Chapter 10 - reaction rates Flashcards
what is the rate of a chemical reaction in real terms
how fast a product is being made or how fast a reactant is being used up
what is the generalized equation for ROR
rate = change in concentration/time
what determines the rate of a reaction
the frequency of successful collisions
what determines if something is a successful collision
- correct energy, Ea or higher
- correct orientation
name the main 4 factors on ROR
concentration, temperature, surface area, catalysts
define activation energy
the minimum energy required for a reaction to take place
what are the two ways of measured ROR
- measuring formation of a product
- measuring removal of a reactant
how can ROR be measured for a gas release
- use a gas syringe, take the volume of gas at regular intervals
- use an upturned measuring cylinder and take vols at regular intervals
- plot graph and take grad
- OR measure loss of mass over time
how can ROR be measured for a precipitate
measure how long it takes for a cross underneath a container to no longer be visible where ROR is INVERSELY proportional to time
define what a catalyst is
“a catalyst is a substance that changes the rate of a chemical reaction without undergoing any permanent changes itself by providing an alternative reaction pathway with a lower activation energy”
how do catalysts increase ROR
they reduce Ea by providing an alternative reaction pathway
what is a homogenous catalyst and name an example of a reaction with one
a catalyst which is in the same physical state as the reactants, it reacts with the reactants to form an intermediate then breaks down again to form the products and reform the catalyst
e.g.
Cl*(g)
2O3(g), 3O2(g)
what is a heterogenous catalyst and give an example of a reaction with one
a catalyst which is in a different physical state to the reactants, it usually works by the reactants weakly bonding to its surface/ adsorbtion where the reaction takes place then they desorb from the surface after the reaction
e.g.
Fe(s)
N2(g) + 3H2(g) 2NH3(g)
why do reactions with catalysts tend to be more sustainble/ why are they used in industrial processes
- they reduce the energy required
- they can also reduce costs
- they make the reaction faster/more product can be made
- the catalyst isn’t used up
- reactions can take place at lower temperatures
what does the Boltzmann distribution represent
- the spread of molecular energies in gases
what are the labels of the axes on a Boltzmann distribution
y-axis = number of molecules with given energy x-axis = energy
how does the distribution change with temperature and what effect does this have
the peak moves down and to the right, thus given the area under the graph is finite, the shaded area increases, more particles have greater than or equal to the Ea and thus the FOSC increases so ROR increases
why does ROR increase with concentration
- there are more particles per given volume
- thus particles are more likely to collide
- this means FOSC increases so ROR increases
why does ROR increase with temperature
- the particles move faster so they are more likely to collide and more likely to collide with the Ea so FOSC increases
why does a catalyst increase ROR
- reduces Ea by providing an alternative reaction pathway
- more particles react >= Ea so FOSC increases
why does increases the S.A. of the reactants increase the ROR
- greater area for collisions to occur on/more available molecules
- increases FOSC
how does the Boltzmann distribution change with the addition of a catalyst
- the shaded line is drawn at a lower energy, more molecules have greater than or equal to the Ea
what what are some key features of dynamic equilibrium
- the rates of the forwards and backwards reactions are equal
- the concentrations of the reactants and products do not change
- there must be a closed system
state le chatalier’s principle
“when a system in dynamic equilibrium is subjected to an external change the system readjusts and the position of equilibrium changes to minimize the effect of that change”
what is the effect of concentration changes on equilibrium
- for A + B –> C + D
- if the conc’s of A+B are increased then the position of equilibrium will move right in order to form more products to reduce the conc of A+B
- and vice versa
how can concentration changes with le chatalier’s be investigated
- if the reactants and products are different colours e.g.
2CrO4^2- + 2H+ —> Cr2O7 ^2- + H2O
yellow orange - if the forward reaction is quicker then the solution will appear more orange
- if the backwards reaction is quicker then the solution will appear more yellow
describe how temperature affects the position of equilibrium
- if forwards reaction is exothermic (-deltaH) then a decreased temperature will increase the forward reaction and an increased temperature will decrease the forwards reaction
- and vice versa
- a similar thing as before can be done to investigate this
how does a change in pressure affect the position of equilibrium
whichever side of the reaction has fewer moles of gas will benefit from increased pressure
effect of a catalyst on the position of equilibrium
- a catalyst doesn’t affect the position of equilibrium because it increases the rates of the forwards and backwards reactions BY THE SAME AMOUNT
- it will mean that equilibrium is reached quicker
what does the equilibrium constant mean and what is its formula
it provides the actual position of equilibrium
for reaction aA + bB —> cC + dD
Kc = [C]^c [D]^d / [A]^a [B]^b
what does <1, =1 and >1 mean in relation to Kc
if Kc < 1 then the backwards reaction is favored
if Kc = 1 then equilibrium is halfway
if Kc > 1 then the forwards reaction is favored
why does changing concentrations or pressured not affect Kc
the system equals out to give the same ratio of concentrations and pressures
what does affect Kc
changing temperature- it benefits one of the reactions, this permanently changes the position of equilibrium
what are the key features of a Boltzmann distribution
- there are no particles with no energy (line passes through origin)
- the line doesn’t retouch the x-axis but gets close (there aren’t many particles with lots of energy)
- most particles have average amounts of energy
- area under the graph = number of particles
- higher temperature shifts distribution right and down
things to remember on catalyst questions
- when asked what the purpose of a catalyst is or effect of a catalyst always mention BY PROVIDING AN ALTERNATIVE REACTION PATHWAY WITH A LOWER ACTIVATION ENERGY
- what should you say if asked the effect of a catalyst on dynamic equilibrium – No effect because it increases the rate of both reactions BY THE SAME AMOUNT
why does rate of reaction get slower as a reaction progresses and when does the reaction stop
- as the reaction proceeds, the concentrations of the reactant particles decreases (because they are used up in the reaction)
- hence the frequency of successful collisions decreases
- the reaction stops when ALL OF ONE REACTANT is used up
what is the effect on a concentration of products/vol of gas against time graph if a strong acid is replaced by a weak acid or an acid of lower concentration (but same number of moles)
the rate of reaction (gradient) is lower but the final volume is the same
what is the effect on a concentration-time graph if less of a reactant is used but the same concentration
If the reactant is still in excess then there’s no change
if the reactant is not in excess/ is limiting then the rate of reaction is mostly the same as before but it finishes at a lower value
why does adding a catalyst to a forming dynamic equilibrium decrease the time taken for the equilibrium to be established
- it decreases the activation energy so the rates of both reactions increase (by the same amount)
