SC6 - Dynamic Equilibria Flashcards
How do you calculate percentage yield?
Percentage yield = (actual yield / theoretical yield) x 100
Why is the actual yield less than the theoretical yield?
- the reaction may be incomplete or it may be reversible
- some of the product is lost
- there may be other side reactions
How do you calculate atom economy?
Atom economy = relative formula mass of the useful product / sum of the relative formula masses of all the reactants
How do you calculate concentration?
Concentration in g/dm3 = mass of solute in g / volume of solution in dm3
Concentration in mol/dm3 = number of moles of solute / volume of solution in dm3
How do you convert between g/dm3 and mol/dm3?
To covert an answer from mol/dm3 to g/dm3, multiply by the relative formula mass.
To convert from g/dm3 to mol/dm3, divide by the relative formula mass.
How do you calculate gas volume?
Volume (dm3) = moles x 24
Volume (cm3) = moles x 24000
What are the three mole equations?
Moles = mass/RFM
Moles = concentration x volume
Moles = volume (dm3) / 24
What is a reversible reaction?
A chemical reaction in which the product can react to reform the reactants (i.e. there is a forward and a reverse reaction)
What is dynamic equilibrium?
The forward and reverse reactions are occurring at the same rate, so the net amount of reactant and product doesn’t change.
What is the industrial process used to manufacture ammonia called?
The Haber Process
What are the conditions used for the Haber Process?
Temperature = 450 degrees C
Pressure = 200 atmospheres
Catalyst = iron
What is the balanced symbol equation for the Haber Process?
N2 (g) + 3H2 (g) —> 2NH3 (g)
<—
What happens to the position of equilibrium if the temperature is increased?
If the temperature is increased, the endothermic reaction will be favoured (to try and make it colder)
What happens to the position of equilibrium if pressure is increased?
If pressure is increased, equilibrium will shift to the side of the equation with fewer moles (to try and decrease the pressure)
What happens to the position of equilibrium if the concentration is increased?
It will shift to the side with lower concentration (so the substance of higher concentration gets used up)
What is Le Chatelier’s principle (equilibrium rule)?
If the concentration, temperature or pressure of an equilibrium is changed the position of a dynamic equilibrium will shift to reduce the effect of the change.
Where does the nitrogen used in the Haber Process come from?
The air
Where does the hydrogen used in the Haber process come from?
Natural gas (methane)
How is the yield of ammonia maximised in the Haber Process?
- drain off ammonia as it is made (decreases its concentration so forward reaction is favoured to make more ammonia)
- low temperature (forward reaction is exothermic so a lower temperature will result is more ammonia being made (although in reality a very low temperature would result in a slow rate of reaction so temperature is a compromise))
- high pressure (there are fewer moles on the RHS so increased pressure means more ammonia)
What are NPK fertilisers and why are they important?
N nitrogen
P phosphorus
K potassium
They are important as they promote plant growth by replacing mineral ions in the soil.
Can equilibrium be reached in an open system?
No
What does a chemical cell consist of?
Chemical cells must have:
- two different metals, each dipped in a solution of one of their salts
- a ‘salt bridge’ to allow dissolved ions to pass from one solution to the other
How do you increase the voltage of a chemical cell?
The further apart in the reactivity series the metals are, the greater the voltage.
What is a Daniell cell?
A simple chemical cell using zinc in zinc sulfate solution, copper in copper sulfate solution and filter paper soaked with concentrated potassium nitrate solution (the salt bridge)
Why do batteries go ‘flat’?
One of the reactants gets used up, so the voltage is no longer produced.
Can fuel cells go ‘flat’?
No, they produce a voltage for as long as they are supplied with the reactants.
What happens at the cathode in a hydrogen-oxygen fuel cell?
Hydrogen atoms lose electrons to form hydrogen ions.
H2 (g) —> 2H+ (aq) + 2e-
Electrons flow through the external circuit towards the anode. Hydrogen ions pass through a membrane to the anode.
What happens at the anode in a hydrogen-oxygen fuel cell?
Oxygen atoms react with hydrogen ions to form water.
4H+ + 4e- + O2 (g) —> 2H2O (g)
What are some positives and negatives of hydrogen-oxygen fuel cells?
P - only produce water so very environmentally friendly
N - hydrogen has to be stored, which is difficult to do
How can you collect the gas from a reaction?
- upturned measuring cylinder
- upturned burette
- gas syringe
What is the method for a titration?
- fill burette with acid (make sure jet below tap is also full)
- note initial volume of acid
- use glass pipette to measure alkali and place in a conical flask
- add a few drops of a suitable indicator (phenolphthalein or methyl orange) and place flask of a white tile (makes colour change easier to see)
- add the acid to the alkali, swirling as you go
- stop adding acid when the indicator changes colour (this is the end point)
- note the final volume of acid
- repeat until concordant result are obtained (within 0.2 cm3 of each other)
