Chemistry notes

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1.   Demonstrate an understanding of the terms-  RATE OF REACTION,  RATE EQUATION,  ORDER OF REACTION,  RATE CONSTANT,  HALF-LIFE,  RATE DETERMINING STEP,  ACTIVATION ENERGY,  HETEROGENOUS AND HOMOGENOUS CATALYST.

The rate of reaction is the rate of change of the concentration of the reactants or products with time. Its unit is moldm^-3s^-1

The rate equation describes how the rate of reaction depends on the concentration of the reactants each raised to the appropriate power.
Example : Consider the following chemical equation : pP +qQ ----> rR + sS
                    Rate = K[P]^m[Q]ⁿ where m and n are obtained experimentally and K = rate constant

The order of reaction with respect to a particular reactant is the power raised of the concentration in terms of the reactants in the rate equation.
Example : Consider the following rate equation.
Rate = K[P]^m[Q]ⁿ
The reaction is m order with respect to P
The reaction is n order with respect to Q

The rate constant is a constant of proportionality between the reaction rate and the concentration of the various species that appear in the rate equation.
NOTE : Rate constant has no fixed unit. Its unit will depend on the power of the concentration in the rate equation.

Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half.

The rate determining step is a slow step out of a series of steps which take place in chemical reactions.

Activation energy is the minimum amount of energy required by reacting particles for a chemical reaction to take place.

A homogenous catalyst is one that is in the same physical state as the reactants and it operates by a change in oxidation number.

A heterogenous catalyst is one that is not in the same physical state as the reactants. It operates by absorbing the reactants on its surface, weaking the bonds and accelerating the formation of the products.

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6.   Deduce from experimental data for reactions with zero, first and second order kinetics:-
i.   half-life ( the relationship between half-life and rate constant will be given if required)
ii.   order of reaction
iii.   rate equation
iv.   rate-determining step related to reaction mechanisms
v.   activation energy (by graphical methods only, the Arrhenius equation will be given if needed)

Consider the following rate equation

Rate = K [P][Q]²[R]⁰

1. If a reaction is zero order with respect to a particular reactant, then a graph of concentration of that reactant against time will be a straight line with decrease in concentration. It will have fixed gradient indicating that it is not affecting the rate od reaction with change of concentration.

2. For reactions with first order, second order, third order their graph will be a curve.

However the graph can be used to identify whether the reaction is first order or not because first order reactants will have a constant half-life.

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7.   Investigate the activation energy of a reaction, eg- OXIDATION OF IODIDE IONS BY IODATE (V)

Activation energy is the minimum amount of energy required by reacting particles for a chemical reaction to take place.

The reacting particles in a chemical reaction can be represented by Boltzmann distribution graph.


From the graph we can deduce that most of the reacting particles do NOT take part in the chemical reaction since they do not have energy equal or greater than activation energy (Ea)

Effect of temperature on rate of reaction : http://www.chemguide.co.uk/physical/basicrates/temperature.html

Rate of reaction is also affected by the presence of a catalyst. The latter accelerate the rate of reaction without itself being chemically involved in the reaction.

A catalyst operates by lowering the activation energy thus providing more reacting particles with energy greater to the new activation energy. This can also be represented by Boltzmann distribution curve.


Here are the different ways along with graph that can affect rate of reaction