C5 Flashcards
Yield of a product
Mass of that product made in a chemical process
Theoretical yield
The maximum mass it is possible to make from a given mass of reactants
Actual yield
Mass of product actually made in a chemical reaction
Percentage yield
Actual yield
______________ X 100
Theoretical yield
What factors affect percentage yield ?
-reactants reacting in a different way than expect
-reaction may not go to completion ( often happens in reversible reactions)
-may lose some of the product when separating from the reaction mixture of purifying it
Atom economy
Sum of Mr of desired product
—————————————— X 100
Sum of Mr of all products
What factors do engineers consider when choosing a reaction pathway
-yield of product
-atom economy of reaction
-usefulness of by product
-rate of reaction
-equilibrium position , if it’s a reversible reaction
Epoxyethene
C2H4O
What happened to the process of forming epoxyethene ?
Original :
Ethanol + chlorine + calcium hydroxide = calcium chloride + water + epoxyethene
Yield of this was 80% but atom economy was very low at 25.4#
Modern :
Ethene + oxygen = epoxyethene
Using silver as a catalyst , yield is 80% but atom economy is 100%
How can by products improve atom economy of a process
If they are useful and can be sold
For example , calcium chloride is a useful product in the original process of epoxyethene
How to convert from cm3 to dm3
Divide by 1000
How to convert from dm3 to cm3
Multiply by 1000
Concentration in g/dm3
Mass of solute in g
—————————
Volume of solution in dm3
Concentration in mol/dm3
Amount of solute in mol
————————————
Volume of solution in dm3
Molar volume at RTP
24dm3/mol
What is molar volume
One mole of any substance in the gas state occupies the same volume at the same temperature and pressure
Volume in dm3 at RTP
Amount in mol x 24dm3/mol
Amount in mol at RTP
Volume in dm3/24dm3/mol
Rate of reaction
Amount of reactant used
————————————
Time taken
Rate of reaction
Amount of product formed
—————————————
Time taken
How do you measure volume of gas
Use an upside down measuring cylinder in water or use a gas syringe
Reaction can only happen if
React at particles collide with each other and haven enough energy to react
A collision that leads to a reaction is
A successful collision
A collision won’t be successful
If the particles have less energy than the activation energy
As the temperature of a reaction increases
Particles move more quickly colliding more often , so a greater proportion of colliding particles have the activation energy or more.
What is directly proposition to the rate of reaction
1/t = rate of reaction
What happens to the energy stored in a reaction if temperature / pressure / surface area increases
Energy stored in the particles does not change , but , because the rate of collisions increases , the rate of successful collisions increases
How do you investigate the effect of concentration on reaction rate ?
-reaction between magnesium ribbon and HCL , measure the time taken for magnesium to be used up
-add water to reduce its concentration
- when recording reactions times , you can then calculate 1/time to obtain reaction rates
In the reaction between magnesium and HCL , what is the control
The magnesium ribbons length and therefore size
What happens to a reaction if pressure is increased
Particles in the gas state become more crowded , so collide more often
What do industrial chemical processes often use to achieve high rates of reaction ?
High pressures
Why do reactions occur slower with solids
Particles in a substance can only vibrate around fixed positions , and cannot move place to place .
-only particles at the surface can take part in collisions
As the SA increases
So does rate of reaction
Why do reactions go faster with powders ( larger SA) ?
More reactant particles are available for collisions , collisions are more likely so particles collide more often
In a solid , as the size of the lump decreases
The surface area to volume ratio increases
What is a catalyst ?
A substance that increases the rate of a reaction but remains unchanged at the end of the reaction
If u add 1g of a catalyst to a reaction mixture
There will still be 1g left when the reaction has finished
True or false : catalysts can catalyse any reaction
FALSE , they are specific to particular reactions
Why are catalysts helpful
Even though they are very expensive , only a small amount of catalyst will catalyse the reaction between large amounts of reactants
What are biological catalysts
Enzymes are proteins that catalyse different reactions in biological systems
Catalyse
Enzyme produced by the liver cells , causes the decomposition of hydrogen peroxide
How do catalysts work
Catalysts work by providing an alternative reaction pathway with a lower activation energy
How do catalysts affect reactions
Greater proportion of colliding particles have the activation energy or more
The rate of successful collisions increases compared to the rate in an uncatalysed reaction
Reversible reaction
The products can react together to from the original reactants
Example of a reversible reaction
Reaction between Copper (II) sulfate and water
-blue crystals of copper (II) sulfate are hydrated as each cu(II) ion is surrounded by 5 water molecules
-removing these water molecules by heating , you are left with white ANHYDROUS CuSO4
What is the forwards reaction hydrated copper sulfate = copper sulfate + water
Endothermic reaction , vice versa
In the forwards reaction , why is steam produced
Because enough energy is transferred by heating
What are the conditions required to reach dynamic equilibrium
Must be in a closed system
Equilibrium
When a reversible reaction happens in a closed system , the rates of the forward and backward reaction become equal
Dynamic equilibrium
Forwards and backwards reactions still happen ( concentrations remain constant ) , but at the same rate
Equilibrium position is on the right when
Concentration of reactants is greater than the concentration of products
When the concentration of reactants is less than the concentration of the products
Equilibrium position lies on the left
If pressure is increased
Equilibrium moves to the side where there are fewest moles of gas
If concentration is increased
Equilibrium position moves in the direction away from the substance
If temperature is increased
Equilibrium moves in the direction of the endothermic change
Equilibrium yield
Amount of desired product present in a reaction at equilibrium
Depends on the pressure and temperature and concentration of the reactants
Why do you need to supply plenty of reactants to make plenty of product
So equilibrium position moves to the right
Le Chateliers principle
When a change is made to a reaction at equilibrium , the position of equilibrium moves to oppose that change
What are the disadvantages of using a very high pressure
-requires expensive equipment
-lot of energy to run
-hazardous
-requires tough reaction vessels to withstand pressure
What are the disadvantages of using a low temperature
-rate of reaction is too low to be profitable
Volume
Moles x 24
Volume
Moles x molar volume
