EXPERIMENT #5: CHEMICAL KINETICS
The reaction of magnesium metal with hydrochloric acid produces hydrogen gas and aqueous
magnesium chloride. Magnesium chloride is an important coagulant and is used in the process of
making tofu from soy milk. It is also the precursor to several important magnesium compounds,
including magnesium hydroxide.
Mg(s) + 2HCl(aq) H2(g) + MgCl2(aq)
Your group needs to monitor the rate of reaction of magnesium metal and determine the rate
law for the reaction. You will also need to determine the impact of temperature on the rate.
In chemistry, sometimes it is very important to know how long a reaction will take. How
long will it take for reactants to combine and form products? The costs of maintaining an
expensive manufacturing process is very dependent on how long it takes to make the product.
Profit margins and how much the product will sell for are dependent on the manufacturing time.
How long a reaction will take is called its rate of reaction. The rate of a chemical reaction
depends on several factors including: the concentration of the reactants, the temperature, the
pressure, the surface area, the presence of catalysts, and the type of solvent used. The area of
chemistry that studies chemical reaction rates and the mechanism of chemical reactions is called
Chemical Kinetics. Kinetics concerns itself with the reaction rates of chemical reaction and a
kinetic study “experimentally explores the factors affecting reaction rates.”
Given the following hypothetical chemical reaction,
aA + bB cC + dD
The rate of this reaction can be described in terms of either the reactants or products. If
reactants are use to describe the rate, an expression that shows how fast the reactant disappears is
needed. This is because as the reaction proceeds, the amounts of the reactants are getting smaller
as they are changed into products. By the same approach, if the products are used to describe the
rate, an expression of how fast the product appears is needed. Since the entire reaction can only
proceed at one rate, the rate of disappearance of each of the reactants and the rate of appearance
of each of the products are relative and can be described by the following expression.
Rate of the reaction =−
= − 𝒃∆𝒕 =
The rate is equal to the change in concentration of each reactant or product divided by the
change in time (time it took the reaction to occur). Notice that a negative sign is used to show the
rate of disappearance of the reactants A and B. Also, notice that rate is made equal relative to the
number of moles of each reactant or product.
Since the rate of the reaction depends on and is influenced by the concentration of the
reactants, the rate can be experimentally determined to fit the following relationship.
Rate of the reaction = k[A]x[B]y
This says the rate is proportional to the initial concentration of the reactants and how often
they come together to make the products. The exponents, x and y, describe the rate order of the
reaction (x with respect to reactant A and y with respect to B). The overall rate order is the sum
of the rate orders of the reactants used in the rate expression. In this case that would be x + y.
For heterogeneous reactions such as the reaction between Mg and HCl, pure solids and pure
liquids are not included in the rate law because their concentrations are considered constant.
Therefore, the rate law of the above reaction can be written as
where k’ is the apparent rate constant.
The rate constant, k, is constant for the set of conditions that the reaction occurs under
including temperature and pressure. It has a relationship with the temperature and the activation
energy of the reaction. This relationship is called the Arrhenius reaction model and is described
by the Arrhenius equation.
𝑘 = 𝐴𝑒 𝑅𝑇
Where k is the reaction rate constant, A is the frequency factor and has to do with the total
number of potential collisions between the reactants, e is the natural number, Ea is the energy of
activation, and R is the gas constant.
This equation shows how the reaction rate constant can vary with temperature.
Procedure: You will be given strips of magnesium metal and a 2.0 M solution of HCl.
• Design experiments to measure the rate of disappearance of magnesium
metal as a function of time.
• Determine the effect of the initial concentration of HCl on the reaction
• Determine the rate law
Rate = k’[HCl]y
• Determine how temperature affects the rate of this reaction (activation
• Summarize your findings in a written report.
(Attach a detailed description of the experiment or laboratory procedure.)
1. The main goal/purpose of the experiment is (what are you trying to discover in this lab):
2. The hypothesis(es) we seek to test in this experiment is(are) (what is the basis of your
3. The key question the experiment seeks to answer is (what problems are you
4. The controls involved in this experiment are (a control is used to minimize the unintended
influence of other variables on the same system):
5. The key concept(s) or theory(ies) behind the experiment is(are) (define terms like
volatile, ideal gas law, etc. can you explain the relevant theory):
6. The important technique(s) used in this experiment is(are):
7. The experiment is based on the following assumptions (identify your assumptions and
determine if they are justifiable):
8. The data that will be collected in the experiment are (what do you plan to record in your
9. The potential consequences of the experiment are:
10. The point of view behind the experiment is (make sure it’s scientific).
11. What special safety precautions do you need to be aware of for this experiment?
Objective: To determine the molecular weight of an unknown compound by using the freezing
Equations: ΔTf = iKfm
1. Set up the balance: Be sure to level the balance. Place the vial on the balance and set
it to zero. (AM)
2. Prepare the cyclohexane: Add about 10ml of the cyclohexane to the graduated
cylinder. Bring the stopper, and the graduated cylinder of cyclohexane to the balance.
Add the cyclohexane to the test tube (remove test tube from the balance when adding
the pure solvent). Let it measure up to nearest 0.001g. (BM)
3. Prepare the apparatus and the freezing water bath: Use a beaker and set it on the
apparatus. Place the test tube in the clamp. Make sure the clamp is secure. Ensure that
the test tube is not touching the bottom of the beaker, but close enough. Place the
thermometer in the test tube well enough to see the ten-zero degree mark. Fill the
beaker with ice, then add water to an inch of the top of the beaker. (AN)
4. The display of the freezing point: Once the test tube submerges in the freezing
water bath, stir the pure solvent. Check the test tube periodically to see any display of
crystals of solid. Be sure to record the temperature of each phase. (AM)
5. Take the mass of the test tube: Measure the mass of the test tube. determine the
mass of the solvent from the final mass of the flask with vapor. (BM) Pour the water
of the ice in the beaker. Add more ice to bring the level to within an inch of the top.
Pour the rock salt from the 50 ml beaker onto the ice. Stir the ice/salt mix well with a
stirring rod. Add enough water to bring the liquid level to just under the ice level.
Weigh out between 0.5 and 0.6 grams of paradichlorobenzene on a weighing paper.
Make sure you zero out the weight of the paper. Record the weight to the nearest
0.001 g. Add this to the cyclohexane in the test tube. Stir until all of the pdichlorobenzene dissolves. Warm the tube in your hand if necessary. Place the test
tube in the ice/salt mix, and stir the contents of the test tube constantly. Look at the
contents of the test tube often. Try to catch the temperature when the first solid or
crystals appear. Record that temperature to the nearest 0.1°C. Warm the tube to melt
the crystals, and repeat the freezing point observation. Record the second value. If the
data is not consistent, use the average value of your data. (BM)
6. Molar mass of the cyclohexane: Reweigh the empty test tube and the stopper in the
vial, zeroing the vial weight as before. Record the weight. Add 10 ml of cyclohexane.
Weigh again and record. Add between 0.5 and 0.6 g of one of the unknown solids.
Record the weight to the nearest 0.001g. Record which unknown you have. Now
proceed as in Part 2 to dissolve the solid in the cyclohexane. Determine the freezing
point of the solution, taking two measurements as directed in Part 2. (AN)
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