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NOTE THAT THE SPEED!

 

In chemistry there is a special area that studies the rate and mechanisms of various reactions- chemical kinetics.

Although the chemical theory can explain much, to predict theoretically the speed of any response as of yet. It is studied experimentally in the laboratory, and then develop ways this speed change. There are a lot of reactions important to industry that go too slowly, you must be able to accelerate. Other reactions, in contrast, have to slow down, because they are harmful.

In short, chemical kinetics and experimental science. In the justice of its laws, you can verify by putting a few simple experiments.

For a start, make sure that the speed is the same reaction may indeed change, and quite significantly. (However, it can be assumed on the basis of chemical and life experience; for example, products in the cold deteriorate slower than in the heat, because at different temperatures the same biochemical reactions proceed at different speeds.)

To verify repeat the experiment from Chapter "Chemical clock", but change at this time is not the concentration of substances (this you already know), and temperature. If both the original solution of sodium sulfate and potassium Iodate with sulfuric acid is poured into water with ice, the time until blue coloration will be significantly longer than when using warm water. Just note that in very hot water, the color does not appear at all as colored compound of iodine with starch unstable.

So, you have found by experience: the higher the concentration and the temperature, the faster the reaction. But some of the reactions at first glance seem to be the exception to the rule. Here is an example.

Pour into a test tube to a height of 1-2 cm of acetic acid and throw it in a few pieces of zinc. Zinc must be pre-cleaned by soaking it twenty seconds in a solution of hydrochloric acid and washing with water.

Acetic acid is a weak, and zinc dissolved in it very slowly - bubbles of hydrogen barely stand out. How to speed up the reaction? Heat the solution. Correctly. Isn't there any way? Do: slowly going to add in a test tube of pure water, each time stirring well. Carefully follow the bubbles. It's amazing: acid diluted twice, three times, and the reaction instead of slow down, go faster!

If you put this experience in the classroom mug, replace zinc with a small piece of magnesium turnings and does not handle. With diluted acetic acid magnesium reacts more vigorously than zinc.

This "exception" to the rule is understandable when it is well to study. Our experience with acetic acid may be explained as follows. The speed with which the zinc or magnesium interact with the acid depends on the concentration of hydrogen ions in solution. These ions are formed when dissolved in water of any acid. But when the waters a little, weak acetic acid remains in solution almost exclusively in the form of medicationabana molecules. As dilution water more molecules of acetic acid dissociates into ions, and the reaction goes faster. But if you add too much water, then the reaction will again slow down already for another reason: because of the strong dilution of the concentration of hydrogen ions will again decrease. Fastest reacts with zinc 15%acetic acid.

Of course, we have discussed this experience is not for the sake of simply to show what are unusual chemical transformations. We wanted to draw your attention to the following: to control the speed of the reaction is necessary to know how it goes.

Any reaction begins with the fact that the molecules of substances collide with each other. Let's see how the reaction begins.

Take not very wide glass tube with a length of several tens of centimeters and pick up her two tubes. On the inner side facing the tube, insert both plugs on a small glass rod and reel them in cotton wool. One piece moisten with a few drops of concentrated hydrochloric acid, and the other concentrated ammonia solution. At the same time insert the tube with cotton into the tube from both ends. After a few minutes - depending on the length of the tube in it, closer to the fleece with hydrochloric acid, appears white ring of ammonium chloride, NH4Cl.

The reaction of hydrochloric acid with ammonia

Usually during chemical reactions the mixture is stirred, so that the process went faster. We deliberately did not do and did not try to help the molecules to meet, they moved themselves. Such independent movement of molecules in a particular environment are called diffusion. Steaming with wool, molecules of both substances were tested billions of collisions per second with the air molecules and with each other. And although the speed of molecules is very large, it amounts to hundreds of meters per second, at 0°C and normal pressure free path, i.e. the distance that goes through the molecule from one collision to another, is for these substances only about 0.0001 mm . Therefore, the ammonia and hydrogen chloride (hydrochloric acid) was slowly moving in the tube. Equally slowly spreads around the room with still air odorous substance.

But why white ring appeared in the middle of the tube? Because ammonia molecules less they move through the air faster. If from the tube to pump the air, the molecules of ammonia and hydrogen chloride will occur in a fraction of seconds - the length of the free path of the molecules will increase significantly.

I advise you to do some research to find out what is the impact on the diffusion of gravity and temperature. To do this, place the tube vertically and obliquely, as well as heat selected portions (including the place where it settles ammonium chloride). Conclusions try to do it themselves.

From gases turn to liquid. In them, the diffusion is much slower. Check it out on the experience.

On a smooth and clean glass plate drip next several drops of three liquids: in mid - water on both sides of solutions of caustic soda and hydrochloric acid. Fluid prior experience should not touch. Then very carefully, avoiding mixing wand connect solutions. Should stand out carbon dioxide, but it will not happen immediately. And when the gas starts to stand out, the bubbles will be located along the border that separates the area of diffusion of the acid and soda.

Instead of the soda and acid, you can take any two water-soluble substances which, when mixed painted or give a precipitate. However, in such experiments it is difficult to avoid fluid flows, distorting the picture, so it's best to experiment in thickened solutions. And sagemath they can gelatinous.

Prepare a 4%solution of gelatin, dropping it into hot water (not boiling!). The hot solution pour into a test tube and, when it cools, in the center of the tube quickly, in one motion, type tweezers crystal of potassium permanganate, copper sulphate or other brightly colored and water-soluble substances. Tweezers immediately remove careful, but quick movement. Within a few hours you can see a very nice picture of diffusion. Dissolved material is distributed in all directions with the same speed, forming a colored sphere.

With thickened solution can be put another experience. Pour in the hot gelatin solution into two test tubes and add in one little alkali solution, and in the other of phenolphthalein. When the contents of the test tubes hardens, tweezers quickly type in the center of the first tube piece tablet phenolphthalein, in the center of the second lump of soda ash. In both cases appears raspberry color. But note: in the second test tube color spreads much faster. Ions hydroxide, formed by dissociation of alkali, much smaller and lighter than the complex organic molecules of phenolphthalein, and so they move in the solution faster.

Now let's move to solids. In the reactions between them (or between solid liquid or gas) molecules can collide only on the surface. The more surface area, the faster the reaction. Verify this. Iron in air is not lit. However, this is true only for iron objects. For example, the nails, the surface of contact with air is small, the oxidation reaction is too slow. Iron filings react with oxygen much faster: cold before turning into rust, and in the flame can be ignited. The smallest of grains can flare up without heating. This iron is called pyrophoric. It cannot be planed even the small file, so get it by chemical means, for example, decomposing the salt of oxalic acid - oxalate of iron.

Mix aqueous solutions of some salts of divalent iron, such as iron sulphate and oxalic acid or its soluble salts. A yellow precipitate of oxalate of iron filter and fill in the test tube is not more than one fifth of total. Heat the substance in the flame of the burner, and hold the tube horizontally or slightly inclined, with the opening facing down and away from yourself. Eye-catching water droplets remove a bundle of filter paper or cotton wool. When oxalate decompose and turn into black powder, close the tube and refrigerate it.

Slowly and very carefully empty the contents of the tubes on the metal or asbestos sheet: powder will ignite bright sparks. Particularly spectacular experience in a darkened room.

Important caution: pyrophoric iron cannot be stored, it can cause fire! At the end of the experience powder will flame the bacteriological air or treat acid that remained unburned particles - they can catch on fire by itself.

Next, investigate the impact of the size of the surface of a solid substance on the rate of its reaction with the liquid. Take two of the same piece of chalk and one of them grind into powder. Place both samples in a test tube and pour the same volume of hydrochloric acid. Melkorazdroblennyh chalk, as expected, will dissolve much faster. Another piece of chalk is put in a test tube with sulfuric acid. Began was a vigorous reaction soon calms down, and then completely stops. Why? Because sulfuric acid is not weaker salt.

The reaction of chalk with hydrochloric acid to form calcium chloride СаСl2 which is easily soluble in water and does not impede the flow to the surface of the chalk of new portions acid. When interacting with sulfuric acid produces calcium sulfate CaSO4, and it is very poorly soluble in water, remains on the surface of the chalk and closes it. To the reaction went on, it is necessary from time to time to clean the surface of the chalk in advance or turn it into a powder. Knowledge of such details of the process are very important for chemical technology.

And another experience. Mix in a mortar two solids, giving a colored reaction products: lead nitrate and potassium iodide, iron sulfate, and red blood salt, etc. and grind the mixture with the pestle. Gradually, as the grinding, the mixture will begin to turn, as the surface interaction of substances is increasing. If you pour the mixture a little water, it will immediately appear intense colour because in solution the molecules move much easier.

And in conclusion, experiments on the kinetics put a quantitative experiment; the only tool you will need is a stopwatch or watch with a second hand.

Prepare 0.5 l of 3%aqueous solution of sulfuric acid (to pour acid into the water!) and as much as a 12%solution of sodium thiosulfate. Before the dissolution of thiosulfate add water a few drops of ammonia.

Two cylindrical flask (glasses, stacks) with a capacity of 100 ml put labels on level 50; 25; 12.5 and 37.5 ml, sequentially dividing the height in half. Mark the bottles and pour them in to the top labels (50 ml) prepared solutions.

Conventional thin glass with a capacity of 200 or 250 ml put on dark paper and pour the solution of thiosulfate, and then acid. Immediately note the time and stir the mixture for one to two seconds. In order not to break the glass, it is better to use a wooden stick. As soon as the solution begins to thicken, record the time elapsed from the beginning of the reaction. Experience convenient to have two: one is watching the clock, and the other merges solutions and indicates that the turbidity.

Wash the glass and experiment three more times; the solution of thiosulfate pour into a glass until the third (37,5), second (25) and the first (12.5 ml) labels, adding each time the water up to the top division. The amount of acid in all experiments remains constant, and the total volume of the reaction mixture always equal 100 ml.

Now draw the graph: the dependence of the reaction rate on the concentration of thiosulfate. Concentration is conveniently expressed in arbitrary units: 1, 2, 3 and 4. Put them on the x-axis. But how to calculate the reaction rate?

Just do not least because the point of turbidity, we define the eye, to a certain extent subjective. In addition, the turbidity shows only that the smallest particles of sulfur that are released during the reaction, has reached such a size that they can be seen. And yet, for want of a better, take the beginning of turbidity at the end of the reaction (which, incidentally, is not very far from the truth). Let's make another assumption: the rate of reaction is inversely proportional to its duration. If the response was 10 seconds, we will assume that the rate equal to 0.1. Put the speed on the y axis.

Four experiences have given four points, the fifth - origin. All five points will be located approximately on a straight line. Its equation is written as:

v == k [Na2S2O3]

where v - is the velocity of the reaction, the brackets - made in chemical kinetics designation of concentration, and k is the rate constant, which is easy to find from the graph.

But the reaction rate should depend on the concentration of sulfuric acid. Leaving the amount of thiosulfate unchanged and diluting sulfuric acid, check how the reaction rate. Oddly enough, it never changes!

Such cases are not uncommon. In our experience there is a complex reaction, and the product sulfur is not right with the direct collisions of the molecules of thiosulfate and acid. And in General, reactions where the products are obtained immediately, not so much.

In complex sequential reactions some stage runs slower than the other. In our case, the latter, in which sulfur is formed. It's the speed we, in fact, and measured.

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986

 

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