The Determination of a Rate Equation
Length: 938 words (2.7 double-spaced pages)
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The purpose of this experiment is to develop a method to determine the
rate equation for the reaction between Magnesium ribbon and 2.0mol dm
Hydrochloric acid, HCl.
Hypothesis and Theory
When I react the magnesium ribbon with hydrochloric acid they will
undergo the reaction according to the equation below:
Mg(s) + 2HCl(aq) à MgCl (aq) + H (g)
For a reaction to be successful the molecules must collide with
sufficient kinetic energy in order to surmount the energy barrier,
also known as the activation energy. When there is an increased
concentration of reactants then there is a greater chance of these
successful collisions occurring therefore speeding up the reaction (As
How far, How fast?).
According to the collision theory, I should expect that as I increase
the concentration of hydrochloric acid then the reaction rate would
also increase because a higher frequency of collisions will increase
the rate of reaction. Therefore increasing the time taken for the
magnesium to dissolve.
When the magnesium ribbon is added to hydrochloric acid effervescence
is expected as they begin to react providing me with a reliable method
of measuring the reaction rate, hence the time taken for the magnesium
wire to completely dissolve.
Accuracy measures and Varying Factors
Ø The length of the magnesium ribbon will have to be kept to 0.03g on
Ø The temperature of the hydrochloric acid should remain constant
throughout the trials and each concentration temperature should be
checked with a thermometer of 0.5 accuracy. Varying temperatures
increase kinetic energy therefore increasing the vibrations and then
the frequency of successful collisions. Any variations in temperature
may skew results as I am investigating the effect of hydrochloric acid
concentration on the rate and not temperature.
Ø Start the clock as soon as the magnesium ribbon is added to the
Ø Measure solutions as accurately as possible by making sure the
bottom of the meniscus touches the line.
Before starting ensure that:
1. Safety goggles are worn;
2. Protective footwear is worn i.e. not open toed shoes;
3. Long hair is tied back;
4. Plastic gloves are worn when using high concentrations of acid;
5. Lab coat is buttoned up at all times.
To decide the best length of magnesium ribbon and concentrations of
hydrochloric acid to use and also the length of the intervals a pilot
experiment was conducted.
100 cm measuring cylinder x2
100 ml beaker
2.0 mol dm Hydrochloric acid
* Solutions of 0.5, 0.25, 0.75, 1.0, 1.25, 1.5, 1.75 and 2.0 mol dm
were made up using two separate measuring cylinders for the water
and hydrochloric acid using table 1 as a guide and the water and
acid were placed into a beaker.
* Firstly the 0.01g of magnesium was reacted with 0.5mol dm
hydrochloric acid and observed.
* The same solution was made up again and instead 0.02g of ribbon
was used and the same method repeated for 0.03g and 0.04g of
* This method was then repeated for the remaining concentrations of
Analysis and Conclusion (Summary)
It was finally decided to use 0.03g magnesium ribbon, this was seen as
most appropriate because anything below reacted much too fast at high
acid concentrations to be observed and above this took too much time
(which I do not have) when reacted with low acid concentrations.
Hydrochloric acid concentrations of 0.5, 0.75, 1.0, 1.25, 1.5, 1.75
and 2.0 mol dm . The 0.25 concentration was eliminated as the reaction
time was far too long and the remaining concentrations were seen as
Final Equipment and Explanations
100 cm measuring cylinder x2- to measure out the acid and water
100 ml beaker- allows enough area for the reaction to take place
2.0 mol dm Hydrochloric acid
Stop clock- to time the reaction
Distilled water- to make the different concentrations
Sand paper- to remove any dust or other impurities from the magnesium
Electronic scales- to weight the magnesium ribbon
1) Carefully measure 30ml distilled water using one of the two
measuring cylinders being careful not to mix it up with the other one
and keeping a close eye on the meniscus, use diagram 1. (Use of labels
may help) Place this in a beaker.
2) Measure the 10ml hydrochloric acid using the other measuring
cylinder and add this to the water in the beaker. Swirl the beaker 3
times to ensure mixing of the contents.
3) Lightly rub both sides of the magnesium ribbon 5 times with the
sandpaper to remove any impurities. Then accurately weigh 0.03g
magnesium ribbon using the electronic scales.
4) Place the Magnesium ribbon into beaker and then quickly start the
stop clock and swirl the beaker three times to initiate the reaction.
The help of another student may be necessary to ensure accuracy.
5) As soon as there is no longer effervescence and it is evident that
all of the magnesium ribbon has finally dissolved then stop the clock.
6) Record the time taken on the table of results (Table 1) provided.
7) Make sure the beaker is rinsed and dried thoroughly before re-use.
8) Continue to follow steps 1 to 7 substituting the figures for the
ones on the table.
9) Repeat the experiment once more for increased accuracy.
Table 1: Table of results for the time taken for the reaction of
Hydrochloric acid and Magnesium Ribbon.
Mass of Mg ribbon (g)
Concentration of HCl (mol dm )
Amount of HCl (ml)
Amount of water (ml)
(1) Time taken for Mg to dissolve
(2) Time taken for Mg to dissolve
Once both trials are complete, plot the results on two separate
rate-concentration graphs. Work out the rate using the equation:
A2 Analysis: The determination of a rate equation
* The elements of Physical Chemistry by P.W.Atkins
* A level Chemistry by E.N.Ramsden
* Advanced level Chemistry by Michael Clugsten et al
* Chemistry 2 by Brian Ratcliff et al