Kinetics, Equilibra And Redox Reactions Flashcards
Formula for rate of reaction
Rate of reaction=
Amount of reactant used or product formed/ time
What are particles in liquids and gases always doing?
Moving and colliding with each other
Do they react every time they collide?
No only when conditions are right
What two things must be true according to collision theory for a reaction to occur?
Collide in the right direction they need to be facing each other in the right way
Collide with at least the activation energy required
What’s activation energy?
The minimum amount of kinetic energy that particles need to react
Why is activation energy required to start off a reaction?
Need at least the energy needed to break their bonds and start the reaction
What reactions happen easily?
Ones with low activation energy
What reaction happen don’t happen easily?
Ones with a high activation energy so you need to give particles extra energy by heating them
How much energy do molecules in a gas have?
Some don’t have much kinetic energy and move slowly
Others have loads of kinetic energy and whizz along but molecules are sometimes in between
If you plot a graph of number of molecules in a gas with different kinetic energy what do you get?
Maxwell-Boltzmann distribution
Area under Maxwell-Boltzmann distribution?
The total number of molecules
Where does the curve of maxwell-Boltzmann begin?
(0,0) no molecules have zero energy
Peak of curve maxwell-Boltzmann?
Represents most likely energy of any single molecule
Mean (average) energy is where?
All the molecules a bit right to a peak
Which molecules react?
Some molecules have more than the activation energy. These are the only ones that can react
Increasing the temperature of a reaction does what? Maxwell-Boltzmann
The particles will on average have more kinetic energy and move faster
What does a higher temperature mean for the proportion?
Greater proportion of molecules will have at least the activation energy and be able to react
Changes shape of maxwell-Boltzmann distribution curve pushing it over to the right
Total number of molecules is the same for?
Each of these reactions meaning the area under the curve must be the same
Why does a slight increase in temperature result in quite a large increase in reaction rate?
Molecules move faster so collide more
More molecules have the activation energy required
Together can lead to large increase in reaction rate
What does increasing pressure do the rate of reaction?
Increase concentration of reactants in a solution the particles will be on average closer together
If closer they will collide more often. If collisions occur more frequently they’ll have jute chance to react meaning increasing concentration increases reaction rate
Pressure does what to the rate of reaction?
If a reaction involved gases, increasing pressure works in just the same way as increasing concentration
Raising pressure pushes all gas Particles together making them more likely to collide so collisions take place more frequently and reaction rate increases
What’s a catalyst?
A substance that increases the rate of reaction providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction
Why are catalysts great?
They don’t get used up so you only need a tiny bit of catalyst to catalyse a huge amount of stuff. They do take part in reactions but they’re remade at the end
Why are catalyst fussy about which reactions they catalyse?
They often only work on a single reaction
Why do catalysts save heaps of money in industrial processes?
Allow you to make the same amount of product faster and often at a lower temperature too
Why can a catalyst work?
The catalyst lowers the activation energy meaning there’s more particles with enough energy to react when they collide. It does this by allowing the reaction to go via a different route so certain amount of time more particles react
How is rate of reaction defined?
As the change in concentration (or amount) or reactant or product over time
How can you follow the rate of reaction?
Either by how fast reactants are used up or how fast the products are formed.
Three methods to measure reaction rate?
Timing how long a precipitate takes to form
Measuring a decrease in mass
Measuring volume of gas given off
Timing how long a precipitate takes to form when can you use this method?
When the product is a precipitate which clouds a solution.
Timing how long a precipitate takes to form what do you do?
Watch a mark through the solution and time how long it takes to be obscured.
Timing how long a precipitate takes to form how can you compare it?
If the same observer uses the same mark each time you can compare the rate of reaction because roughly the same precipitate will have formed when the mark becomes obscure
Timing how long a precipitate takes to form what’s wrong with this method?
Method is subjective- different people may not agree on the exact same moment the mark disappears
Measuring a decrease in mass when can this be done?
When or more products is a gas you can measure the rate of formation using a mass balance
Measuring a decrease in mass how does it work?
As gas is given off the mass of the reaction mixture decreases
Measuring a decrease in mass disadvantage?
Method is accurate and easy to do but does release gas into the atmosphere so usually done in a fine cupboard
Measuring volume of gas given off involves?
Using a gas syringe to measure volume of gas being produced
When can you use measuring the volume of gas given off?
Only when one or more of the products is a gas
How accurate is measuring the volume of gas given off?
Gas syringes usually give volumes to the nearest 0.1cm3 so this method is accurate
What are sodium thiosulfate and hydrochloric on their own?
Clear, colourless solutions
What colour are sodium thiosulfate and hydrochloric acid when they react together?
They form a yellow precipitate of sulfur
How can you measure the reaction between sodium thiosulfate and hydrochloric acid?
Amount of time that it takes for the precipitate to form as a measure of the rate of the reaction
What is reaction between sodium thiosulfate and hydrochloric acid often demonstrate?
The effect of increasing temperature on reaction rates
Step 1 for reaction between sodium thiosulfate and hydrochloric acid?
Measure out fixed volumes of sodium thiosulfate and hydrochloric acid using a measuring cylinder
Step 2 for reaction between sodium thiosulfate and hydrochloric acid?
Use water bath to gently heat both solutions to desired temperature before you mix them
Step 3 for reaction between sodium thiosulfate and hydrochloric acid?
Mix solutions in conical flask. Place flask over a black cross which can be seen through the solution. Watch the black cross disappear through cloudy sulfur and time how long it takes
Step 4 for reaction between sodium thiosulfate and hydrochloric acid?
The reaction can be repeated for solutions at different temperatures. The depth of liquid must be kept the same each time. The concentration of solutions must also be kept the same
Step 5 for reaction between sodium thiosulfate and hydrochloric acid?
The results should show that higher temperature the faster the reaction rate and therefore the less time it takes for the mark to disappear
What are a lot of chemical reactions?
Reversible they go both ways
⇌
What happens as the reactants are used up?
The forward reaction slows down as more product is formed the reverse reaction speeds up
What’s dynamic equilibrium
The forward reaction going at the same rate as the backwards reaction so amount of product and reactants won’t be changing
What happens at equilibrium?
Concentration of reactants and products stay constant
When can dynamic equilibrium happen?
Only in a closed system
What happens if you change the concentration, pressure or temperature of a reversible reaction?
You will alter the position of equilibrium. Meaning you end up with different amounts of reactants and products at equilibrium
What happens if the equilibrium moves to the left?
You’ll get more reactants
What happens if the position of equilibrium moved to the right?
You get more products
What does Le Chatelier’s principle say?
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
What does the Le Chatelier’s principle basically mean?
If you raise the temperature the position of equilibrium will shift to try to cool things down. If you raise the pressure or concentration the position of equilibrium will shift to try to reduce it again.
What can you use the Le Chatelier’s principle to work out?
What effect changing the concentration, pressure or temperature will have on the position of equilibrium.
What does the Le Chatelier’s principle apply to?
Homogeneous equilibria meaning reactions where every species is in the same physical state (all liquid or all gas)
What happens if you increase the concentration of a reactant?
The equilibrium tries to get rid of extra reactant. It does this by making more product. So the equilibrium shifts to right.
What happens if you increase the concentration of the product?
The equilibrium tries to remove extra product making the reverse reaction go faster. The equilibrium shifts to the left
What happens if you decrease the concentrations?
It will have the opposite effect
Pressure when does this change anything?
Only affects equilibria involving gases
Increasing the pressure does what?
Shifts the equilibrium to the side with fewer fas molecules reducing pressure
Decreasing the pressure does what?
Shifts the equilibrium to the side with more gas molecules raising pressure
Increasing the temperature means what?
Adding heat
What happens if the temperature is increased?
The equilibrium shifts to the endothermic (positive 🔼H) direction to absorb this heat
Decreasing the temperature means what?
Removing heat
What does decreasing temperature do?
Shifts the equilibrium in the exothermic (negative 🔼H) direction to produce more heat in order to counteract the drop in temperature
What will the reverse reaction be if the forward reaction is endothermic?
Exothermic
What will the reverse reaction be if the forward reaction is exothermic?
Endothermic
What will catalysts do to equilibrium?
Nothing on the position of equilibrium
Can’t increase yield
But do mean equilibrium is reached faster
What do companies have to think about?
How much it costs to run a reaction and how much money they can make from it. Meaning they have a few factors to think about when choosing the right conditions
Give example
Ethanol can be produced via a reversible exothermic reaction between ethene and stream
C2H4(g) + H2O(g) C2H5OH (g)
🔼H=-46kJmol-1
Reaction is carried out at pressures of 60-70 atomspheres and a temperature of 300 degrees with a phosphoric acid catalyst
Perfect temperature
Exothermic reaction lower temperature favours forwards reaction meaning at lower temperature more ethene and stream converted to ethanol. You get a better yield.
Compromise temperature
Means slower rate of reaction
No point getting a very high yield of ethanol if it takes you 10 years so 300 degrees is a compromise between a reasonable yield and a faster reaction
Perfect pressure
High pressure shifts equilibrium to the side with fewer molecules which favoured the forwards reaction here. High pressure increases rate of reaction so pressure of 60-70 atomspheres is used
Compromise pressure
Very high pressure are very expensive to produce and you need string pipes and containers to withstand high pressures
So 60-70 atomspheres is a compromise giving a reasonable yield for lowest possible cost
If you know the molar concentration of each substance at equilibrium what can you work out?
The equilibrium constant Kc
Equilibrium constant equation
aA+bB dD+eE
Kc=(D)^d(E)^e/ ((A)^a(B)^b)
What can you use the equilibrium constant equation for?
Equilibrium constant and it’s units
Kc valid for?
One particular temperature
If you change the temperature of the system?
You will also change the equilibrium concentrations of the products and reactants so Kc will change
If the temperature means more product?
Kc will rise
If temperature means less product at equilibrium?
Kc will decrease
Changing the concentration of a reactant or product?
Will not affect Kc
Catalyst does what to Kc?
Nothing
They’ll speed up the reaction in both directions by the same amount so they just help the system to reach equilibrium faster
Loss of electron
Oxidation
Gain of electron
Reduction
Reduction and oxidation happening stimultaneously?
Redox reaction
What does an oxidising agent do?
Accepts electrons and gets reduced
A reducing agent does what?
Donates electrons and gets oxidised
What does the oxidation state of an element tell you?
The total number of electrons it has donated or accepted
Rules you can use to work out oxidation state of an atom when it’s in a compound, in an ion or on its own
1) uncombined elements like He and Ar have oxidation of 0
2) elements just bonded to identical atoms have oxidation state of 0
3) oxidation state of a simple monatomic ion is same as its charge
4) compound ions the overall oxidation state is the ion charge
5) sum of oxidation states for a neutral compound is 0
6) combined oxygen is nearly always -2 except in peroxide where it’s -1
7) combined hydrogen is +1 except in metal hydrides where’s -1
What are Roman numberials in the chemical name
Oxidation number
What do ionic half-equations show?
Oxidation or reduction
What do you show in an ionic half-equation?
Electrons that have been lost or gained in a half- equation
E.g. Na->Na+ + e-
What can you do with half-equations?
Combine them for different oxidising or reducing agents together to make full equations for redox
Magnesium burns in oxygen to form magnesium oxide
Oxygen reduced to O2- O2+4e- => 2O2- Magnesium oxidised to Mg 2+ Mg=> Mg2+ +2e- You need both equations to contain same number of electrons so double everything in the second equation 2Mg=> 2Mg2+ + 4e- Combine to make 2Mg+ O2 -> MgO
Aluminium reacts with chlorine to form?
Aluminium chloride Aluminium oxidised to Al3+ Al-> Al3+ + 3e- Cl2+ 2e- => 2Cl- Make sure equations have same number of electrons Combine the two equations 2Al+3Cl2=> 2AlCl2
