Description of the Experience
Chemical reactions are processes in which the breaking of chemical bonds existing between the atoms that form the reacting substances occurs, forming new chemical bonds between the same atoms of departure, but arranged in a different way, to form the molecules of the substances which are said final products of the reaction.
It is well known that chemical reactions occur with a significantly different speed one from the other. Some are very slow, while others are very fast.Given any reaction:
A + B → C + D
The reaction rate is defined as the variation of the concentration of a substance in time unit (time interval); the substance can be chosen between the reactants or the products, therefore the variation of concentration will be positive or negative; According to this definition, the mathematical expression of the speed of reaction is gold:
v = - ∆ [A] o ∆ [B] / ∆ t = ∆ [C] o ∆ [D] / ∆ t
where the symbol ∆ indicates the variation in magnitude, the square brackets indicate the molar concentration and the negative sign for the reagents indicates the decrease in their concentration.
The factors that affect reaction rates are:
a) the nature of the reactants.
b) the physical state of the reactants
c) the concentration of the reactants.
d) temperature
e) catalysts
The first point is evident when one considers the different speed with chemical reactions, depending on the nature of the substances reagents and chemical bonds involved in the reaction. As the molecules of the reactants must come into intimate contact to react, the physical state in which they are is also important: if they are in the gas phase or in solution, the molecules or ions can easily come into contact, but if one or both are solid state, it is necessary that the contact surface is the as large as possible for the reaction to take place. To explain the influence of other factors (concentration, temperature, catalyst) one should bear in mind the collision theory, which states that the condition required for a reaction to occur is that the molecules of the reactants with kinetic energy are in contact with each other, bumping into each other.
It is evident that an increase in temperature and in concentration favors such a contact.
Catalysts are substances that increase the speed of a reaction even if it will not fit in the composition of the products, their only function is to lower the activation energy of the latter.
The following is an experiment carried out by students of the I.T.I.S A. Meucci of Florence on the factors that influence the reaction rate. The change in color, index of reaction, has contributed to increase the attention and curiosity of children.
Experience
Reagents:
. H2SO4 al 96%
. KMnO4 0,1 M
. Ice
. MnCl2
. C2H2O4 0,1 M
Materials:
. Pipette Pasteur
. 3 becher
Proceedings:
. Stopwatch (instrument)
. magnetic stirrer and stir bar
The reaction, which must be balanced, it's a redox:
KMnO4 + C2H2O4 + H2S04 → k2SO4 + MnSO4 + CO2 + H2O
- Located in a beaker on the magnetic stirrer, set up 10 ml of potassium permanganate
- Add 10 ml of oxalic acid and 2 ml of sulphuric acid were mixed. Operate at room temperature
- With a stopwatch to monitor and record the reaction time from the starting color violet until the discoloration of the solution
- Cool with ice in a Bain-Marie another beaker and pour 10 ml of potassium permanganate. Place the beaker on the stirrer and add 10 ml of oxalic acid and 2 ml of sulfuric acid formerly mixed
- With a stopwatch to monitor and record the reaction time from the starting color violet until the bleaching solution
- Cool with ice in a Bain-Marie another beaker and pour 10 ml of potassium permanganate. Place the beaker on the stirrer and add 10 ml of oxalic acid and 2 ml of sulfuric acid, previously mixed with a spatula tip of MnCl2
- Monitor and record with a stopwatch the reaction time from the starting color violet until the bleaching solution.
Finally, return the data obtained in a table
While boys did this experience, there were no special difficulties as sequential operations carried out did not require particular familiarity and workshop dexterity.
What was harder to understand were the theoretical principles behind this experience: difference between reactants and products, chemical reaction and reaction speed.
This problem was solved by showing and providing simple examples of chemical reactions that occur in everyday life.
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