F7 Essay Kinetics

Only available on StudyMode
  • Download(s) : 261
  • Published : May 12, 2011
Open Document
Text Preview
F7 Essay Writing (Kinetics)

Q.Write an essay on factors which affect the rate of reactions and discuss the uses of kinetic studies.

Outlines:

(I)Factors affecting the Rate of Reaction
(a)temperature
----collision of molecules with different velocities and kinetic energies ----Collision Theory and Maxwell-Boltzmann distribution ----Arrhenius equation and Activation energy

(b)concentration
----frequency of collisions and effective collisions

(c)pressure
----for reaction involving gases only
(solids and liquids are not compressible)
----frequency of collisions and effective collisions

(d)surface area
----state of subdivision and the area of contact opened to reactant molecules

(e)light
----e.g. photosynthesis and formation of silver salts on photographic film

(f)catalyst
----definition of catalyst
----providing new reaction pathway with lower activation energy ----effect shown by Maxwell-Boltzmann distribution

(II)Uses of Kinetic Studies
---optimization of industrial production of chemicals
(max. yield, min. raw materials, min. fuel and shortest time)

---strategy of slowing down some undesirable reactions
(e.g. rusting, deterioration of food)

---archaeological investigations (e.g. carbon-14 dating)

---understanding of reaction mechanisms (e.g. SN2)
~ Sample Essay ~

Chemical kinetics is the studies of the rates of chemical reactions and the factors affect them. Chemical reactions proceed at a rate which is influenced by a number of factors depending on the experimental conditions. In this essay, a brief review on these factors and the applications of kinetic studies will be presented.

Temperature

The most well-known method to make reactions go faster is to heat the reactants. Reactant particles are moving at different velocities, some collision between them are high-energy ones (effective collision) while others are low-energy ones. It is found that only a small fraction of particles has a certain minimum amount of energy, known as activation energy (EA), for them to collide with each other to form products. An increase in temperature increases the number of reactant particles having an energy greater than the EA, thus producing some fruitful collisions.

As shown in the Maxwell-Boltzmann distribution above, when temperature is raised, the fraction of collisions with the required activation energy increases dramatically. Therefore, more molecules will take part in reaction and the rate of reaction will significantly speed up.

In fact, the fraction of effective collisions increases exponentially with temperature, as shown by the Arrhenius equation: k = A exp([pic])
where k is the rate constant of the reaction, A is Arrhenius factor, R is the universal gas constant and T is the absolute temperature of the reaction mixture.

Concentration

Many chemical reactions take place in solution, and the concentration of reactant may affect the speed of reaction. According to the Collision Theory, frequency of collisions increases with increasing concentration, and higher collision frequency leads to a higher chance of a collision having sufficient energy for a reaction to occur. Hence, the rate of reaction generally increases with the concentration of reactant(s).

Pressure

For reactions involving gases, increasing pressure usually speed up the reaction rate. If the pressure of the gases is increased by compression, molecules are pushed closer together. Consequently, the concentration of gases increases, i.e. the molecules collide more frequently and react more rapidly. Again, the rate of gaseous reaction increases. However, it is noticeable that pressure has little effect on the reaction involving solids and liquids, because they are not compressible.

Surface Area of Reactant

In...
tracking img