The rate of reaction and concentration of reactant

The rate of reaction and concentration of reactant

The rate of reaction and concentration of reacting molecule describes the effect of reaction rate on changing of concentration of reactant molecules. It is a page for introduction of chemical kinetics. In the different types of reaction involved in gases or liquid, rate of reaction increases continuously with increase in concentration of reactant molecules. The fact when concentration is in collision, the frequency of reactant molecules direct effect on rate of reaction. Generally rate of reaction increase with increase in concentration of reactant molecules but we should not image that rate of reaction may not be exactly increased by twice, if we double the concentration of reactant molecules. In the mathematical reaction or double displacement reaction, both the reactant molecules rather than single one. In some reaction in which equilibrium shift towards back direction the rate of reaction does not increases or constant with concentration of reactant molecules.

The reaction between zinc and hydrochloric acid is,

Zn(s) + 2HCl (l) → ZnCl₂(l)+ H₂(g)↑

The rate of reaction becomes slower if zinc crystal in lab reacts with diluted hydrochloric acid when we use concentration hydrochloric acid with zinc crystal or when we increase the concentration of hydrochloric acid rate of reaction is found to be fast.

The catalytic decomposition of hydrogen peroxide

2H₂O₂(l)^MnO₂→ 2H₂O(l) + O₂(g)↑

When manganese (iv) oxide is used as catalyst in this reaction rate of reaction is found to be faster in use of concentration hydrogen peroxide rather than diluted hydrogen peroxide.

Reaction between sodium thiosulphate solution and hydrochloric acid

Na₂S₂O₃(l) +2HCl(l)→ 2NaCl(l) + H_2O (l) +H₂O(l) +S(s) + SO₂(g)

When sodium thiosulphate is used in diluted form it takes ore and more time to form precipitate or longer time is consumed to form yell precipitate but when we used concentration sodium thiosulphate in case of diluted one, yellow color precipitate form immediately or within certain time period.

The collision involving between two particles has major role in rate of reaction. This concept is also applied in the reaction where the reaction involved between two different particles or two of same particle. In order or rule to form a reaction the reactant molecules first collide in proper orientation with sufficient energy. This concept is true whether the reaction takes place in liquid or gas phases. If concentration is higher which cause the increase in collision frequency between two molecules.

If reaction is involved in a single reactant or particle which splitting up in same way, then the number of collision is found to be irrelevant. In general matters have enough energy to equal or exceed the activation energy. If we had millions of particles only 100 of them would react. If we had 200 million particles in same volume, 200 of them could be reacted. Which shows that rate of reaction nearly becomes double, if concentration of reactant doubled. There is a same causes in which rate of reaction does not effect when concentration of reactant molecules increased. First one is in case of that type of reaction where reaction is already catalyzed by catalyst for this fact, we are using a small amount of solid catalyst in the reaction and catalyst had already clustered up the total reacting surface of reacting particles. In this case the increase in concentration of reactant molecules rate of reaction remain unchanged. Another case is in multi-step reaction which support an A level point of view. Suppose you have a reaction which completes with series of small steps. These steps are fallowed widely with different rates some fast and some slow. To support it consider a following reaction which occurred between two reactants molecules A and B.

A(aq) → C(aq) + D(aq)

C(aq) + B(aq) → Othere product

As we considering overall steps, rate of reaction is controlled by splitting tendency of a to make intermediate C and D. the slow steps explains in term of rate determining step or rate of reaction is directly proportional to the first slow step reaction. If we increase the concentration of B as in second step reaction by double, the rate of reaction becomes unchanged. It became hardly to change overall rate of reaction. The second step reaction in above considering reaction seems to be waiting for completion of first step reaction so it also seems to be second step reaction directly depend upon first step reaction. That the reason in which rate of reaction depend upon slow step reaction of first step reaction. The best example of above comes on organic chemistry as SN1 reaction. This reaction completed with the tertiary substances including hydroxide ion. These examples are involved in nucleophitic substitution reaction which is.

Reference

Bahl, B.S. Essential of Physical Chemistry. New Delhi: S. Chand & Company Pvt. Ltd, 2005.

Bahl, B.S., Arun Bahl and G.D. Tuli. Essential of Physical chemistry . New Delhi: S. Chand & Company Pvt. Ltd., 2005.