Kinetics - topic 6 Flashcards
(28 cards)
what is kinetics
Study of how fast a rxn goes
What is the rate of a chemical rxn
Increase in the concentration of products OR decrease in concentration of reactants pero unit time, in mol.dm-3. s-1
rate of rxn (equation)
rate of rxn= (Δ [products])/ Δtime = - (Δ[reactants])/ Δtime
what does the - sign in the rate of rxn equation indicate
that the concentration of reactants decreases over time. Given this, the value would be negative, but for it to be expressed as positive, the - sign is added before
Why are graphs curved and not straight lines
The gradient is not constant, ans therefore can only be given for a particular value of time.
This shows that rate of rxn are dependant on concentration.
As time goes by, the reactant concentration decreases as it gets consumed. This causes a decrease in collision frequency , which decreases rate of rxn
what is the rate of rxn measured at a given time
instantaneous rate of rxn
what is the average rate of rxn
measure of the total change in concentration over the total time taken for the rxn to occur
what is the collision theory
In order for a chemical rxn to occur, particles must collide with sufficient energy and correct orientation
What is the activation energy (Ea)
Minimum value of kinetic energy which particles must have before they are able to react
exithermic rxns
They release heat energy to their surroundings, therefore resulting in products with a lower energy value.
endothermic rxns
They absorb heat energy from their surroundings, therefore resulting in products with a higher energy value.
factors that affect the rate of rxn
temperature, concentration of reactants, surface area, pressure of the rxn mixture, addition of a catalyst
how does temperature affect the rate of rxn
Increasing temperature causes an increase in the average kinetic energy of the particles, meaning that the frequency of collisions increases, and so does the rate of rxn. Also, because particles have a higher average kinetic energy, they are more likely to surpass the Ea.
how does concentration of reactants affect the rate of rxn
Only gaseous and aqueous species have concentration
As the concentration of reactants increases, the number of reacting particles per unit volume also increases, so the frequency of collisions increases. This causes the rate of rxn to increase.
how does surface area affect the rate of rxn
only valid for rxns between (s)+(aq/l/g) or (aq/l)+(g)
as the surface area od solid particles increases, there is more area available for the reaction to occur, thus the frequency of collisions increases , causing rate of rxn to increase
how does pressure of the rxn mixture affect the rate of rxn
Only valid for rxns containing gases as reactants
an increase of pressure at constant temperature causes the concentration of gaseous species to increase. This increases the amount of gaseous particles per unit volume, causing an increase in the frequency of collisions which increases the rate of rxn
how does the addition of a catalyst affect the rate of rxn
the addition of a catalyst lowers the Ea of the rxn, meaning that more particles will be able to overcome the Ea. A catalyst provides an alternative pathway for the rxn to occur, at a lower Ea. A catalyst is not consumed in a chemical rxn.
elements to monitor rates of rxn
change in volume (or pressure) of a gas produced, change in mass, change in colour of the rxn mixture measured by colorimetry, measuring electrical conductivity, changes in pH
how to monitor rates of rxn by the change in volume (or pressure) of gas produced
for this, a gas syringe or water displacement can be used
how to monitor rates of rxn by the change in mass
this method can be used in rxns producing gases (in open system) or rxns involving solid species
method not valid for rxns that produce H2 gaseous (very light, indifferent)
either you take the solid and weight as the rxn occurs, or the whole rxn in a scale and see the change in weight
how to monitor rates of rxn by the change in colour of rxn mixture by colorimetry
this can be used if one of the reactants or products is coloured
the amount of light absorbed is proportional to its concentration
coloured substances include halogen molecules (F2, Cl2, etc.) and transition metal compounds (KMnO4, K2Cr2O7, CuCl2, etc.)
how to monitor rates of rxn by measuring electrical conductivity
it can be measured directly with a conductivity meter
can be used whenever there is free movement of charges. This include soluble salts like NaCl, acids (H+), bases (OH-) and any explicit ions
how to monitor rates of rxn by changes in pH
valid for rxn mixtures that include acids and/ or bases
sometimes acids may be represented as H+ or OH-
what is the kinetic molecular theory of gases
particles in a substance (particularly a gaseous) move randomly as a result of the KE that they possess
because of these random movements, not all particles have the same value of KE
a convenient way to describe the KE of the particles is to take the average of this values and this is directly proportional to it’s absolute Temperature
