SC14,SC15 & SC16 Flashcards
How do you calculate the theoretical yield?
RAM x balancing number
What is percentage yield?
Compares the actual yield and the theoretical yield
Percentage yield= actual yield/theoretical yield x 100
Name 1 reason why the yield is less than expected?
The reaction may be incomplete so not all reactants are used up
Possibly because reaction has been left for too long or reaction reached equilibrium
Name the second reason why the yield is sometimes less than expected?
Some of the product is loss
Name a third reason why the yield is less than expected?
May be unwanted side reactions taking place
E.g. some reactants may react to make a different product so compete with the main reaction
What does a high percentage yield mean?
Higher percentage yield=more useful the reaction
Fewer raw materials are used to make the small amount of product
What is atom economy?
A method of showing how efficiently a particular reaction makes use of the atoms in the reactants
What is the formula to work out atom economy?
RAM of useful product/sum of RAM of all reactants x 100
Can reactions have a high percentage yield but a low atom economy?
How can you improve atom economy if so?
Yes they can and it means that waste by products are formed
Atom economy can be improved by finding uses for the by products of the reaction
What are some of the other factors when scientists must consider when deciding a reaction pathway?
Raw materials, temperature, pressure, catalyst, rate of reaction, purity
What is the concentration of a solution?
It is the amount of solute dissolved in a stated volume of solution
How do you calculate concentration in g dm-3
Mass of solute in g/volume of solution in dm3
What do you do if the volume of the solution is given in cm3?
Divide by 1000
What is the equation triangle for working out concentration?
Mass (g)
Concentration (g dm-3) x volume (dm3)
How do you calculate concentration in mol dm-3?
Number of moles on solute/ volume of solution in dm3
What equation relates both types of concentrations?
Concentration in g dm-3/ RAM of solute
What is the equation triangle for converting concentration?
Concentration (g dm-3)
Concentration (mol dm-3) x RAM
What does Avogadro’s law state?
If the temperature and pressure are the same, equal volumes of different gases contain an equal number of molecules
What is the molar gas volume?
It is the volume occupied by one mole of molecules of any gas
It is 24dm3 or 24000cm3 at room temperature and pressure
What is the equation for the volume of gas?
Amount of gas (mol) x molar volume
How do you calculate the amount of gas if you know the volume?
Volume of gas/molar volume
What do fertilisers do?
They replace the mineral ions needed by plants and so promote plant growth
What are some of the important elements used in fertilisers?
Nitrogen, phosphorus and potassium
Fertiles must provide them as soluble compounds so the root hair cells only absorb mineral ions dissolved in water
How is ammonium nitrate produced?
Salt manufactured by reacting ammonia solution with dilute nitric acid
How is nitric acid manufactured?
Ammonia + oxygen -> nitric acid + water
How is ammonium sulfate produced?
Made in the laboratory by reacting ammonia sulfate with dilute sulfuric acid
How does the production of ammonium sulfate differ in a laboratory to it being produced on an industrial scale?
Laboratory - small scale
Start with ammonia sulfate and dilute sulfuric acid
Made through titration, then crystallisation in a batch process
Industrial- large scale
Raw materials for making ammonia and sulfuric acid
Made in several stages in a continuous process
How does the preparation differ in each process?
Laboratory
Ammonia solution and dilute sulfuric acid bought from chemical manufacturers
Industrial
Substances made on site from raw materials
What is the difference between a batch process and a continuous process?
Batch (difficult to automate)
Small amount made, apparatus cleaned, then repeated
Continuous (automated)
Reactants are constantly fed into reactors and products removed
Need fewer people to make a given amount of product
What happens in dynamic equilibrium?
The forward and backward reactions still happen, at the same rate
The concentrations of all reacting substances don’t change
What happens if the temperature is increased?
Moves in the direction of the endothermic reaction
Time taken it reach equilibrium decreases
What happens if the pressure is increased (in a reaction involving gas)?
Moves towards the side of the balanced equation with the fewer molecules of reacting gas
Time take to reach equilibrium decreases
What happens if the concentration of a reacting substance increases?
It moves away from the reacting substance in the balanced equation
Time taken to reach equilibrium decrease
What happens if a catalyst is added?
There in no change in the position if equilibrium
Time taken to reach equilibrium decreases
What happens if the pressure is increased in the barber process?
The position of equilibrium moves to the right as there are fewer molecules on this side
How do high pressures effect the equilibrium yield?
It increase the equilibrium yield of ammonia and increase the rate of reaction
However, very high pressures are too expensive to maintain
If the temperature increases in the harder process, does the equilibrium move towards the left or right?
It moves to the left in the direction of the endothermic reaction
What is the temperature used in the Harber process?
450 C
Low temperature increase equilibrium yield of ammonia but reduce rate of reaction
Why is an iron catalyst used in the Harber process?
It doesn’t change the position of equilibrium but increases the rate of reaction
What are factors taken into account when choosing reaction pathways?
Availability and cost of raw materials and energy supplies
Rate of reaction and the equilibrium position
Atom economy, yield and usefulness of by products
What does a simple chemical cell have?
2 different metals, each dipped into a solution of one of their salts
A ‘salt bridge’ to allow dissolved ions to pass from 1 solution to the other
When is the voltage greater?
The voltage is greater when the further apart in the reactivity series the 2 metals are
