Kinetics: Hydrolysis of Methyl Formate

Computer-Interfaced Experiments - Conductivity Measurement
Kinetics
Hydrolysis of Methyl Formate using an Acidic Ion Exchanger - First Order Reaction

Objectives: Determination of Rate Constants, Temperature Effect on Rate

Peter Keusch

Datalogging using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
methyl formate (m.w. = 60.05 g / mol, d = 0.97 g / mL)
methanol (m.w. = 32.04 g / mol, d = 0.79 g / mL)
ion exchanger I, strongly acid cation exchanger, H⁺-form (Merck)

Apparatus and glass wares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish 190 × 90 (for water bath)
2 beakers 250 mL
contact thermometer
conductivity measuring cell
temperature probe
graduated measuring pipet 15 mL
volumetric pipet 50 mL, 25 mL
tea bags
pipet bulb

Hazards and safety precautions

Methyl formate is extremely flammable. May act as a narcotic. May be harmful if inhaled or swallowed.

Methanol may be a reproductive hazard. Ingestion may be fatal. Risk of very serious, irreversible damage if swallowed. Exposure may cause eye, kidney, heart and liver damage. Chronic or substantial acute exposure may cause serious eye damage, including blindness.

Safety glasses and gloves must be worn. The experiment should be performed in a laboratory fume cupboard!

Theoretical background:

The hydrolysis of methyl formate using an acidic ion exchanger results in the formation of methanol and formic acid.

equation

Hence the reaction can be monitored by following the change in the conductance of the reaction mixture with time.

Inorganic zeolites function as ion exchangers. They are of great technical interest. Always the anti ions are catalytically effective. The cation exchangers (H⁺-form) and the anion exchangers (OH^--form) catalyze reactions speeded by the presence acids or bases. The network of the exchanger acts merely as a carrier for the catalyst. In particular, synthetic resin ion exchangers are qualified for the acid and base catalysis, since they can be transformed easily into H⁺-form and OH^--form. Due to their porosity and their swelling property they have a sufficient large surface so that larger organic molecules can penetrate into the inside of the resin too. The catalysis with a strongly acidic cation exchanger takes place just as fast as e.g. with sulfuric acid.

Experimental procedure:

In addition to a conductivity measuring cell (1) a temperature probe (2) is connected to the CHEMBOX via input Sensor2 (Fig. 1).

Fig. 1: Experiment set-up

94 mL of dist. water and 49 mL of methanol (1.21 mol) are pipetted into a beaker placed in a water bath. A tea bag containing 20 g ion exchanger is submerged into the solution.

Using a hotplate stirrer and a contact thermometer the aqueous methanol solution is warmed up in the water bath to the desired temperature (45 or 55 °C).

When thermal equilibrium is reached, 12.3 mL of methyl formate (0.198 mol) are added to the aqueous methanol solution while stirring constantly. After 30 seconds the sensing software is started.

The data are logged at one-second intervals.

The change in the conductivity and the constancy of the temperature are displayed simultaneously on the measuring screen (Fig. 2).

Fig. 2: Multigraph screen conductivity curves determination of half-life t_1/2
1: 45.7 °C 2: 54.7 °C

Data analysis:

When dealing with first-order reactions, the use of a half-life rather than a rate constant is often convenient. The half-life t_1/2 of a reaction is the time required for the reactant concentration to drop to one-half of the initial value. By positioning the mouse cursor at the appropriate point on the conductivity curve (Fig. 2 cursor: nw-resize) the half-life is shown in the title bar on the top of the measuring screen. The rate constant k is computed according to equation (8) Kinetic equations (Download PDF file).

	45.7 [ °C ]	54.7 [ °C ]
t_1/2 [ s ]	296.4	192.6
k [ s^-1 ]	0.00233	0.0036

Tab. 1: Half-life t_1/2 and rate constant k

Data analysis using Excel - determination of the rate constants

Excel table

Tab. 2: Measured values k(t) conversion according to y = ln (1.275*(1 - k / k_¥))
The initial concentration of methyl formate is 1.275 M.

After the import of the data measured into Excel the conductivity values are converted (Tab. 2) according to equation (6) Kinetic equations (Download PDF file).

The conductance measured at the end of the reaction k_¥ corresponds to [ A ] ₀ (initial concentration of methyl formate)and k_¥ - k corresponds to [ A ] (concentration of methyl formate at time t).

Equation (6) Kinetic equations (Download PDF file) becomes ln( k_¥ - k) / k_¥)) = - kt. Thus a plot of ln(1.275*(1 - k / k_¥)) against t is enabled (Fig. 3).

rate constants

Fig. 3: First order kinetics plot determination of the rate constant k
1: 45.7 °C 2: 54.7 °C)

The data were logged at one-second intervals. A dialog module programmed in Visual basic allows the hiding of data pairs in the data sheet (Tab. 2). For the reason of clearness only every twelfth data point is plotted in the graph ( Fig. 3).

	45.7 [ °C ]	54.7 [ °C ]
k [ s^-1 ]	0.0023	0.0037

Tab. 3: Rate constants

Discussion:

Reaction rate constants are usually temperature dependent; the rate of a reaction usually increases as the temperature rises.

Only a small temperature change is needed to increase the number of effective collisions. Much more molecules collide with kinetic energy higher than a certain threshold energy, react and get converted into products.

The test result confirms the Vant-Hoff rule: Reaction rates are roughly doubled when the temperature increases by 10 degrees Celsius. This means the rate can be quadrupled if the temperature is raised by 20 degrees Celsius. Vant-Hoff rule is applicable in the narrow interval of temperatures only.

Note: In dilute aqueous solution, water is present in such excess that its concentration remains unchanged during the reaction, and if the experiment is carried in a solution of a strong acid catalyst, the formation of carbonic acid will not effectively change the concentration of hydronium ions. Under these condtions the reaction is pseudo first order.

Reference:
Computer-Interfaced Experiments Kinetics: Kinetics: Alkaline Hydrolysis of Ethyl Acetate
Computer-Interfaced Experiments Kinetics: Alkaline Hydrolysis of Esters
Microscale Projection Experiments Reactivity of Aromatic Esters
Esterification of Acetic Acid and Isoamyl Alcohol with a Heterogeneous catalyst

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