SC6 - Dynamic Equilibria

Question 1

How do you calculate percentage yield?

Answer 1

Percentage yield = (actual yield / theoretical yield) x 100

Question 2

Why is the actual yield less than the theoretical yield?

Answer 2

• the reaction may be incomplete or it may be reversible
• some of the product is lost
• there may be other side reactions

Question 3

How do you calculate atom economy?

Answer 3

Atom economy = relative formula mass of the useful product / sum of the relative formula masses of all the reactants

Question 4

How do you calculate concentration?

Answer 4

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

Question 5

How do you convert between g/dm3 and mol/dm3?

Answer 5

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.

Question 6

How do you calculate gas volume?

Answer 6

Volume (dm3) = moles x 24
Volume (cm3) = moles x 24000

Question 7

What are the three mole equations?

Answer 7

Moles = mass/RFM
Moles = concentration x volume
Moles = volume (dm3) / 24

Question 8

What is a reversible reaction?

Answer 8

A chemical reaction in which the product can react to reform the reactants (i.e. there is a forward and a reverse reaction)

Question 9

What is dynamic equilibrium?

Answer 9

The forward and reverse reactions are occurring at the same rate, so the net amount of reactant and product doesn’t change.

Question 10

What is the industrial process used to manufacture ammonia called?

Answer 10

The Haber Process

Question 11

What are the conditions used for the Haber Process?

Answer 11

Temperature = 450 degrees C
Pressure = 200 atmospheres
Catalyst = iron

Question 12

What is the balanced symbol equation for the Haber Process?

Answer 12

N2 (g) + 3H2 (g) —> 2NH3 (g)
<—

Question 13

What happens to the position of equilibrium if the temperature is increased?

Answer 13

If the temperature is increased, the endothermic reaction will be favoured (to try and make it colder)

Question 14

What happens to the position of equilibrium if pressure is increased?

Answer 14

If pressure is increased, equilibrium will shift to the side of the equation with fewer moles (to try and decrease the pressure)

Question 15

What happens to the position of equilibrium if the concentration is increased?

Answer 15

It will shift to the side with lower concentration (so the substance of higher concentration gets used up)

Question 16

What is Le Chatelier’s principle (equilibrium rule)?

Answer 16

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.

Question 17

Where does the nitrogen used in the Haber Process come from?

Answer 17

The air

Question 18

Where does the hydrogen used in the Haber process come from?

Answer 18

Natural gas (methane)

Question 19

How is the yield of ammonia maximised in the Haber Process?

Answer 19

• 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)

Question 20

What are NPK fertilisers and why are they important?

Answer 20

N nitrogen
P phosphorus
K potassium

They are important as they promote plant growth by replacing mineral ions in the soil.

Question 21

Can equilibrium be reached in an open system?

Answer 21

No

Question 22

What does a chemical cell consist of?

Answer 22

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

Question 23

How do you increase the voltage of a chemical cell?

Answer 23

The further apart in the reactivity series the metals are, the greater the voltage.

Question 24

What is a Daniell cell?

Answer 24

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)

Question 25

Why do batteries go ‘flat’?

Answer 25

One of the reactants gets used up, so the voltage is no longer produced.

Question 26

Can fuel cells go ‘flat’?

Answer 26

No, they produce a voltage for as long as they are supplied with the reactants.

Question 27

What happens at the cathode in a hydrogen-oxygen fuel cell?

Answer 27

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.

Question 28

What happens at the anode in a hydrogen-oxygen fuel cell?

Answer 28

Oxygen atoms react with hydrogen ions to form water.

4H+ + 4e- + O2 (g) —> 2H2O (g)

Question 29

What are some positives and negatives of hydrogen-oxygen fuel cells?

Answer 29

P - only produce water so very environmentally friendly
N - hydrogen has to be stored, which is difficult to do

Question 30

How can you collect the gas from a reaction?

Answer 30

• upturned measuring cylinder
• upturned burette
• gas syringe

Question 31

What is the method for a titration?

Answer 31

• 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)