Chapter 10 - Rates of Reaction Flashcards
rate of reaction is?
the change in the conc of r or p in a given time
how is rate calculated?
Change in conc/ time
- rate in moldm(^-3)s(^-1)
collision theory?
in order for a reaction to occur, 2 molecules must collide:
- with sufficient energy to overcome Ea
- w the correct orientation
factors affecting the ROR?
- temp
- pressure
- conc
- SA
- Presence of catalyst
factor: temp?
⬆temp =⬆KE of molecules
as KE ⬆, no. of successful collisions ⬆, R.O.R ⬆
factor: pressure?
- ⬆ P means molecules are closer together, more likely to collide and react w energy in excess of Ea, rate of collisions ⬆, ROR V
INCREASING THE PRESSURE ONLY INCREASES THE RATE OF GASEOUS REACTIONS
what does collisions are more freq mean?
more coliisons occur in a given period of time
factor: conc?
- ⬆ conc of a r ⬆ no. of moleculs of that paticular r
- ⬆ conc means molecules are closer together and there is a ⬆ chance of them colldiing w sufficient energy to overcome Ea
- ⬆ in the no. of successful collisons in a given period of time
- ROR ⬆
factor: SA?
⬆ sa of the r ⬆s the area over which a reaction can occur
ROR ⬆
factor: presence of a catalyst?
adding a catalyst to a rection provides an an alternate pathway w a lower Ea
- more particles have energy > or equal to the Ea of the catalysed reaction
- ⬆ successful collisons take place per unit time, so adding a catalyst ⬆ ROR
industrially, the yield of a reaction must be balanced against?
the economic feasibility
Haber Process equation?
N2(g) + 3H2 (g) (reversible reaction arrow) 2NH3 (g)
ΔH = -92
Haber Process optimum conditions?
- high pressure : L: 4 moles R: 2 moles so equil. moves to tight
- low temp: forwards reaction is exothermic so ⬇ temp will cause equi to shift to right
BUT THERE ARE SOME ISSUES WITH USING THESE CONDITIONS IN INDUSTRY
prblms with using high pressure?
- increasing pressure increases the conc of gases which increases ROR
- But, lots of energy is required to maintain these high pressures - expensive
- storing gases at such high pressure is unsafe
issues w low temp?
Decreases the ROR
what does the use of the Iron catalyst mean for the Haer Process?
Equil is established quickly at lower temp
what allows almost a 100% conversion rate in Haber Process?
unreacted reactants are recycled
At equil, the concs of r+ p are?
constant, but that does not mean they are of equal conc
what does the Equil. constant indicate?
- where the equil lies
- Ratio of reactants and products
what does the magnitude of Kc indicate?
the extent of reaction
in order to monitor the ROR we can monitor?
- conc of r/p -e.g. titration
- gas volume of product e.g. using gas syringe
- change in the mass of substances formed - e.g. using a balance to change in mass
ROR graph:?
X axis: time
y: change in (conc of e.g.)
calc the IROR from a graph?
- IROR shown at t=0
- draw tangent that is straight line to curve
when the graph plateaus of?
rate = 0
gradient = ?
rate
what does a reaction profile diagram show?
the progress of reaction plotted against energy
homogeneous catalyst e.g.?
- enzymes within saliva
heterogeneous catalysts e.g.?
catalytic converter in the exhaust pipes of cars
the importance of catalysts in industry?
- by lowering Ea, lower energy demands of the process so:
- process requires less energy ➡reduces cost
- lower demands for fossil fuels ➡ lower greenhouse gas emissions ➡ better for envir
Boltzmann distribution curve shows?
the distribution of molecular energies in a gas at constant temp
BDC - molecules?
- some mols move fast & have high energy
- some move slow and have a low energy
- but majority have an average energy
BDC: area under curve =?
the total no. of molecules in sample
why does the BDC never touch the x axis?
no molecules have 0 energy
the BDC starts?
at origin
effect of temp on BDC?
- curve moves to right, lower peak
- AREA UNDER CURVE REMAINS SAME AS THE NUMBER OF MOLECULES IN THE SYSTEM REMAINS CONSTANT
effect of catalyst on BDC?
addition of a catalyst does not CHANGE THE DISTRIBUTION OF MOLECULAR ENERGIES (CURVE DOES NOT MOVE, ONLY Ea LINE DOES)
list the factors altering the ROR?
- Conc
- temp
- catalyst
- SA
describe how conc alters the ROR?
Increasing the conc increases the ROR as it inc the amount of particles per dm3 thfr collision freq is the highest - increases the no. of successful coliisons
describe how temp inc ROR?
An increase in temp inc the proportion of particles with E> or equal to Ea which means more successful collisions thus a higher rate
describe how a catalyst increases ROR?
Catalysts provide a new mechanism for the reaction with lower Ea so that the proportion of particles w E greater than or equal to Ea is inc thus more successful collisions so a higher ROR
Describe how SA increases ROR?
A larger SA from small particles means more reactant particles can come into contact, thfr more successful collisions so ROR is inc.
pressure is abt? but conc is abt?
pressure is gas, conc is solution
why use a gas syringe when CO2 is produced?
- to measure V of CO2 produced using gas syringe
- CO2 is denser than air, so will get into syringe + don’t need to bubble thru H20 - would dissolve
when drawing a gradient triangle, make it as ?
large as possible - tangent
⭐at higher conc, there is the largest amount of?
particles PER DM3
Steps to calculating the ROR from gradients of graphs?
1) draw tangent using SHARP PENCIL
2) determine change in y and change in x
3) gradient = change in y/ change in x (rise/run!!)
4) use the axis units to determine units for rate
5) make gradient triangle as large as possible
what is a catalyst?
a substance that inc the ROR w/o getting used up in the reaction. A catalyst provides an alt mechanism w a lower Ea
examples of heterogeneous catalysts?
- iron
- aluminium oxide
- nickel
- Rhodium, Platinium, Paladium
examples of homogeneous catalysts?
acid
chlorine radicals
what is activation energy?
the minimum energy required for a particle to be involved in a successful collision
homogeneous catalysts are?
catalysts and reactants are in the same state
heterogeneous catalysts are?
catalyst and reactants are in different states (heterosexual like diff sex remember )
catalytic convertors?
- are heterogeneous catalysts
- hydrocarbon fuels are burnt in the internal combustion engines of many vehicles
- in limited O2, CO - a toxic gas can form, high temps can also cause oxides of N to form from N in air: N2(g) + O2 -> NO(g)
catalytic convertors allow?
- the NO and CO to react. Harmless N gas and non-toxic CO2 is formed
- 2NO + CO -> CO2 + N2
catalyst = platinum
catalytic convertors - what happens to the unburnt hydrocarbons?
they’re oxidised to water and CO2
heterogeneous catalysis - how exactly does it work?
- step 1: ADSORPTION: weak bonds form between r + catalyst, weakens bonds in reactants
- 2: the reaction - bonds break and form
3: DESORPTION: weak bonds to catalyst break + products leave surface
what does a Boltzmann distribution show?
the range of energies of all of the molecules in a gas at a constant temp. We can use em to explain how changes like adding catalysts or changing the temp affect the ROR
When drawing a boltzmann distribution for T higher make sure?
SAME AREA UNDER BOTH CURVES
What does a higher temp Boltzmann distrubution curve look like?
further right & lower peak
make sure BDC does not touch?
x axis at end
start at origin.
why does temp inc rate?
at a higher temp, the proportion of particles with E> or = to Ea has inc - more sucessful collisions -> higher rate
the effect of catalyst on a BDC?
- NO EFFECT ON ACTUAL CURVE
- Ea line shifts to left ( becomes lower)
which carbonates are soluble ?
only aqueous carbonates are G1, rest are insoluble
how does the use of catalysts help chemical companies make their processes more sustainable ?
- reduced Ea so lower temp so less fossil fuels -> lower demand for fossil fuels -> less global warming
- catalysts aren’t used up so can be used over and over
- cataylsed reactions can be caused to produce other products/ more products (e.g. catalytic convertors)
- bio-enzymes - biodegradable
what is dynamic equilibrium?
- EQUILibrium is when the rate of forwards and reverse reactions are EQUAL
- dynamic means the f+b reactions are still taking place at a particle level
- the amounts of the reactants and products don’t change (but the actual atoms, molecules or ions do) but THE AMOUNTS DO NOT NEED TO BE EQUAL
for dynamic equilibrium to occur u must have a closed system. what is meant by this?
no chemicals can be taken out or added in in a closed system
which factors affect the position of equilibrium (yield of product)?
- pressure
- temp
- adding/ removing a chemical (not conc)
state ‘Le Chatelier’s principle’
if the conditions of an equilibrium are changed, the position of the equilibrium moves to minimise (readjust) the change
what effect does a catalyst have on the position of equilibrium?
NONE- it speeds up the forwards and reverse reaction equally
how could the position of equilibrium be monitored in an equilibrium reaction?
colour changes
inc the temp shifts the equilibrium in?
the endothermic direction
inc the pressure shifts the equilibrium to?
the side w the fewer moles of gas
Dynamic definition?
the reaction is taking place at a molecular level
equilibrium definition?
when the forwards and reverse reactions are taking place at the same rate
what does it mean if equilibrium lies to the left?
there are a higher conc of r than p
what does it mean if equil lies to the right?
there are a higher conc of product than reactant at equil, higher yield of product
if an acid/ alkali is being added?
be careful, bc the acid will react w the alkali or vice versa using it up, so the equil will shift to make more of the acid/alkali
y is graphite more thermodynamically (heat) stable than diamond?
it has less energy/ enthalpy
Equil- sometimes a ? has to be made
compromise between yield and ROR
Equilibrium constant (Kc) can be calculated w the equation?
[products] / [reactants]
Kc - [] means?
the conc of (in mol dm-3)
Kc- [X] means
the concs of r and products in moldm-3
Kc - []^y means
the molar amounts in the equation - (big numbers)
to calc Kc what do we need to know?
the concs of reactants and products at equilibrium
Kc - working out the units?
write them out and do the cancelling to see what’s left
what can Kc values be used to do?
estimate the position of equilibrium from the value/ size of Kc
if Kc = 1 then?
equal amounts of reactants and products
if Kc > 1 then?
high yield of product, equil is shifted forwards (to right)
if Kc < 1 then?
low yield of product, equil is shifted backwards (to left)
what is meant by ROR?
THE rate of a chemical reaction measures how fast a reactant is being used up or how fast a product is being formed
units of rate ?
moldm-3s-1
the progress of a chemical reaction can be followed by?
- monitoring the removal (⬇ in conc) of a r
- following the formation ( ⬆ in conc) of a p
apart from conc, what else may change as the reaction proceeds?
gas V, mass of r or p and colour
if the reaction produces a gas, 2 methods that can be used to determine the ROR?
- Monitoring the V of a gas produced at regular time intervals using gas collection
- monitoring the loss of mass of r using a balance
IROR is when?
t=0
how does a homogeneous catalyst work?
- the catalyst reacts w the r to form an intermediate
- the intermediate then breaks down to give the product and regenerates the catalyst
features of the boltzmann distribution?
- no molecules have 0 energy- the curve starts at the origin
- the area under the curve is equal to the total no. of mols
- there is no max E for a mol- the curve does not meet the x axis at high energy. The curve would need to reach infinite energy to meet the x axis
why do the concs of reactants and products not change in an equilibrium?
- as fast as the r are becoming ps, the ps are reacting to become rs
- thr, the concs remain unchanged even tho the f+b reactions are still taking place
