DETERMINATION OF THE ORDER OF REACTION
INTRODUCTION:
General Equation:
Mg (s) + 2HCl (aq) ⇌ MgCl 2 (aq) + H2 (g)
1. Write the balanced net ionic equation for the reaction between magnesium and
hydrochloric acid. How would you expect the concentration of the acid and the
amount of magnesium metal to affect the rate of reaction? Be sure to discuss in
qualitative terms.
Net Ionic Equation:
Mg (s) + 2H+ (aq) ⇌ Mg2+ (aq) + H2 (g)
● When the concentration of the acid and/or the amount of magnesium metal
increases, the rate of the reaction will increase.
● When the concentration of the acid and/or the amount of magnesium metal
decreases, the rate of the reaction will decrease.
2. Why would it be important that the length of magnesium used throughout the
experiment be kept constant?
It is important that the length of magnesium used throughout the experiment be kept
constant, because if length of magnesium increases when width is constant, the
surface area of magnesium increases. When surface area increases, hydrochloric acid
will react more intensely and rate of the reaction will be affected. Therefore,
magnesium needs to stay constant because it will ultimately change the rate of the
reaction.
3. Write the general rate equation for this reaction. Since rate can be expressed as the
reciprocal of time taken for the reaction, rewrite this rate equation in terms of time.
Rewrite this rate equation after taking the natural log of both sides of the rate
equation.
General Rate Law & Reaction Mechanism:
This reaction happens in two elementary steps, which are written below:
1. H2O + HCl ⇋ H+ + Cl - + H2O
2. H+ + H+ + Mg ⇋ Mg 2+ + H 2 RDS
The overall reaction is written below.
Mg (s) + 2HCl (aq) ⇌ MgCl 2 (aq) + H2 (aq)
Since the second step is RDS:
Rate(rxn) = k * [H+]2 * [Mg]0
Rate(rxn) = k * [H+]2
General Rate Law in Terms of Time:
Δt = 1 / (k * [H+]2)
Logs of General and Time Rate Law:
log (Rate(rxn)) = log (k * [H+]2)
log (Δt) = log (1 / (k * [H+]2))
4. If this equation were graphed, what type of graph would be obtained?
Rate order of the reaction:
The rate order can be determined by the sum of exponents of the rate law, for this
experiment the rate law is written below :
Rate(rxn) = k * [H+]2
Therefore the rate order is 2nd order.
The graph will have a raising quadratic curve. Moreover, when we take the square
root of concentration, the graph will be linear. Both indicates that it’s a second order
reaction. The graph below is a general representation of a second order reaction that is
expected when we do our experiment.
5. How can the order of reaction and the value of the rate constant be obtained from this
graph?
The rate order is determined by the curve of a graph. The following rules
determine the rate order:
● If it was a line with zero slope (constant curve) it will be 0th order.
● If it was a line with a slope (linear curve) it will be 1st order.
● Else if the curve looks like exponential or any other curve, we will have to do
some editing to the curve until it becomes liner then we know what order it is
but generally it will be more than one.
The rate order is determined by taking the slope of the linear line that
make from editing.
(1/Δt) VS Concentration
6. Design an experimental procedure involving at least six 25.00 mL aliquots of
hydrochloric acid or varying concentration prepared from stock 6.0 M HCl.
FIRST PROCEDURE:
1. Make different concentrations of hydrochloric acid by adding different amount of water.
2. Use six Erlenmeyer flask with the same amount of hydrochloric acid but different
concentrations for each tube. The use of pipets is crucial for this step to insure accuracy.
3. Use an accurate timing device (eg. stopwatch) to get the Concentration of hydrogen gas
in 30 second intervals.
4. Connect the beaker with the tubes which are inside water in a flat bucket.
5. Cut and measure equal size strips of magnesium to put in erlenmeyer flasks.
6. Measure the volume of hydrogen gas in the gas measuring tube every 30 seconds with
a time measuring device until the reaction is finished.
7. Make a table with time in seconds and the volume of hydrogen gas for each one of the
six constartions.
8. Graph the data as Volume vs Time for each one of the six constartions.
9. Draw a tangent line at time equals (0) for each one of the six constartions to get the
initial rate of reaction.
10. Make a table of initial rate of reaction vs initial HCl concentration.
11. Draw the graph.
12. Then take the log of each side initial rate of reaction and initial HCl concentration.
13. Then take the slope of a straight line which will be the order of the reaction.
Rate = k * [HCl]n
log (Rate) = log (k * [HCl]n)
log (Rate) = log (k) + log ([HCl]n)
log (Rate) = log (k) + n * log ([HCl])
n = (log (Rate) - log (k)) / log ([HCl])
SECOND PROCEDURE:
1. Different concentrations of hydrochloric acid but with same amount.
2. Use six erlenmeyer flask with the same amount (25.00 mL) of hydrochloric acid but
with different concentrations.
3. Connect the beaker with the tubes which are inside the water in a flat bucket.
4. Cut and measure equal size strips of Mg to put in Erlenmeyer flasks.
5. Measure the volume of hydrogen gas in the gas measuring tube every 30 seconds with
a time measuring device until the reaction is finished.
6. Make a table with time in seconds and the volume of hydrogen gas for each one of the
six constartions.
7. By now the data that you need is collected , you should start to analyze it .
8. From the table we can have Δ[] / Δt of HCl real
Δ[HCl] / Δt = k * [HCl]n
9. First take the change in concentration of HCl of first Tube and the change in time ,
you can use the following equation.
log(Δ[HCl] / Δt) = log (k) + n * log([HCl])
10. Repeat Step 9 for five more times.
11. Using some knowledge of algorithms we can rearrange to make the following
equation.
log(Δ[]1 / Δt) - log(Δ[]2 / Δt) = n * (log[]1 - log[]2)
12. Now it’s clear that the value of “n” is equal to the following equation.
n = (log(Δ[]1 / Δt) - log(Δ[]2 / Δt)) / (log[]1 - log[]2)
Safety:
Hydrochloric acid is a hazardous solution which must be used with care. The acid itself is
corrosive, and concentrated forms release acidic mists that are also dangerous. If the acid or
mist were come in contact with the skin, eyes, or internal organs, the damage can be
irreversible or even fatal in severe cases.
A product of this reaction, magnesium chloride solution, also has some hazards properties.
Repeated exposure to magnesium chloride solution may cause skin dryness or cracking. May
cause drowsiness or dizziness. Ingestion of this substance causes burns of the upper digestive
and respiratory tracts.
Magnesium ribbon is a flammable solid. Hydrochloric acid is a corrosive liquid. Hydrogen
gas is explosive. However, the very small quantities and low dilutions used in or produced by
this demo presents little hazard. Wear safety goggles. Keep flammables and open flame away
from the vicinity of the demo.
The use of personal protective equipment like vapor respirator, rubber gloves and goggles is
essential.
Some personal safety equipment like:
● Vapor respirator
● Rubber gloves
● Boots
● Full suit
● Face shield
Wear protective glasses and an apron at all times. Avoid skin contact with solids and
solutions. Hydrochloric acid is a very corrosive reagent. Wash any spills copiously with
water. If you need to dilute hydrochloric acid, remember AAA--"Always Add Acid" to water.
No Bunsen burners should used during this activity due to the production of potentially
explosive hydrogen gas. Wash your hands before leaving the laboratory.
PREDICTION:
Mg (s) + 2HCl (aq) ⇌ MgCl 2 (aq) + H2 (g)
The reaction is a second-order reaction.
Theoretical Rate Law:
Through researching the web, results have shown:
Rate(rxn) = 0.0272 * [HCl]2
(Reference:
http://www.cma-science.nl/activities/en/chemistry/Chemical%20kinetics/Magnesium%20and
%20hydrochloric%20acid%20(model)/Magnesium%20and%20hydrochloric%20acid%20(mo
del)%20-%20Science%20background.docx)
