Rates of Reaction Flashcards
what is needed for a reaction to occur?
- particles must collide with sufficient energy which overcomes the activation energy.
- particles must collide with the correct collision geometry.
activation energy definition
the minimum kinetic energy required for a reaction to occur.
activated complex definition
an unstable arrangement of atoms. It is an intermediate between reactants and products.
what is an enzyme
a biological catalyst
homogeneous catalyst
a catalyst in the same physical state as the reactants
heterogeneous catalyst
a catalyst in a different physical state from the reactants.
how does using a catalyst increase the rate of reaction?
it provides an alternative reaction pathway with a lower activation energy.
negative effect if a reaction is too slow
it will not be economically viable
negative effect if a reaction is too fast
there will be a risk of explosion
average rate of reaction equation
average rate = change in q/change in t
relative rate equation
relative rate = 1/t units s^-1
do reactants with energy greater than that activation energy always collide?
no, they also need the correct collision geometry.
what happens to catalysts at the end of a reaction?
they are regenerated and therefore can be used again.
temperature definition
a measure of the average kinetic energy of all the particles in a sample.
effect of increasing temperature
the number of particles with Ek above Ea increases
when drawing graph: Ea stays the same, area under graph stays the same so draw to the right and slightly lower down
potential energy diagram purpose
shows the progress of a reaction from reactants to products
decreasing particle size
decreasing particle size increases surface area of the reactant.
more particles exposed results in greater chance of successful collisions, therefore increased rate of reaction.
increasing concentration
increases the number of particles. more particles result in greater chance of successful collisions, therefore increased rate of reaction
increasing temperature
increase speed of particles
particles moving at a greater speed result in more successful collisions, therefore increased rate of reaction.
more particles have energy greater than or equal to the activation energy.
increasing pressure
reactant particles are pushed closer together (same number of particles in a smaller volume). Therefore, more chance of successful collisions and increased rate of reaction.
when is relative rate calculation used
when it is difficult to measure the change in a chemical reaction (colour change etc)
enthalpy change equation
Hp-Hr
enthalpy change for exothermic reactions
negative
enthalpy change for endothermic reactions
positive
why a small change in temperature can produce a large change in reaction rate
because increasing the temperature increases the number of successful collisions because there are more molecules with energy greater than the activation energy.
how heterogeneous catalysts work
- adsorption: reacting molecules are adsorbed onto active sites.
- reaction: reaction occurs on the surface with less energy needed to form activated complex.
- desorption: product molecules are formed and leave the catalysts surface free for further reactions.
catalyst poisoning
heterogeneous catalysts are easily poisoned to the extent that their activity can be greatly reduced or stopped completely
the poison particles form strong bonds with the surface of the catalyst blocking the active sites.
examples of catalyst poisoning
if lead poisons a catalytic converter in a car it would be rendered useless.
what are energy distribution diagrams also known as?
Maxwell-Boltsman distribution curve
what does area under energy distribution diagram equal?
number of molecules in sample
general rule of thumb when increasing temperature by 10 degrees
rate of reaction doubles
enzyme optimum temperature
enzyme activity increases until it reaches a peak
this is the temp at which enzyme works best
what happens to enzyme after optimum temperature
rate will fall drastically to zero
hydrogen bonds break and protein unravels and changes shape (denaturing)
denatured enzyme
enzymes no longer work and so their activity (rate) falls to zero.
examples of enzymes in nature
-catalase: breaks down hydrogen peroxide
examples of enzymes in industry
-zymase: converts glucose into alcohol in the brewing industry
