R2.2 How fast? The rate of chemical change Flashcards
rate of reaction
the change in concentration of a particular reactant/product per unit time
rate of reaction units
mol dm^-3 s^-1
due to what do species react
- Collisions of sufficient energy (kinetic energy due to temperature)
- Proper orientation (collision geometry)
collision theory
- Particles must collide with sufficient kinetic energy in the correct orientation
- Considers particles to be hard spheres that react with each other with sufficient kinetic energy
successful collision
- Reactant particles have the correct orientation, enough kinetic energy (equal to or exceeding Ea) and collide
- Change into product particles when they collide
- Results in a chemical reaction and chemical change
unsuccessful collision
- Reactant particles collide
- Bounce off each other
- Wrong orientation, not enough kinetic energy
steric effect
The rate/path of a chemical reaction depends on size/arrangements of groups in a molecule
how does pressure influence the rate of reaction
- Increase pressure = increase rate
- Forces the particles closer together
- More particles per unit volume
- More frequent successful collisions
how does concentration influence the rate of reaction
- Increase concentration = increase rate
- Particles are closer together as there are more pre unit volume
- Frequency of successful collisions between reactant particles in solution increases
how does surface area influence the rate of reaction
- Increase surface area = increase rate
- More particles on the surface that can react
- More frequent successful collisions
how does temperature influence the rate of reaction
- Increase temperature = increase rate
- Molecules have an average greater velocity → greater distance per unit time travelled → more successful collisions per unit time
- Larger proportion of the colliding molecules have kinetic energy equal to or exceeding activation energy → larger proportion of successful collisions
what is a catalyst
A substance that increases the rate of reaction but remains chemically unchanged at the end of the reaction
how does the presence of a catalyst influence rate of reaction
- Provides an alternate reaction pathway with a lower activation energy
- Therefore, a greater proportion of sample particles have sufficient kinetic energy to react when they collide
- Rate is directly proportional to the concentration of the catalyst
homogeneous catalyst
Catalyst and reactants in the catalysed reaction are in the same phase
heterogeneous catalyst
- Catalyst and reactants in the catalysed reaction are in a different phase
- Provides a surface on which the reaction can occur
biological catalyst
enzymes
activation menergy
- the minimum energy that colliding particles need for a successful collision leading to a reaction
- the minimum amount of combined total kinetic energy required by a colliding pair of ions, atoms or molecules for a chemical reaction to occur; the energy barrier that has to be overcome to form the transition state
energy barrier of a fast reaction
low, low activation energy
energy barrier of a slow reaction
high under standard conditions
transition state
- Forms before the products, cannot be easily isolated and studied as it is unstable
- The partially bonded, short lived chemical species of highest potential energy, located at the top of the activation energy barrier as a reaction proceeds from reactants to products
what is distributed over a Maxwell-Boltzmann distribution
the distribution of kinetic energies of molecules at a constant temperature
what is the total area under the curve of a MBD directly proportional to
directly proportional to the total number of molecules
what is the area under any portion of the curve is directly proportional to
the number of molecules with kinetic energy in that range
do particles in a substance all have the same energy, explain
- no
- energy constantly changes
- when particles collide with each other, they gain/lose energy
- particles with insufficient energy may gain enough to react in a later collision
graph of concentration vs rate
directly proportional
graph of pressure vs rate
directly proportional
graph of surface area vs rate
directly proportional
graph of temperature vs rate, explain
- exponential
- a small temperature increase has a large effect on rate (higher kinetic energy value)
how can light (involving reactions of gas mixtures including halogens) influence rate of reaction
Sunlight/UV can potentially cause a very large increase to rate of reaction
rate determining step
- the slowest elementary step in a chemical reaction that determines the rate of reaction
many reactions occur in…
…a series of elementary steps
what can energy profiles be used to show
the activation energy and transition state of the rate-determining step in a multistep reaction
molecularity of an elementary step
the number of reaction particles taking part in that step
unimolecular
- 1 chemical specie
A –> B
bimolecular
- 2 chemical species
A + A –> B
A + B –> C
termolecular
- 3 chemical species
2A + B –> C
intermediate
made in one step and used in another
what do rate equations depend on and how can they be determined
- depend on the mechanism of the reaction
- can only be determined experimentally
order of a reaction with respect to a reactant
- the exponent to which the concentration of the reactant is raised in the rate equation
- can describe the number of particles taking part in the rate determining step
overall reaction order
the sum of the orders with respect to each reactant
rate constant (k)
- temperature dependent
- units are determined from the overall order of the reaction
graph of rate vs concentration, 0 order
never (0,0)
graph of rate vs concentration, 1st order
- directly proportional
- positive linear gradient
- (0,0)
graph of rate vs concentration, 2nd order
- directly proportional to [A]^2
- (0,0)
graph of concentration vs time, 0 order
- negative linear gradient
- half life decreases
- constant half life decrease at a constant rate
graph of concentration vs time, 1st order
- constant half life
graph of concentration vs time, 2nd order
- half life increases with time
- concentration
- half life is inversely proportional to concentration
what does the Arrhenius equation use, and what does it do with this
- uses the temperature dependence of the rate constant
- to determine the activation energy
what does the Arrhenius factor (A) take into account
the frequency of collisions with proper orientations
what happens to rate of reaction and rate constant when temperature increases
increase exponentially
frequency factor (A)
- combination of the steric factor (p) and the collision number (z)
- an account of the frequency of collision with the correct orientation
- how often the activation energy barrier is met
