C5 - Monitoring And Controlling Chemical Reactions Flashcards
Describe the yield of a reaction
The yield of a reaction is the mass of the desired products which is made
Describe the theoretical yield of a reaction
The theoretical yield is the maximum possible mass of desired products you can make from a given mass of reactants
Describe the actual yield of a reaction
The actual yield is the mass of products you actually make
How do you calculate percentage yield?
Percentage yield = Actual yield / Theoretical yield x 100
What factors affect percentage yield?
- Reactants reacting in an unexpected way
- Reaction not going to completion
- Losing products when separating or moving the substance
What is atom economy?
Atom economy is a measure of how many atoms in the products make up the desired product
How do you calculate atom economy?
Atom Economy = Mr of desired products / Mr of all products x 100
What factors should be considered when making a substance?
- Cost
- Rate of reaction
- Equilibrium
What are by products?
By products are the other products formed in a reaction that aren’t the desired products
Important conversions for concentration:
1dm3 = 1L
1cm3 = 1ml
cm3 /1000 = dm3
dm3 x 1000 = cm3
Equation for concentration:
Concentration = Amount in moles / Volume
What are the units for concentration?
g/dm3
mol/dm3
How do you calculate a new concentration from a given concentration and volume?
To calculate the new concentration:
C1V1 = C2V2
Worked example:
20cm3 of 2mol/dm3 HCl was added to 80cm3 of water - what is the new concentration?
- First assign the values for C1, V1, C2, V2 - C1 = 2mol/dm3 V1 = 0.020 dm3 C2 = ? V2 = 0.1dm3
- Substitute values and rearrange - 2 x 0.02 = C2 x 0.1 C2 = 2 x 0.02 / 0.1 = 0.4 mol/dm3
What is a titration?
A titration is a method used to find an unknown concentration
Define a ‘titre’
Volume of solution added from a burette during a titration
What are concordant results?
Titres which are within 0.1cm3 of each other
What is molar volume?
1 mol of any substance in the gas state occupies the same volume at the same temperature and pressure
At room temp and pressure, the molar volume is 24dm3
What equation links gaseous moles and molar volume?
Amount (mol) = volume (dm3) / 24
Define ‘rate of reaction’
How quickly reactants are used up or products are formed
How do you calculate rate of reaction?
Rate of reaction = Amount of products produced / time
Rate of reaction = Amount of reactants used / time
What is ‘collision theory’?
In order for two particles to react with each other, two things need to happen:
- Reacting particles need to collide
- They need to have enough energy (more than activation energy)
If these two things happen: a successful collision will occur
Something will increase the rate of reaction if it:
- Increases the number of collisions
- Increases the percentage of successful collisions
When the temperature of a reaction is increased:
- The number of collisions increase as the particles are moving more quickly
- The percentage of collisions which are successful increases because all the particles have more energy
This results in an increase in rate of reaction
How does an increased concentration affect rate of reaction?
Increasing concentration increases the reaction rate because the particles have become more crowded which increases the number of collisions
What do catalysts do?
Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway.
They have no effects on the number of collisions however they do increase the percentage of successful collisions.
How does pressure affect the rate of reaction?
Increasing pressure increases the reaction rate because the particles have become more crowded which increases the number of collisions
How does surface area affect rate of reaction?
Increasing the surface area increases the reaction rate because more particles are available to collide with, increasing the number of collisions.
What is a reversible reaction?
A reversible reaction is one in which the products are able to react with each other to change back into the reactants without any additional changes.
Define ‘equilibrium’
When the forward rate of reaction is equal to the backwards rate of reaction in a reversible reaction.
How does the rate change over time during equilibrium?
- The rate of the forward reaction decreases
- The rate of the backward reaction increases
- Until they are the same - the rate stops changing
How does the concentration change over time during equilibrium?
- The concentration of the reactants decreases
- The concentration of the products increase
- The concentration stops changing until equilibrium is reached
What is the position of equilibrium?
The position of equilibrium is related to the ratio of the concentration of products to the concentration of reactants
It is on the left if the reactant concentration is greater than the product concentration
It is on the right if the product concentration is greater than the reactant concentration
Position of equilibrium on the left:
Position of equilibrium on the right:
What is Le Chatelier’s principle?
‘Any change made to a reaction which is in equilibrium, will result in the equilibrium position moving to minimise the change made’
What effect does temperature have on dynamic equilibrium?
If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat
If you increase the temperature, the equilibrium will move in the endothermic direction to absorb the extra heat
What effect does pressure have on dynamic equilibrium?
If you increase the pressure, the equilibrium will move towards the side that has fewer moles of gas to reduce pressure.
If you decrease the pressure, the equilibrium will move towards the side that has more moles of gas to increase pressure.
What effect does concentartion have on dynamic equilibrium?
If you increase the concentration of the reactants, the equilibrium will move to the right to use up the reactants.
If you increase the concentration of the products, the equilibrium will move to the left to use up the products.
Decreasing the concentrations will have the opposite effect.
