Physical factors affecting rate of reaction.

1. Reaction Rates (Chemical Kinetics)
Key Concepts
The rate of a chemical reaction is the speed with which reactants are converted to products.
Collision Theory is used to explain why chemical reactions occur at different rates.
Collision Theory states that in order for a reaction to proceed, the reactant particles must collide.
The more collisions there are per unit of time, the faster the reaction will be.
In order for a reaction to proceed, the reactant particles must:
collide with sufficient energy to break any bonds in the reactant particles.
The activation energy is the minimum amount of energy the colliding reactant particles must have in
order for products to form.
be in an orientation favourable for breaking those bonds.
THE EFFECT OF TEMPERATURE ON REACTION RATES
Usually, an increase in temperature is accompanied by an increase in the reaction rate.
Temperature is a measure of the kinetic energy of a system, so higher temperature
implies higher average kinetic energy of molecules and more collisions per unit time.
A general rule of for most (not all) chemical reactions is that the rate at which the
reaction proceeds will approximately double for each 10°C increase in temperature.
Once the temperature reaches a certain point, some of the chemical species may be
altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop.
With the exception of some precipitation reactions involving ionic compounds in
solution, just about all chemical reactions take place at a faster rate at higher
temperatures. The question is why?
Temperature (in Kelvin degrees) is proportional to
the kinetic energy of the particles in a substance.
For example, if the Kelvin temperature of a
substance is doubled, then the average kinetic
energy of the particles in that substance is
doubled.

2. At higher temperatures, particles collide more frequently and with greater intensity.
Examples
Some reactions are virtually instantaneous - for example, a precipitation reaction
involving the coming together of ions in solution to make an insoluble solid, or the
reaction between hydrogen ions from an acid and hydroxide ions from an alkali in
solution. So heating one of these won't make any noticeable difference to the rate of
the reaction.
Almost any other reaction you care to name will happen faster if you heat it - either in
the lab, or in industry.
The explanation
Increasing the collision frequency
Particles can only react when they collide. If you heat a substance, the particles move
faster and so collide more frequently. That will speed up the rate of reaction. Collisions
only result in a reaction if the particles collide with enough energy to get the reaction
started. This minimum energy required is called the activation energy for the reaction.
It turns out that the frequency of two-particle collisions in gases is proportional to the
square root of the kelvin temperature.
The key importance of activation energy
In a sample of substances, at a given temperature, the particles will not all possess the
same amount of energy as each other.A few will have a relatively small amount of
energy.A few particles will have a relatively large amount of energy.Most particles will
have an amount of energy somewhere in between.The distribution of energies at a

3. given temperature can be shown on a graph called boltzman distribution.
You can mark the position of activation energy on a Maxwell-Boltzmann distribution to
get a diagram like this:
Only those particles represented by the area to the right of the activation energy will
react when they collide. The great majority don't have enough energy, and will simply
bounce apart.
To speed up the reaction, you need to increase the number of the very energetic
particles - those with energies equal to or greater than the activation energy. Increasing
the temperature has exactly that effect - it changes the shape of the graph.
In the next diagram, the graph labelled T is at the original temperature. The graph
labelled T+t is at a higher temperature.
If you now mark the position of the activation energy, you can see that although the
curve hasn't moved very much overall, there has been such a large increase in the

4. number of the very energetic particles that many more now collide with enough energy
to react.
Remember that the area under a curve gives a count of the number of particles. On the
last diagram, the area under the higher temperature curve to the right of the activation
energy looks to have at least doubled - therefore at least doubling the rate of the
reaction.
Summary
Increasing the temperature increases reaction rates because of the large increase in the
number of high energy collisions. It is only these collisions (possessing at least the
activation energy for the reaction) which result in a reaction
Increasing the temperature of a reaction increases the kinetic energy of the particles which increases
the number of collisions so the reaction rate increases.
Increasing the kinetic energy of reactant particles also means more of the reactant particles will have
the minimum amount of energy required to form products (ie, activation energy) which leads to more
successful collisions and therefore increases the reaction rate.
Increasing the temperature will increase the reaction rates of both endothermic and exothermic
reactions, it will also, by Le Chetalier's Principle, affect the equilibrium position.
pH
pH is a measure of the concentration of hydrogens ions (= H+) (= protons) in a solution.
Numerically it is the negative logarithm of that concentration expressed in moles per liter (M).

5. Pure water spontaneously dissociates into ions, forming a 10-7 M solution of H+ (and OH-). The negative
of this logarithm is 7, so the pH of pure water is 7.
Solutions with a higher concentration of H+ than occurs in pure water have pH values below 7 and are
acidic.
Solutions containing molecules or ions that reduce the concentration of H+ below that of pure water
have pH values above 7 and are basic or alkaline.
The Effect of pH on Enzyme Activity
pH is a measure of the concentration of hydrogen ions in a solution.
The higher the hydrogen ion concentration, the lower the pH. Most
enzymes function efficiently over a narrow pH range. A change in pH
above or below this range reduces the rate of enzyme reaction
considerably.
Changes in pH lead to the breaking of the ionic bonds that hold the
tertiary structure of the enzyme in place. The enzyme begins to lose
its functional shape, particularly the shape of the active site, such
that the substrate will no longer fit into it, the enzyme is said to
be denatured. Also changes in pH affect the charges on the amino acids
within the active site such that the enzyme will not be able to form
an enzyme-substrate complex.
The pH at which an enzyme catalyses a reaction at the maximum rate is
called the optimum pH. This can vary considerably from pH 2 for pepsin
to pH 9 for pancreatic lipase
particle size
Smaller reactant particles provide a greater surface area which increases the chances for particle
collisions so the reaction rate increases.

6. presence of a catalyst
A catalyst lowers the activation energy for the reaction so more reactant particles will have the
minimum amount of energy required to form products so the reaction rate increases.
Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of a
chemical reaction without being consumed in the process. Catalysts work by increasing the frequency of
collisions between reactants, altering the orientation of reactants so that more collisions are effective,
reducing intramolecular bonding within reactant molecules, or donating electron density to the
reactants. The presence of a catalyst helps a reaction to proceed more quickly to equilibrium. Aside
from catalysts, other chemical species can affect a reaction. The quantity of hydrogen ions (the pH of
aqueous solutions) can alter a reaction rate. Other chemical species may compete for a reactant or alter
orientation, bonding, electron density, etc., thereby decreasing the rate of a reaction.
intensity of light affects some reactions
Some reactions occur very slowly in the dark but much more quickly in light.
eg, methane reacts very slowly with chlorine in the dark, but the rate of reaction is much faster in the
presence of ultraviolet light.Light provides the activation for a reaction to occur. Radiation of proper
frequency and sufficient energy must be absorbed to activate the molecules.
Energy unit of radiation is photon and is equal to Quantum of energy. Photochemical reactions are
independent of temperature. After a molecule has absorbed quantum of radiant energy,it will raise their
kinetic energy and temperature of system increases. Example of photosynthesis.

7. Ionic Strength
Ionic strength is a characteristic of an electrolyte solution (a liquid with positive and negatively
charged ions dissolved in it). It is typically expressed as the average electrostatic interactions
among an electrolyte's ions. An electrolyte's ionic strength is half of the total obtained by
multiplying the molality (the amount of substance per unit mass of solvent) of each ion by its
valence squared.
The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic
compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in
solution will affect important properties such as the dissociation or the solubility of different
salts. One of the main characteristics of a solution with dissolved ions is the ionic strength.
Quantifying ionic strength
The ionic strength, I, of a solution is a function of the concentration of all ions present in that
solution.
where ci is the molar concentration of ion i (mol·dm-3), zi is the charge number of that ion, and
the sum is taken over all ions in the solution. For a 1:1 electrolyte such as sodium chloride, the
ionic strength is equal to the concentration, but for MgSO4 the ionic strength is four times higher.
Generally multivalent ions contribute strongly to the ionic strength.
Ionic strength is closely related to the concentration of electrolytes and indicates how
effectively the charge on a particular ion is shielded or stabilized by other ions (the so-called
ionic atmosphere) in an electrolyte. The main difference between ionic strength and electrolyte
concentration is that the former is higher if some of the ions are more highly charged. For
instance, a solution of fully dissociated (broken down) magnesium sulfate (Mg+2 SO4-2) has 4
times higher ionic strength than a solution of sodium chloride (Na +Cl -) of the same
concentration. Another difference between the two is that ionic strength reflects the
concentration of free ions, and not just of how much salt was added to a solution. Sometimes a

8. salt may be dissolved but the respective ions still bound together pairwise, resembling uncharged
molecules in solution. In this case the ionic strength is much lower than the salt concentration.
Importance
It is also important for the theory of double layer and related electrokinetic phenomena in
colloids and other heterogeneous systems.. Increasing the concentration or valence of the
counterions compresses the double layer and increases the electrical potential gradient.
Media of high ionic strength are used in stability constant determination in order to
minimize changes, during a titration, in the activity quotient of solutes at lower
concentrations.
Ionic strength is an important factor in biochemical reactions like metabolism to respiration.
Ionic strength is a key factor in these reactions because it affects the rates at which ions react
with each other and, thus, the extent to which the reaction occurs. Enzymes, protein molecules
that catalyze and regulate reactions important to life, can also be extremely sensitive to ionic
strength and may become insoluble or inactive if the organism's ionic strength is too high or too
low, much in the same way that they are extremely sensitive to temperature.
Take for example, the case of a person running. When he or she begins to perspire they will lose
moisture as well electrolytes, or ions. This loss of electrolytes is a practical example of how ionic
strength works. If the runner does not replace those lost electrolytes, he or she will become
thirsty, sluggish, and overheated.
Ionic strength is one of the basic characteristics of an organism's chemical makeup that
determines whether that organism can exist in a state of homeostatis, or internal stability. In
higher animals, the kidneys regulate the body's ionic strength by maintaining electrolyte and
water balances.
Acetylcholine, a positively charged ion that organisms release at the ends of certain neurons.
Acetylcholine's jobs are to serve as a bridge between neurons, passing along nerve impulses from
one to the next, and to start muscular contractions. Ionic strength determines the rate at which
acetylcholine reacts with other chemicals in an organism, so if the ionic strength of the
organism's electrolytes was too high, the acetylcholine would react at a rate too slow or too fast,
or may bind too strongly or too weakly to its receptor for the organism to function normally.
Ionic strength is a also a useful parameter in the laboratory. If a researcher knows the ionic
strength of an electrolyte, it can tell him or her a great deal about the dynamics of specific
chemical reactions.