Jessica Schroeder and Chloe Choi
General Chemistry
Mrs. Knowles
Molar Concentration of Sulfuric Acid and Its Effect upon the Rate of Reaction
Research Question:
When magnesium is reacted with sulfuric acid, how does the molar concentration of the acid affect the
rate of reaction between the two substances?
Mg + H2 SO4  MgSO4
+ H2
In other words, magnesium ribbon is reacted with sulfuric acid to yield magnesium sulfate and hydrogen
gas.
Hypothesis:
As the concentration of the sulfuric acid is increased, the rate of reaction and intensity of reaction will
also increase. This is due to there being a greater amount of sulfuric acid molecules present to react
with magnesium strip, allowing the collision between the two substances to occur in greater abundance
within a period of time. As the concentration decreases, the rates of change will decrease in a
comparably linear manner.
Variables:
Independent variable- The concentration of the sulfuric acid, measured in
Dependant variables
- The time needed for a certain point in the reaction to be reached, or the rate of reaction.
-The intensity of the reaction (how vigorously bubbles are formed, the thickness of the steam produced,
the amount of heat felt after a certain time)
Controlled variables
- The mass of magnesium (measured in grams) added to the acid.
- The amount of sulfuric acid used to react against the magnesium (measured in ml)
Apparatus:
Sulfuric Acid ( 0.1 M, 0.2 M, 0.5 M, 1.0 M, 2.0 M)
Magnesium strips
Timer
Electronic balance
Safety goggles and safety apron
Two 200 Ml beakers
Graduated cylinder, with 1 ml increments
Scissors
Metal Scrubber
Procedure:
Be sure to use safety goggles and a lab apron throughout the entirety of the experiment.
1. Scrub the magnesium ribbon using the metal sponge until shiny.
2. Measure 0.25 grams of magnesium ribbon using the electronic balance- use scissors to cut to
the proper length.
3. Measure 30 ml of 0.1 M sulfuric acid using a 10 ml graduated cylinder, and pour it into the
beaker.
4. Place both the beaker and the magnesium strip on the balance, record the initial mass.
5. Using a calculator, determine what mass is exactly 0.05 grams less than that mass. This is the
mass that will signal to stop the timer.
6. Place the magnesium into the beaker, and immediately begin the stop watch. Observe the
change in mass, and record the time when the scale reads a decrease in 0.05 grams, this certain
mass being calculated previously.
7. Record observations and the number of seconds read by the timer.
8. Repeat steps above using 0.2 M, 0.5 M, 1.0 M, and 2.0 M.
Data Collection
Quantitative Data:
Table 1: Trial 1 Results
Type of Chemical (# of
mol)
0.1M H2SO4
Initial Weight (g)
(+/- 0.001 grams)
106.028 +/- 0.001
Final Weight (g)
(+/- 0.001 grams)
105.978 +/- 0.001
Time Taken (seconds)
(+/- 1. Seconds)
265. +/- 1.
0.2M H2SO4
106.130 +/- 0.001
106.080 +/- 0.001
161.+/- 1.
0.5M H2SO4
106.830 +/- 0.001
106.780 +/- 0.001
33. +/- 1.
1M H2SO4
107.836 +/- 0.001
107.786 +/- 0.001
16. +/- 1.
2M H2SO4
108.910 +/- 0.001
108.860 +/- 0.001
6. +/- 1.
Qualitative Data:
In every experiment, a similar reaction took place. However, depending upon the molar concentration,
the reaction happened with less or more potency.
-
Instant bubbling of the magnesium ribbon, forming collections of white foam.
-
A distinct sizzling noise, much like that of placing raw meat upon a hot pan.
-
The magnesium ribbon visibly bobbed about the sulfuric acid (it would float at the top, sink the
bottom, then return to the surface)
-
With any concentration of acid equal or greater to 0.5 M, light steam rose from the reaction.
When 2 M acid was used, the intensity of the steam was significantly greater.
-
An acidic, corrosive odor could be detected.
-
Remaining magnesium ribbon was much more lustrous than it was previous to the reaction.
Data Processing
(See attached graph, Figure 1)
An obvious correlation between the two variables reveals that indeed, the time needed for a certain
point in the reaction to be met decreases with the increasing of concentration. However, what we
viewed that we previously did not take into account was in what manner this would happen.
Rather than a linear, constant correlation, we viewed an exponential change. The graph can be roughly
represented by the equation [y= -ln (x)], rather than the expected [y= x].
To investigate this further, we can explore each point on the graph numerically.
For 0.1 M:
Approximately 265. 0 seconds were required for 0.05 grams of H2 to be released into the air. Taking this
into account:
(0.05 +/- 0.001 grams) / (265. +/-0.1 seconds) = 0.0001887 grams (+/- 20.38 % uncertainty) H2 released
per second
For 0.2 M:
(0.05 +/- 0.001 grams) / (161. +/- 0.1 seconds)= 0.0003116 grams H2 released per second
(+/- 20.62 % uncertainty)
For 0.5 M
(0.05 +/- 0.001 grams) / (33. +/- 0.1 seconds)= 0.001515 grams H2 released per second
(+/- 23.03 % uncertainty)
For 1.0 M:
(0.05 +/- 0.001 grams) / (16. +/- 0.1 seconds)= 0.003125 grams H2 released per second
(+/- 26.25 % uncertainty)
For 2.0 M:
(0.05 +/- 0.001 grams) / (6. +/- 0.1 seconds)= 0.008333 grams H2 released per second
(+/- 36.67 % uncertainty)
The rate of H2 production using different concentrations can be viewed using these values. To
comprehend the difference in rate, we can calculate the ratio:
(0.1M rate)/ (2.0 M rate) = (0.0001887 g/s)/ (0.008333 g/s) = 0.022644
Thus 0.1 M rate is 0.002264 of 2.0 rate of reaction.
However, to truly understand the data, we must take into account the differences in instantaneous
slope amongst the several points. Referring to Figure 2, it can be seen that the instantaneous slope of
the point graphed at 0.1 M has a slope nearing that of undefined. I contrast, the point measured at 2.0
M has a slope nearing that of 0.
The curve withstood a sharp turn at approximately 0.5 M, as change in concentration is depicted to be
less significant to the brevity of the experiment.
Note: The instantaneous slopes are used simply as a rough estimation and reference. Due to the rough
nature of the line of best fit, to use such a curve to find a precise slope would be inappropriate and
vastly inaccurate.
Conclusion:
Our aim was to investigate how the concentration of acid affects a rate of reaction, specifically sulfuric
acid reacting with magnesium ribbon.
This aim was achieved- we concluded that concentration has a clear effect upon the rate of reaction
(the graphs best exemplified this). As the concentration (independent variable) is increased, the rate of
reaction (dependent variable) consequently increases. Also, just as we predicted, the intensity of the
physical results (another dependant variable) became greater as the concentration (independent
variable) was increased. However, we also concluded that the rate of reaction does not change at a
constant rate. This was not our original prediction- we believed before the experiment that a constant
change in reaction rate would be viewed. Concentrations greater than 1 M saw a much less significant
change in time as higher concentrations were tested. Thus concentration, as it is raised, has a less and
less noticeable effect upon the rate of reaction in an experiment.
This of course raises many new questions in our minds- does this mean that with the use of for example
3 M and 4 M, a difference will be negligible? Is this curve in the graph due to there being a limited
amount of surface area for magnesium to react with the acid, or is it because we simply did not use the
quantities of magnesium and acid on a large enough scale to gain an accurate perspective on how time
and concentration affect each other. These are all questions that I believe could be explored had we
had more materials and unlimited time in the lab to conduct several tests and trials under different
conditions.
Evaluation:
Deviating from the original hypothesis that the reaction rates will increase linearly along with
the increasing number of moles, the reaction rate changed forming a curved line. The reaction rate did
not increase with a constant slope.
For instance: had the rates of reaction all changed in the expected manner, and assuming that the 0.1 M
data point and 2.0 M data point are accurate, then a general slope would be calculated as:
(0.1 M, 265. seconds) and (2.0 M, 6. seconds)
--- > Rise/Run --- > (Y2 – Y1)/( X2- X1)
(6. – 265.)/ (2.0 – 0.1) = - 136.31578
In contrast to this, at 2.0 M, dy/dx = approximately 0.
This demonstrates the clear disparity between what we expected, and what was true in actuality.
Throughout the process, we failed to keep the mass of magnesium and the initial mass of each solvent
constant. Although the volumes of the solvents were accurately measured to be 30mL, the masses
greatly deviated from each other. We found it quite ironical the way mass greatly increased as the
number of moles, the concentration of the solvent, increased. Another amendment that we can make is
to calculate the percent change, instead of the mass change. As changes in mass do not show the
proportional aspect of the solution, it is more correct to use percent change to accurately show the
increase in mass. However, we used 0.05g as the mass change that indicates a reaction; accordingly we
failed to take into account the proportional aspect, and used a wrong standard to measure the time
taken for solvents to complete the reactions.
Even though, there are some few flaws to the experiment, the experiment successfully shows
the relationship between the concentration of a solvent and its reaction rate. It provides the data
necessary to indicate the overall trend, and the most importantly supports the hypothesis.
Bibliography
"Chemical Kinetics: Reaction Rates." Davidson College Chemistry Resources. N.p., n.d. Web. 1
Mar. 2010. <http://www.chm.davidson.edu/vce/kinetics/ReactionRates.html>.