Unit 1: Section 5 - kinetics, equilibria and redox reactions MDY CDS * Flashcards
reaction rates more on reaction rates reversible reactions The equilibrium constant redox reactions
what is the rate of reaction?
the change in concentration of the reagent per unit time
what is the formula for rate of reaction?
rate of reaction = amount of reactant used or product formed/ time
what conditions must be met for particles to react?
they must collide in the correct orientation
they must collide with the minimum amount of kinetic energy
what is activation energy?
the minimum amount of kinetic energy that particles need to react
why do particles need activation energy?
this is the amount of energy needed to break their bonds
what happens in an enthalpy profile diagram?
the bonds within each particle are stretched
if they have the activation energy the bonds will break
what is enthalpy?
heat change at constant pressure
what is a Maxwell-Boltzmann distribution?
a graph for the number of molecules with a certain kinetic energy
what is the area under the curve in a Maxwell-Boltzmann distribution?
the area under the curve is the total number of molecules
what is the peak of the curve in a Maxwell-Boltzmann distribution?
it represents the most likely energy of any single molecule.
most molecules are moving at a moderate speed - their energies are somewhere in the middle
where is the mean energy of a Maxwell-Boltzmann distribution?
the mean energy of all the molecules is a bit to the right of the peak. because the curve is asymmetrical
which molecules can react on the Maxwell-Boltzmann distribution curve?
some molecules have more than the activation energy (a point to the far right of the curve) these are the only molecules that can react
where does the curve start in the Maxwell-Boltzmann distribution?
the curve starts at (0, 0) because no particles have zero energy
what happens to particles when you increase the temperature of a reaction?
the particles will on average have more kinetic energy and will move faster.
so they’ll collide more often which increases the chance of successful collisions
small increase leads to large increase in reaction rate
what happens on the Maxwell-Boltzmann curve when the temperature of a reaction is increased?
a greater proportion of molecules will have at least the activation energy and be able to react. the curve is pushed to the right and the most likely energy is higher. the area under the graph stays the same
what happens if you increase the concentration of reactants in a solution?
the particles will be closer together on average.
if they’re closer, they’ll collide more often. so collisions are more frequent so there’s a higher chance of successful reactions
what happens if you increase the pressure of reactants in a solution?
the gas particles will be closer together on average.
if they’re closer, they’ll collide more often. so collisions are more frequent so there’s a higher chance of successful reactions
what is a catalyst?
a catalyst is a substance that increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. the catalyst is chemically unchanged at the end of the reaction
why are catalysts so good?
a small bit of catalyst catalyses a large amount of stuff
save lots of money in industrial processes
large surface area
thin
cheap mesh coated in catalyst
what is a disadvantage of catalysts?
they are suited for specific reactions so some only work on a single reaction
how do catalysts increase rate of reaction?
they lower the activation energy so there’s more particles with enough energy to react when they collide. so increases chance of successful collisions
they can also change the particles’ orientation and make the particles collide more frequently
what are some different ways to measure rate of reaction?
timing how long a precipitate takes to form
measuring a decrease in mass
measuring the volume of gas given off
what is involved in timing how long a precipitate takes to form?
watch a mark through the solution and time how long it takes to be obscured
if the same observer uses the same mark rates of reaction for different reactions can be recorded because the same amount of precipitate will obscure the mark each time
when can rate of reaction be measured by the time taken for a precipitate to form?
when the product is a precipitate which clouds a solution
what is a disadvantage of measuring rate of reaction by measuring the time taken for a precipitate to form?
this method is subjective - different people might not agree on the exact moment the mark disappears
what is involved in measuring a decrease in mass?
you can measure the rate of formation of product using a mass balance
as gas is given off, the mass of the reaction mixture decreases
when can rate of reaction be measured by measuring a decrease in mass?
when 1 or more of the products is a gas
what are the advantages and disadvantages of measuring rate of reaction by measuring a decrease in mass?
this method is accurate and easy to do but it does release gas into the atmosphere so its usually done in a fume cupboard
what is involved in measuring the volume of gas given off?
a gas syringe is used to measure the volume of gas being produced
when can rate of reaction be measured by measuring the volume of gas given off?
when 1 or more of the products is a gas
what are the advantages of measuring rate of reaction by measuring the volume of gas given off?
they give volumes to the nearest 0.1cm3 so this method is accurate
what is an example of timing how long a precipitate takes to form?
the reaction between sodium thiosulfate and hydrochloric acid
what happens when sodium thiosulfate reacts with HCl?
they are both clear, colourless solutions and they react to form a yellow precipitate of sulfur.
how to measure the rate of reaction between sodium thiosulfate and hydrochloric acid?
measure out fixed volumes of both using a measuring cylinder
use a water bath to gently heat both solutions
mix the solutions in a conical flask and place it over a black cross and watch it and time it until the cross disappears through the cloudy sulfur.
repeat for solutions at different temperatures
what should be kept the same for a reaction between sodium thiosulfate and HCl?
the depth of liquid
the concentrations of the solutions
what should the results for the reaction between sodium thiosulfate and HCl show?
the higher the temperature, the faster the reaction rate and therefore the less time it takes for the mark to disappear
what happens in a reversible reaction as the reactants get used up?
as reactants get used up, the forward reaction slows down
and as more product is formed, the reverse reaction speeds up
what happens after a while in a reversible reaction?
the forward reaction will be going at exactly the same rate as the backwards reaction. the amount of products and reactants doesn’t change as dynamic equilibrium is reached
what is a dynamic equilibrium?
both reactants and products present
forward and backwards reactions happening at the same time at same rate
concentrations of both products and reactants stay constant
closed system
reversible reaction
produces a mixture
doesn’t go to completion
what is Le Chatelier’s principle?
if a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change
when can Le Chatelier’s principle be used to predict the effects the environment will have on a reversible reaction?
when it’s a homogenous equilibria - reactions where every species is in the same physical state
what effect does concentration have on the position of equilibria?
if you increase concentration of reactant,
equilibrium position moves to oppose the increase
equilibria will shift to the right to get rid of extra reactant
increasing equilibrium yield of product
what effect does pressure have on the position of equilibria?
(only for gases)
increasing pressure shifts equilibria to the side with fewer moles/ gas molecules, this reduces pressure
and vice versa
what effect does temperature have on the position of equilibria?
increasing temperature makes the endothermic reaction speed up to absorb the heat
decreasing temperature shifts equilibria in exothermic direction
what effect do catalysts have on the position of equilibria?
catalysts have no effect on the position of equilibria.
they can’t increase yield - but equilibria is reached faster
what must be considered when choosing the correct environment for a reaction?
cost and how much money will be made must be considered
e.g. if the forward reaction is exothermic, lower temperature makes more product, but this slows down the rate of reaction so a compromise must be made.
if the product has less moles increasing the pressure will make more product and increase rate but can be very expensive if too high
what is oxidation?
a loss of electrons
what is reduction?
a gain of electrons
what does redox mean?
reduction and oxidation happen simultaneously
what is an oxidising agent?
something that accepts electrons and is reduced
what is a reducing agent?
it donates electrons and gets oxidised
what is the oxidation state of an element?
the total number of electrons it has donated or accepted.
Na+ has an oxidation state of +1
what are the oxidation states of elements?
0
e.g. He, O2
what are the oxidation states of monatomic ions?
its the same as its charge
e.g. Na+ is +1
what are the oxidation states of compound ions?
the overall oxidation state is the same as the charge.
the individual oxidation states add up to that number
e.g. SO4 2-
O=-2
S+4(-2)=-2
S-8=-2
S=6
what is the oxidation state of oxygen in a compound?
-2
except in peroxides where its -1
and in OF2 its +2
and in O2F2 its +1
what is the oxidation state of hydrogen in a compound?
+1
except in metal hydrides its -1
what is the oxidation states of transition metals?
whatever the roman numeral is in brackets is the oxidation state
e.g. iron (III) sulphate the oxidation state of iron is +2
what is a half equation?
it shows oxidation or reduction. it shows the electrons that are being lost or gained.
they can be combined to form redox reactions as long as the electrons on each side cancel out
what does it mean if an element/ ions oxidation state has been decreased?
it has been reduced
it is more negative, so has gained electrons
what does it mean if an element/ ions oxidation state has been increased?
it has been oxidised
what is the equation to work out the equilibrium constant, Kc?
for equation:
aA + bB dD + eE
Kc= [D]^d[E]^e / [A]^a[B]^b
what does [ ] mean?
concentration of species in brackets in mol dm^-3
how to find the units of Kc?
cancel out the number of brackets / powers of the brackets on top and bottom and then the number is the power of the units e.g. [ ]^2[ ]^2/[ ] [ ] = ^2 mol^2 dm - 6 [ ] / [ ] [ ] = ^-1 mol^-1 dm^3
what if the Kc has no units?
it can be calculated with just the number of moles of each substance
how to find the concentrations of substances if volume of container is known?
the volume is the same for all products and reactants so can be divided by the moles of each substance to find conncetration
how to fins number of moles of each substance in a reaction mixture?
unless told otherwise all products start at 0 moles.
the change in moles is proportional is doubled if the substance has a 2 in front.
if reactants lose moles products gain them
e.g.
CH4+2O2 CO2+2H2O
+1 +2 -1 -2
what affect does temperature have on Kc?
if you change the temperature of the system, you change the equilibrium concentrations of the products and reactants so Kc will change
more products at equilibrium = higher Kc
what affect does changing concentration of reactant or product have on Kc?
no affect
what affect does a catalyst have on Kc?
no affect
speeds up reaction in both directions by same amount so equilibrium is reached faster
rules of the Kc constant?
all reactants and products are at their equilibrium concentration
each reactant and product has its concentration raised to the power of its balancing number in the chemical equation
products on top, reactants below
what is ICE?
Initial = initial number of moles of reactants and products Change = change in moles from initial to equilibrium Equilibrium = known moles of equilibrium or calculated moles from ratio
what is a closed system?
nothing can get in or out
what is the generic answer for a question about le Chateliers principle
equilibrium position moves to oppose change
by moving left/right in (endo/exo)(side with more/less moles) direction
increases equilibrium yield of reactant/products
what is the Haber process?
N2(g) + 3H2(g) 2NH3(g) Δ=-76 kJ/mol
nitrogen from air
hydrogen from electrolysis or crude oil cracking
why is the Haber process dangerous?
hydrogen is flammable and explosive
what temperature is used for the Haber process?
450*C
too high - yield is low, rate is high
too low - yield is high, rate is low
what pressure is used for the Haber process?
20MPa
too high - yield is high, rate is high, high maintenance cost of equipment
too low - yield is low, rate is high
