO4 used is determined by multiplying the molar concentration by the volume of solution in liters. Hence: 0.3 moles/L X 0.01L = 0.003 moles.

From the chemical equation of the reaction, for every 0.003 moles of H2SO4 used, the same amount of moles (0.003) of hydrogen gas (H2) is given off.

The molar concentration of the hydrogen gas evolved can now be determined for each respective time interval, by dividing the number of moles by the volume obtained.

For run 1:

The concentration of the product can now be calculated, using the formula for reaction rate, where: Molarity = Number of moles / Liters of solution

The data for the concentration of the hydrogen gas are tabulated below:

Table 4: Molarity of the hydrogen gas (H2)

time /s	Volume of gas evolved Vtotal/ L	Molarity (mol/L)
	0
30	0.015	0.200
60	0.023	0.130
90	0.032	0.093
120	0.043	0.069
150	0.049	0.061
180	0.053	0.056
210	0.057	0.052
240	0.061	0.049
270	0.0625	0.048
300	0.065	0.048
330	0.067	0.044
360	0.069	0.043
390	0.0695	0.043
420	0.070	0.043
450	0.0705	0.043
480	0.071	0.042
510	0.0715	0.042
540	0.072	0.042
570	0.072	0.042
600	0.0725	0.041
630	0.073	0.041
660	0.073	0.041
690	0.073	0.041
720	0.0735	0.041
750	0.0735	0.041
780	0.0735	0.041
810	0.074	0.041
840	0.074	0.041
870	0.074	0.041
900	0.074	0.041
930	0.0745	0.040
960	0.0745	0.040
990	0.0745	0.040
1020	0.0745	0.040

The molarity values tabulated above are plotted in the graph below:

Graph 4: Concentration vs. Time (Run1)

Graph 5: Concentration vs. Time (Run 1) Showing the Gradient of the Tangent Curve

 

The rate of reaction was calculated by measuring the gradient of the tangent of the curve above, as depicted by the red line. Hence, the gradient of the tangent is equal to 0.12 – 0.05 / 150 = 0.00046. From the rate equation, the rate constant (k) can be obtained by the formula: k = rate x [H2SO4]. So: k = (0.00046) / 0.3 = 0.153. The rate equation is:

Rate = k [H2SO4]

Rate = (0.153) (0.003) = 0. 000459 or 0.0046. Hence, this value is equal to the gradient of the tangent of the graph above. This shows that the rate is directly proportional to the concentration of the sulphuric acid (H2SO4).

Table 5: Concentration of Gas Collected and Rate of Reaction (Run 1)

Time /s	Molarity (mol/L)	Rate (M/T)
0	0	0
30	0.200	0.0067
60	0.130	0.0022
90	0.093	0.0010
120	0.069	0.0006
150	0.061	0.0004
180	0.056	0.0003
210	0.052	0.0002
240	0.049	0.0002
270	0.048	0.0002
300	0.048	0.0002
330	0.044	0.0001
360	0.043	0.0001
390	0.043	0.0001
420	0.043	0.0001
450	0.043	0.0000
480	0.042	0.0000
510	0.042	0.0000
540	0.042	0.0000
570	0.042	0.0000
600	0.041	0.0000
630	0.041	0.0000
660	0.041	0.0000
690	0.041	0.0000
720	0.041	0.0000
750	0.041	0.0000
780	0.041	0.0000
810	0.041	0.0000
840	0.041	0.0000
870	0.041	0.0000
900	0.041	0.0000
930	0.040	0.0000
960	0.040	0.0000
990	0.040	0.0000
1020	0.040	0.0000

 

The plotting of tabulated data above results to the graph below:

Graph 6 Rate vs. Concentration (Run 1)

The Rate –Concentration Graph for Run 1 above shows that the concentration of hydrogen gas (H2) is directly proportional to the rate of reaction. Thus, the concentration of sulphuric acid (H2SO4) is also directly proportional to the rate of reaction, whereby, as the concentration decreases, the rate of reaction also decreases.

Since the Magnesium ribbon is in excess, only the sulfuric acid can be allowed to change. Having it in excess is to make sure that its concentration does not change.

Using the rate of reaction:

Rate = k [H2SO4]

And applying the rate equation to the chemical equation in the reaction:

Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g)

The rate equation is now: Rate = k [Mg] º [H2SO4]¹.

Since Mg concentration does not change in the reaction, it is the zero order. On the other hand, since H2SO4 is in the first order of reaction, the rate is directly proportional to its concentration, as measured in the experiment by the concentration of the H2 gas collected, the overall order of reaction is: 0+1 = 1.

For Run 2, the table below shows its concentration.

Table 6: Molarity of the Hydrogen gas (Run2)

Time /s	Volume of gas evolved (Vtotal in cm3)	Molarity
0	0	0
30	15	0.200
60	25.5	0.120
90	34.5	0.088
120	42.5	0.071
150	47	0.064
180	51.5	0.058
210	55.5	0.054
240	58.5	0.052
270	61.5	0.049
300	63.5	0.047
330	66	0.045
360	67.5	0.044
390	68.5	0.044
420	69.5	0.043
450	70.5	0.043
480	71	0.042
510	72	0.042
540	72	0.042
570	72.5	0.041
600	72.5	0.041
630	73	0.041
660	73	0.041
690	73	0.041
720	73.5	0.041
750	73.5	0.041
780	73.5	0.041
810	73.5	0.041
840	74	0.041
870	74	0.041
900	74	0.041
930	74	0.041

The table of Concentration and Time for run 2 gives the following graph:

Graph7: Concentration vs. Time (Run 2)

The Concentration –Time Graph for run 2 above shows the same trend as in run 1, where it shows an inverse proportionality: as time increases, the concentration of the sulfuric acid ( H2SO4) decreases as measured by the concentration of the hydrogen gas (H2) collected. The rate of re