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The rate and extent of chemical change (Factors which affect the rates of…
The rate and extent of chemical change
Calculating rates of reaction
mean rate of reaction
= (quantity of reactant used)/(time taken)
= (quantity of product formed)/(time taken)
units of rate of reaction = g/s or cm^3/s or mol/s
Factors which affect the rates of chemical reactions
concentration of reactants in solution
pressure of reacting gases
the surface are of solid reactants
the temperature
the presence of catalysts
Collision theory and activation energy
chemical reactions can occur only when reacting particles
collide
with each other with
sufficient energy
the minimum amount of energy that particles must have to react is called
activation energy
increasing the
concentration
of reactants in solution, the
pressure
of reacting gases, and the
surface area
of solid reactants increases the
frequency of collisions
and so increases the rate of reaction
increasing the
temperature
increases the
frequency of collisions
and makes the
collisions more energetic
, and so increases the rate of reaction
Catalysts
change the rate
of reactions but are
not used up
during the reaction
different reactions need
different
catalysts
enzymes
act as catalysts in
biological
systems
catalysts
increase the rate
of reaction by providing a
different pathway
for the reaction that has a
lower activation energy
not included in the chemical equation for the reaction
Reversible rections
the products of the reaction can react to produce the original reactants
Energy changes and reversible reactions
if a reversible reaction is exothermic in one direction it is endothermic in the other
Equilibrium
when a reversible reaction occurs in apparatus which prevents the escape of reactants and products,
equilibrium
is reached when the
forward and reverse
reactions occur at
exactly the same rate
The effect of changing conditions on equilibrium
the relative amounts of all the reactants and products at equilibrium
depend on the conditions
of the reaction
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to
counteract the change
The effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier's Principle
The effect of changing concentration
If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached agin
If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again
If the concentration of a product is decreased, more reactants will react until equilibrium is reached again
The effect of temperature changes on equilibrium
if the temperature of a system at equilibrium in increased:
the relative amount of products at equilibrium increases for an endothermic reaction
the relative amount of products at equilibrium decreases for an exothermic reaction
if he temperature of a system at equilibrium is decreased:
the relative amount of products at equilibrium decreases for endothermic reaction
the relative amount of products at equilibrium increases for an exothermic reaction
The effect of pressure changes on euilibrium
for gaseous reactions at equilibrium:
an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules
a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules