kinetics Flashcards
what is kinetics
Kinetics is the study of the factors that affect the rates of reaction
what is rates of reaction
the rate of reaction is how quickly the reaction takes place
are all rates of reactions the same
There is a large variation in reaction rates
e.g. popping a test tube full of hydrogen is over in a fraction of a second
complete rusting of an iron nail could take several years
how can you change the speed of a reaction
reactions can be sped up or slowed down by changing the conditions of the reaction
what must happen for a reaction to take place
for a reaction to take place between two particles they must collide with enough energy to break bonds
where must the collision take place
The collision must take place between the parts of the molecule that is going to react together.
Therefore orientation has a part to play
How do you get a lot of collisions
you need a lot of particles in a small volume
how do particles get enough energy to break bonds
for the particles to have enough energy to break bonds, they need to be moving fast.
how can we have a fast reaction rate
A fast reaction rate requires rapidly moving particles and a small volume
do all collisions between particles lead to a reaction
most reactions btween molecules or oher particles do not lead to a reaction because:
they do not have enough energy ( to break the bonds )
they are in the wrong orientation
what effects the rates of chemical reactions
increasing the temperature
increasing the concentration of a solution
increasing the pressure of gas reaction
increasing the surface area of solid reactants
using a catalyst
how does increasing the temperature affect the rate of chemical reaction
increases the speed of the molecules which in turn increases their energy and the number of collisions
how does increasing the concentration of a solution affects the rate of chemical reaction
more particles present in a given volume then more collisions are more likely to happen and the reaction would be faster
how does increasing the pressure of a gas reaction effect the reaction
same effect as increasing concentration of a solution
more molecules or atoms in a given volume so more collisions are likely therefore speeding the reaction up
how does increasing the surface area of a solid reactant effect the rate of reaction
the greater the surface area of the solid, the more of its particles are available to collide with molecules in a gas or liquid. This means that breaking a solid lump into smaller pieces increases the rate of its reactions because there are more sites for reaction
what is a catalyst and how does it effect the rate of reaction
a catalyst is a substance that can change the rate of a chemical reaction without being chemically changed itself
what must happen for a collision to result in a reaction
the molecules must have a certain minimum energy, enough to start breaking bonds. This energy is called the activation energy
what is an exothermic reaction
exothermic reactions transfer energy from molecules to the surroundings. This causes the surroundings to heat up
what are some examples of exothermic reactions
- combustion
2. certain oxidation and neutralization reactions
how does a reaction profile/ enthapy diagram look like for a exothermic reaction
Products have less energy than the reactants. This is because energy is being transferred from reaction to the surroundings
The difference between the reactants and the energy in the products tells us the energy that is being released to the surroundings
what are some uses for exothermic reactions
- hand warmers
2. self heating cans e.g. for food and drinks
what is an endothermic reaction
endothermic reactions take in energy from their surroundings therefore, the temperature of the surroundings get decrease [ get colder].
what are some examples of endothermic reactions
thermal decomposition
how does an energy profile/ enthapy look like for a endothermic reaction
Products have more energy energy than reactants because energy is being taken in from the surroundings
The difference in energy between the reactants and products tells is the amount of energy taken in by the reaction
what do both of the reaction profiles of endothermic and exothermic reactions have in common
Both endo and exo energy profiles both have something in common - the energy rise to a peak. This is the activation energy. The activation energy is from the reactants to the peak
what happens if the reaction has a large activation energy
This reaction will take place extremely slowly at room temperature because very few collisions will have sufficient energy to bring about a reaction ( break bonds)
what happens if a reaction has a small activation energy
This reaction will take place rapidly at room temperature because many collisions will have enough energy to bring about a reaction (break bonds)
what is the species
species is a term tat used by chemists to refer to an atom, molecule or ion.
what is the species that exists at the top of an enthapy/ energy profile
The species that is at the top of the curve of an enthapy diagram is called a transition state or activated complex. Some bonds are in the process of being made and some are in the process of being broken.
It has extra energy and is unstable
what is the Maxwell- Boltzmann distribution
Particles in any gas ( or solution) are all at different speed e.g.
- few are moving slowly
- few are very fast
- most are in the middle
Energy of a particles depends on its speeds the particles also have are range of energies.
If you plot a graph of energy against the fraction of particles that have energy, you end up with a curve.
This shape us called the Maxwell- Boltzmann distribution - it tells us the distribution of energy amongst particles
describe the shape of the graph
- The area under the graph represents the total number of particles
- no particles have zero energy hence why the graph always starts at 0-0
- most particles have intermediate energies ( around the peak of the curve)
- a few have very high energies ( the right hand side of the curve) . In fact there is no upper limit
- the most probable energy is at the peak. It is the energy with the most particles
- the average energy is slightly to the left of the probable energy. The average energy is not the same as the probable energy
what is the activation energy and where is it marked on the Maxwell Boltzmann distribution graph
in order for a reaction to take place, collisions between particles must have enough energy to start breaking bonds. This amount of energy is called the activation energy
If you mark Ea on a Maxwell Boltzmann distribution graph the area underneath the graph to the right of the Ea represents the number of particles with enough energy to react
how does temperature affect the shape of the M.B graph
the higher the temperature:
- the peak of the curve is lower and moves to the right
- the number of particles with very high energy increases
- the graph shows that at a higher temperature more molecules have enough energy greater or the same as the Ea. This means a higher percentage of collisions will result in a reaction
why does the reaction rate increases with temp
a small increase in temperature produces a large increase in the number of particles with energy greater than or the same as the activation energy (Ea)
what are catalysts
catalysts are substances that affect the rate of chemical reactions without being chemically changed themselves at the end of the reaction
why are catalysts important in the industry
catalysts are used to speed up reactions so they are important in the industry
It is cheaper to speed up a reaction by using a catalyst than by using high temperatures and pressures. This is even if the catalysts are expensive as they are not used during the reaction.
how do catalyst work
catalyst provides a different pathway for the reaction, one with lower activation energy.
Therefore, they reduce the activation energy ( the minimum amount of energy that is needed to start the reaction)
how can we show the affect of the catalyst on a Maxwell Boltzwell distribution graph
the activation energy marked Ea on the graph will move to the left as the activation energy is lower
what is a catalytic converter
all new petrol cars are now equipped with a catalytic converter in their exhaust systems. This reduces the levels of a number of polluting gases.
catalytic converters are in the shape of a honeycomb. It is made of a ceramic material coated with platinum and rhodium ( the catalysts)
The honeycomb shape provides an enormous surface area, on which the reaction takes place - a little goes a long way
how do catalytic converters work
as the polluting gases pass over the catalysts, they react with each other to form less harmful products by the following reactions:
carbon monoxide + nitrogen oxides = nitrogen + carbon dioxide
hydrocarbons + nitrogen oxides = nitrogen + carbon dioxide + water
