Chemistry · MU Pleven

CHEMICAL KINETICS. MOLECULARITY AND ORDER OF REACTIONS. RATE EQUATIONS

Covers chemical kinetics from the classification of reactions as fast, moderate, slow or very slow, through the definitions of reaction rate, average and instantaneous rates, and the factors that affect rate. It then explains reaction order, molecularity with a comparison table, and first-order rate equations with the half-life relation.

Inside: 7 sections · 11 labelled pictures · 1 table · 27 formulas · 25 flashcards · 22 min

Topic 11 of 47 in the MU Pleven syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 7 sections, about 3,178 words (22 min of reading), with 25 flashcards.

In this note

  1. CHEMICAL KINETICS. MOLECULARITY AND ORDER OF REACTIONS. RATE EQUATIONS
  2. I. CHEMICAL KINETICS
  3. II. RATE OF CHEMICAL REACTIONS
  4. III. FACTORS AFFECTINGREACTIONRATES.Basedonexperimental evidence we know that the rateofareactionisinfluencedbythefollowingfactors:
  5. IV. REACTION ORDER
  6. V. MOLECULARITY OF THE REACTION
  7. VI. RATE EQUATIONS

I. CHEMICAL KINETICS

Chemical reactions vary widely in their rates. There are chemical reactions that complete in a very short time, such as less than 6-10 seconds, they are called fast reactions. Ionic reactions such as the reaction between sodium chloride and silver nitrate are very fast reactions producing precipitates. Burning of fuels like petrol and combustion of magnesium, phosphorus, sulfur etc., are also very fast reactions.

Moderate reactions are those reactions that take place in few minutes to few hours. For example, oxidation of oxalic acid by acidified potassium permanganate and hydrolysis of esters are reactions with measurable speeds:

Chemical reactions that occur very slowly and can take a long time for completion are called slow reactions. Some reactions can take days, weeks and months to complete; they are called very slow reactions. Rusting of iron, formation of crude oil by a geochemical reaction, the reaction between hydrogen and oxygen are very slow reactions.

Usually covalent compounds are involved in slow reactions while reactions involving ions proceed rapidly.

These examples reveal that chemical processes have different rates at the same external conditions. On the other hand, one chemical process may have different rates depending on the conditions. For example, oxygen and hydrogen practically do not interact at room temperature, whereas at 600o C they explode if mix together:

In daily life we encounter many reactions for which the speed of reaction has vital importance and there is a need to be able to measure, control, or predict the rates of chemical reactions.

Thermodynamics allows us to decide whether or not a chemical reaction is favorable, but it cannot tell how quickly reactions may occur.

Chemical kinetics allows you to determine how quickly a reaction occurs.

The branch of chemistry, which studies the reaction rates and the mechanism of chemical reactions, is called chemical kinetics.

Reaction rate is the velocity with which reactants are used up and products are formed.

Reaction mechanism is the pathway (the steps involved in the chemical reaction) by which reactants form products.

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