How to increase reaction speed

In con­duct­ing chem­i­cal ex­per­i­ments, ex­per­i­menters of­ten face the ques­tion of how to in­crease re­ac­tion speed, or how to start the re­ac­tion, whether it’s a re­versible process or not. Here the fol­low­ing meth­ods are re­quired:

• raise the tem­per­a­ture;
• add a cat­a­lyst;
• in­crease the con­cen­tra­tion of reagents;
• change the ag­gre­gate state of reagents, re­duce their size (in­crease the area of con­tact of sub­stances).

Re­ac­tion speed is a char­ac­ter­is­tic of a chem­i­cal re­ac­tion which shows a change in con­cen­tra­tion of one of the reagents in a unit of time.

In­creas­ing the tem­per­a­ture of the re­ac­tion is the most uni­ver­sal and sim­ple method of ac­cel­er­at­ing the in­ter­ac­tion of reagents.

In the in­ter­ac­tion of sub­stances with one an­oth­er, their par­ti­cles col­lide, as a re­sult of which the elec­trons from the out­er or­bitals may move from one atom to an­oth­er. Not all in­ter­ac­tions lead to a change in the state of medi­ums, only par­ti­cles with a high ki­net­ic en­er­gy are ca­pa­ble of “knock­ing out” elec­trons.

The ques­tion of how to in­crease ki­net­ic en­er­gy is solved by heat­ing the sub­stance; the par­ti­cles in­crease their move­ment in medi­ums, and the num­ber of col­li­sions in­creas­es. The re­ac­tion ac­cel­er­ates.

To de­ter­mine the de­pen­dence of the speed of the process on the tem­per­a­ture, you must use the Van ‘t Hoff rule, but it can­not work for all types of chem­i­cal re­ac­tions. It can only be used to give an ap­prox­i­mate es­ti­mate of the in­flu­ence of the tem­per­a­ture in a range from 0 to 100 de­grees Cel­sius.

V₂= V₁х

In the home, you can pre­pare and ex­am­ine the ther­mal de­pen­dence of a chem­i­cal re­ac­tion on the ex­am­ple of the in­ter­ac­tion of a so­lu­tion of io­dine and starch. This is an in­ter­est­ing ex­per­i­ment which can even be rec­om­mend­ed for be­gin­ner chemists. Dis­solve a tea­spoon of starch in a small amount of wa­ter, adding a lit­tle io­dine.

I₂ + (C₆H₁₀O₅)n (starch) ⇄ I₂*(C₆H₁₀O₅)n

The re­ac­tion is re­versible. The com­bi­na­tion ob­tained as a re­sult is un­sta­ble, it col­ors the wa­ter blue, and at high tem­per­a­ture it be­comes col­or­less (the speed of its break­down in­creas­es). When it cools, the wa­ter turns blue once again.

Cat­a­lyst re­ac­tions

In many chem­i­cal ex­per­i­ments, to in­crease re­ac­tion speed, you have to add spe­cial sub­stances – cat­a­lysts. They take part in pro­cess­es as in­ter­me­di­aries, with­out be­com­ing part of the fi­nal sub­stances, re­main­ing un­changed. At the same time their pres­ence, be­cause of their in­volve­ment in the in­ter­me­di­ary stages of re­ac­tion, helps new sub­stances to ap­pear, speed­ing up the re­ac­tion by many times with a min­i­mum en­er­gy loss.

So the pres­ence of a cat­a­lyst is like a “side ma­neu­ver” in an ex­per­i­ment, when reagent sub­stances do not want to in­ter­act or do so too slow­ly.

A dis­tinc­tion is made be­tween ho­moge­nous (when the state, struc­ture of reagents and cat­a­lyst are ho­moge­nous) or het­ero­ge­neous cat­a­lysts. Re­ac­tions of sol­id sub­stances are al­ways het­ero­ge­neous.

You can see an ex­am­ple of a het­ero­ge­neous re­ac­tion at home by burn­ing sug­ar (with­out a cat­a­lyst a sug­ar cube sim­ply caramelizes). To make the ex­per­i­ment, cov­er the sug­ar cube in ash (this is the cat­a­lyst in this case) and set fire to it.

С₁₂Н₂₂О₁₁(sug­ar) + 12О₂ → 12СО₂ + 11Н₂О

For each re­ac­tion, dif­fer­ent types of cat­a­lysts must be used:

• acid;
• al­ka­li;
• met­al ox­ides;
• gas­es;
• com­plex or­gan­ic com­pounds (many cat­a­lyst re­ac­tions take place in our body, and the cat­a­lysts are called en­zymes);
• wa­ter;
• sand.

Here you can see de­tailed in­struc­tion on con­duct­ing this ex­per­i­ment.

Cre­ate an ef­fect of ac­cel­er­at­ed re­ac­tion with change of con­cen­tra­tion

You can make a re­ac­tion take place more quick­ly with­out cat­a­lysts and with­out chang­ing the tem­per­a­ture. A high con­cen­tra­tion of sub­stances is used. Let’s see why this method works:

• the dis­tance be­tween par­ti­cles de­creas­es;
• a greater num­ber of ef­fec­tive col­li­sions be­tween them takes place;
• this leads to an ac­cel­er­a­tion in the re­ac­tion, and a cre­ation of new com­pounds.

The dis­cov­ery of the the­o­ry of ac­tive col­li­sions helped to ex­plain the se­cret of a change in con­cen­tra­tions.

Look at the ex­am­ple of this re­ac­tion:

4НВr + O₂ = 2Н₂О + 2Вr₂

It takes place quite quick­ly at a tem­per­a­ture of over 4,000, and a sim­i­lar re­sult can be achieved by in­creas­ing the con­cen­tra­tion of any of the reagents.