Introduction
In this experiment you will measure the time required for magnesium
metal to react with hydrochloric acid solutions of various concentrations.
From these data, you can calculate the rate of reaction for each experiment
and graphically determine the order of reaction with respect to the hydrogen
ion concentration.
You will clean a 28cm length of magnesium ribbon (to remove any
oxide) and then cut it into seven equal pieces, each 4 cm long. Since
the ribbon is fairly uniform, the surface area is proportional to the length,
and hence the area for reaction will be the same for each piece. i.e., the
concentration of the Mg is constant for each trial. The variations
in rate then will be solely attributable to the variations in acid concentration.
You will make seven aliquots of HCl, 50 mL each, of 0.5 M, 1.0
M, 1.5 M, 2.0 M, 2.5 M, 3.0 M and 3.5 M, using 6.0 M stock HCl solution.
The reaction is: Mg(s) + 2
H^{+}(aq) > H_{2}(g) + Mg^{2+}(aq)
The general rate law expression for this reaction is: rate
= k[H^{+}]^{n}
where n is the order of the reaction. The rate can be expressed
in terms of reciprocal time since the concentration (or surface area) of
the magnesium is a constant. So, rate is directly proportional
to 1/t and 1/t = k'[H^{+}]^{n}
Then take the natural logarithm of both sides, giving ln(1/t)
= n ln [H^{+}] + ln k' which now has the form of a linear
equation, y = mx + b, where the slope (m) is equivalent to the order of the
reaction, n, and the yintercept is the natural logarithm of the rate law
constant, k'.
Procedure
1. 
Prepare the 7 aliquots of HCl as described in the introduction
using the dilution equation. Use graduated cylinders and pipets for
accuracy. 
2. 
Prepare the seven pieces of magnesium metal. 
3. 
Drop the piece of Mg into the acid solution and time how long it
takes for the magnesium to disappear, while stirring gently. Be sure
the metal does not stick to the sides of the beaker. Repeat with the
other six solutions. Record your data in the table below. 
4. 
Plot a full page scatter graph with ln(1/t) (on the vertical axis)
vs ln [H^{+}] (on the horizontal axis), and use a curve of best
fit to draw your line. Using the equation given for your graph, note
the slope of the line and the yintercept. 
5. 
State the order of this reaction relative to the [H^{+}]
and the value of the rate law constant. Then write the completed rate
law expression with k' and n values included. 
6. 
Now complete the last column in your data sheet. Show each
calculation. 
Trial # 
[HCl] 
ln[H^{+}] 
Time, t 
1/t 
ln(1/t) 
Rate 
1 
0.5 M 





2 
1.0 M 





3 
1.5 M 





4 
2.0 M 





5 
2.5 M 





6 
3.0 M 





7 
3.5 M 





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