2.2 rates of reaction Flashcards
What is the collision theory?
The theory that chemical reactions only occur when particles collide with sufficient energy
Define activation energy
the minimum amount of energy needed to start a chemical reaction between two particles
describe and explain the gradient of the curve on a rate of reaction graph
initially the gradient is very steep because the rate of reaction Is very fast at the start (there are more particles so more successful collisions)
the gradient decreases overtime as reactants are used up
the curve eventually levels off when the reaction is complete (one or all of the reactants have been completely used up)
how can you calculate rate of reaction?
Rate of reaction = Amount of reactant used or amount of product formed / Time
What units could be used for rate of reaction?
g/s, cm3/s, mol/s
what conditions can be changed to increase the rate of reaction?
increase In temperature
increase in pressure
increase in surface area of reactants
increase in concentration of reactants
How does temperature affect the rate of reaction?
reactants have more energy so more particles have energy greater than or equal to activation energy so more successful collisions
collisions also occur more frequently because the particles have more kinetic energy
How does surface area affect the rate of reaction?
it increases the rate of reaction because increasing the surface area of solid reactants increases the frequency of collisions
How does concentration affect the rate of reaction?
more reacting particles in the same volume so more frequent successful collisions occur
How does pressure affect the rate of reaction?
The higher the pressure, the more the kinetic energy.
- Creates more collisions between particles
- These collisions are more energetic, so have more kinetic energy and greater ability to react
same number of particles in a smaller volume
draw a reaction profile diagram for exothermic reaction (label activation energy)
check quilzlet
draw a reaction profile diagram for an endothermic reaction
check quizlet
Maxwell-Boltzmann distribution for two temperatures
check quizlet
what can be added to speed up rate of reaction?
a catalyst
what is a catalyst?
substance that speeds up the rate of a chemical reaction without being chemically changed by the end
How does a catalyst affect the rate of reaction?
A catalyst offers an alternative pathway that requires less activation energy in the reaction, as it helps breaks the bonds, but at the end of the reaction it is still there and not used up.
how can catalysts be identified in a reaction?
they are chemically unchanged so can be seen in the product
also not used up
Maxwell-Boltzmann distribution catalyst
same shape, greater number of particles in a higher Ea region
Maxwell - Boltzmann Distribution concentration
check quizlet
how can you measure the rate of reaction when a gas is given off?
collect the gas produced in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the vol of gas produced
- measure the amount of gas collected over regular intervals
how can you measure the rate of reaction when a precipitate is formed?
put a black cross below a beaker containing one reactant. Tim how long it takes for the cross to disappear after the second reactant is added
why is the precipitate method not very accurate?
qualitative
people will disagree when the cross disappeared
Marble chips react with hydrochloric acid to produce calcium chloride, water and carbon dioxide. How could you increase the rate of reaction?
increase the surface area by turning marble chips into powder
- increased concentration of acid
- increased temperature
What is a heterogeneous catalyst?
A catalyst in a different physical state to the reactants
Why are transition metals good catalysts?
They can exist in variable oxidation states, so can provide alternative pathways easily (availability of 3d and 4s shells)
what do heterogenous catalyst do to reactants?
weakens bonds
- brings molecules closer
- more favourable orientation
what are the 4 stages of heterogenous catalyst? (H.A.R.D)
- 1 = the heterogenous catalyst is solid whilst the reactants are gaseous
2 = The reactants adsorb into the active sites on the catalyst and bonds are weakened. Reactant molecules are held in position.
3 = The reactants move closer together and react forming new products which are held on the surface.
4 = The new products desorb from the surface and leave the active site open to react with another molecule.
why are partially filled d orbitals better for catalysts and reactions?
Partially filled d orbitals are able to accept bonding pairs of electrons from reactants. The more d orbital space available to accept electrons, the stronger the bond will be. Less availability will equate in weaker bonds which may not have time for the reactants to react before they are desorbed.
equation 1 of the contact process ( SO2 oxidised by vanadium oxide)
Sulphur dioxide oxidised to sulphur by vanadium (v) oxide
SO2 + V2O5(⁺⁵) → SO3 + V2O4 (⁺⁴)
equation 2 (The vanadium has been reduced, oxidised back to the starting vanadium (V) oxide by oxygen-O)
The vanadium has been reduced to vanadium(IV). It is oxidised back to the starting vanadium (V) oxide by oxygen
2V2O4 + O2 → 2V2O5
The vanadium(v)oxide is unchanged at the end of the reaction.
what is the overall equation of the contact process?
Overall reaction : 2SO2 + O2 → 2SO3
How are catalysts poisoned?
Impurities are strongly adsorbed on the active sites on the surface of the catalyst.
In heterogeneous catalysis the ‘poison’ molecules are adsorbed more strongly to the catalyst surface than the reactant molecules, the catalyst becomes inactive.
what will happen to bonding molecules if active site is poisoned?
Incoming reactant molecules will have nowhere to bond to if the active site is already occupied/bonded to a reactant.
What catalysts are used in catalytic converters?
- Platinum, rhodium, palladium (highly expensive)
what is the support medium of catalytic converters?
honeycomb ceramics
What does a catalytic converter do?
processes the toxic gases in engine exhaust, turning them into carbon dioxide, water, and other less harmful substances
what is the catalyst in the haber process?
iron (Fe)
what is the equation for haber process?
N2(g) + 3H2(g ↔ 2NH3(g) ΔH = -92.4kjmol-1
describe sulphur poisoning in the haber process
the hydrogen is obtained from natural gas which is contaminated by sulphur, which if not removed will poison the Fe catalyst by adsorbing onto the surface of the iron.
how can sulphur poisoning be detected?
sulphur is added to natural gas to give it odour so leaks can be smelt
what is a homogenous catalyst?
The catalyst is in the same physical state as the reactants
what phase do reactions occur with homogenous catalysts?
reaction takes place in aqueous phase, e.g. aqueous acid with aqueous reactants
In this case the catalyst may lead to another mechanism or it may stabilise an intermediate compound. The new mechanism will have a lower activation energy.
what is the overall reaction of persulphate ions?
S2O8 2- (aq) + 2I - (aq) → 2 SO4 2- (aq) + I2 (aq)
why does this reaction have a high activation energy?
they are both negatively charged ions so therefore they repel each other which requires more energy
mechanism stage one equation of S2O8
2 Fe 2+ (aq) + S2O8 2- (aq) → 2 Fe 3+ (aq) + 2 SO4 2- (aq)
mechanism stage two of S2O8
2 Fe 3+ (aq) + 2 I - (aq) → 2 Fe 2+ (aq) + I2 (aq)
Explain why Fe3+ ions are as effective as Fe2+ ions in catalysing this reaction.
because both of them could occur in any order during these reactions
what are the disadvantages of a homogenous catalyst?
catalyst actively involved, get intermediate, transition state, but then keep reacting to reform catalyst
- hard to separate
what is an autocatalyst?
A product of the reaction has to catalyse the reaction
equation for permanganate ( potassium manganate) reacting with ethanedioate ions
2MnO4⁻ + 5C2O4²⁻ + 16H⁺ ͢ 2Mn²⁺ + 10CO2 + 8H2O
what is the catalyst of this reaction?
2Mn²⁺
graph to show auto catalysed
around half way through the catalyst is generated = steeper graph
by end, used up most of reactants so rate is slow again
oxidation of manganate ions p1 equation
4Mn²⁺ + MnO4⁻ + 8H⁺ ͢. 5Mn3⁺ + 4H2O
reduction of manganate ions p2 equation
2Mn³⁺ + C2O4⁻ ͢. 2CO2 + 2Mn²⁺
