C5 Flashcards
What is the ‘yield of a product’?
the mass of that product made in a chemical process
What is theoretical yield?
Maximum mass it is possible to make from a given mass of reactants
What law is used to calculate theoretical yield?
Law of Conservation of Mass
What do you need to calculate theoretical yield?
- mass of limiting reactant
- relative formula masses of reactants and products
- the balanced equation for the reaction
Formula for theoretical yield?
theoretical yield = (mass of limiting reactant / sum of Mr for limiting reactant) x Sum of Mr for all products
What is sustainable development?
The idea that you should meet your needs without damaging the ability of future generations to meet their needs
What does having high percentage yields and atom economies do?
Reduces the use of raw materials and production of waste
What is ‘actual yield’?
The mass of the product you actually make in a chemical reaction
Formula for percentage yield?
Percentage yield = (actual yield / theoretical yield) x 100
What
is the first thing that affects percentage yield?
Reactants may react in a different way than expected e.g. If you burn lithium in air to make lithium oxide, you might make lithium nitride instead
What is the second thing that affects percentage yield?
The reaction may not go to completion e.g. Some of the reactants present do not react
When might a reaction not go to completion?
In reversible reactions
What is the third thing that affects percentage yield?
You may lose some of the product when you separate it from the reaction mixture / purify it e.g. During filtration or when transferring between containers
What is ‘atom economy’?
A measure of how many atoms in the reactants form a desired product
Formula for atom economy
Atom economy % = (sum of Mr of the desired product / sum of Mr of all products) x 100
What is a ‘reaction pathway’?
A reaction or a series of reactions for making a particular substance
What factors do chemical engineers consider when choosing a reaction pathway?
- Yield of the product
- Atom economy of the reaction
- Usefulness or otherwise of by-products
- Rate of the reaction
- Equilibrium position, if it is a reversible reaction
Chemical formula for epoxyethane?
C2H4O
What is epoxyethane made from?
Ethane, a compound obtained from crude oil
How many steps was the original pathway for making epoxyethane?
2
Why was the original pathway for epoxyethane changed?
- It was a complex pathway
* It had high percentage yield (80%) but low atom economy (24.5%)
Overall reaction pathway that was original for epoxyethane
ethene + chlorine + calcium hydroxide -> calcium chloride + water + epoxyethane
What is the modern process for epoxyethane?
Ethene + oxygen -> epoxyethane
What does the modern process for epoxyethane use?
Silver as a catalyst
What is the difference between the original pathway and the modern pathway?
- Both yields are 80%
- Modern process has 100% atom economy
- Both processes waste some reactants
- The higher atom economy of the modern process means it is more efficient
What is a ‘by-product’?
A substance formed in reaction in addition to the desired product
How do you improve the atom economy of a reaction?
By selling the by-products because they then become a desirable product
What is a reason for low atom economy?
Toxic and useless by-products
Convert from dm3 to cm3
x 1000
Formula for concentration in g/dm3
concentration in g/dm3 = mass of solute in g / volume of solution in dm3
Formula for concentration in mol/dm3
concentration in mol/dm3 = amount of solute in mol / volume of solution in dm3
Formula for moles
moles = mass / molar mass
What is a titration?
A technique that uses a neutralisation reaction to find the concentration of an acid or an alkali
How do you calculate the concentration in a titration?
- Find the amount of solute in mol of the standard solution using the concentration formula
- Mol of standard solution / volume (titre value) of unknown solution
What is a standard solution?
A solution with a known accurate concentration and volume, used in titrations
Steps for titration:
- Use a volumetric pipette and pipette filler to measure alkali into a conical flask.
- Add a few drops of phenolphthalein indicator
- Fill a burette to the top with acid until the bottom of the meniscus touches the last line.
- Take an initial burette reading.
- Add the acid.
- Swirl the flask to mix its contents.
- Stop at the end-point.
- Take a final burette reading.
- Calculate the difference between the two readings (titre value).
- Repeat steps 1-8 two more times until you obtain around 2 concordant titres (within 0.10cm3 of each other).
To get a repeatable titre, you need to…
- Swirl the flask to mix its contents
- Do a rough run to get an idea of what the titre is, so you can quickly add the acid within a few cm3 of the rough titre, then add drop by drop
You get accurate readings by…
- Clamping the burette vertically
* Using a volumetric pipette
How do you make your titre values more precise?
By calculating the mean of at least 2 concordant titres
To make the titration more safe you should…
- Wear eye protection
* Use a pipette filler
What is the ‘end-point’?
When the alkali is neutralised and the indicator changes colour
How do you find the amount in mol of a substance with no known amount in mol in a balanced titration equation?
Use the ratio of the moles of the acid to alkali e.g.
If 1 mole of acid reacts with 2 moles of alkali, you would divide the amount in mol of alkali (the known solution) by 2 to find the amount in mol of the acid
What is the ‘molar volume’?
1 mol of any substance in the gas state occupies the same volume at the same temperature and pressure
RTP
Room temperature and pressure
Molar volume sign
Vm
Molar volume equals to…
24dm3/mol
or
24000cm3/mol
What is the relationship between amount and volume in gas?
volume in dm3 = amount in mol x 24dm3/mol
What is another relationship between amount and volume in gas?
amount in mol = volume in dm3 / 24dm3 mol
Ways to measure the volume of gas:
- Gas syringe
- Upside-down measuring cylinder
- Mass lost from reactants
What is ‘rate of reaction’?
A measure of how quickly reactants are used
Or how products are formed
Two equations for rate of reaction:
rate of reaction = amount of product formed / time taken
rate of reaction = amount of reactant used / time taken
Measuring the volume of gas with magnesium and HCl
- Place dilute HCl in a conical flask with a gas syringe attached.
- Add a piece of magnesium, stopper the flask, and start a timer.
- Draw a results table.
- Record the time and volume of HCl at regular intervals.
- Draw a line graph
Cautions when doing the magnesium and HCl practical
- Make sure the syringe reading starts at 0cm3 by pushing the plunger all the way in.
- Stopper the flask as soon as the reaction starts so all of the hydrogen is collected
Calculate mean rate of reaction
Gradient = change in volume / change in time
Calculate instanteous rate of reaction
Draw a tangent to the curve at that time.
Calculate the gradient of the tangent.
What is a successful collision?
A collision that lead to a reaction
What do you need for a successful collision to occur?
Reactant particles need to collide wth each other
The colliding particles need enough energy to react
Rate of reaction and increasing temperature…
- Particles move more quickly -> collide more frequently
* Greater proportion of colliding particles have the activation energy or more
Investigating the effect of temperature using HCl and sodium thiosulfate:
- You can vary the temperature of a reaction mixture by warming up one of the solutions before mixing.
- In a beaker, mix the reactants together.
- Look down through the mixture to a cross drawn on paper -
the longer it takes for the cross to disappear, the slower the rate of reaction
Cautions for investigating the effect of temperature?
- Start the stopwatch as soon as you mix the solutions
* Stop the clock as soon as the cross disappears
How can you use reaction times?
Rate of reaction is inversely proportional to the reaction time
How else can you use reaction times?
1/time is directly proportional to the rate
First formula for rate of reaction
1/time
Second formula for rate of reaction
1000/time
What is ‘concentration’?
A measure of how much solute is dissolved in a solvent
Why do reactions go faster at higher concentrations?
The particles become more crowded, so they collide more often -> more successful collisions
Dependent variable
What you are measuring
Independent variable
What you are changing
Control variable
Any other variable that can change the outcome and needs to be kept constant
Reaction to use as an experiment to investigate the effect of concentration on reaction rate
Magnesium ribbon and HCl
Concentration and reaction rate experiment
- Time taken for a magnesium ribbon to be used up.
- You can add water to dilute the HCl to reduce its concentratio.
- Then do 1/time for reaction rates.
What is the control variable in the concentration experiment of magnesium ribbon and HCL
The length of the ribbon because the mass of the piece of magnesium affects the reaction time, so must be kept constant for a fair test.
Limiting reactants and amount of product
The amount of product is proportional to the amount of limiting reactant (as one increase, the other increases)
Reactions with a reactant gas go faster at higher pressures
If even one of the reactants is on the gas state, the rate of reaction increases as the pressure of the gas increases
How does pressure affect rate of reaction?
The particles in the gas state become more crwoeded, so they collide more often
Why is it bad to react with a lump of substance?
Only particles at the surface take part in the collisions because solid particles cannot move from place to place
Size and rate of reaction
- Rate of reaction increases as surface area increases
* As size decreases, surface area to volume ratio increases
Why do reactions go faster with powders?
- More reactant particles are available for collision
* Collisions are more likely, so particles successfully collide more often
Investigating the effect of particle size
Using calcium carbonate and dilute HCl
- Add dilute HCl to a conical flask and stopper it with cotton wool
- Place the flask on a balance next to a lump of calcium carbonate.
- Tare the balance, remove the cotton wool, drop in the calcium carbonate and start a stopwatch.
- Replace the cotton wool.
- Record the decreasing mass at regular intervals (due to carbon dioxide escaping)
- You can use different sizes of calcium carbonate.
What is a catalyst?
A substance that increases the rate of a reaction butbis unchanged at the end of the reaction
What reactions do catalysts catalyse?
They are specific to particular reactions
How much catalyst should you add?
A small amount as they will catalyse the reactions between large amounts of reactants
Which metals do catalytic converters use?
Platinum, palladium and rhodium
What are catalytic converter metals coated on?
inert ceramic honeycombs
How expensive are the catalytic converter metals?
Expensive
What is the benefit of having catalysts on a ‘honeycomb’?
- they only use a few grams of catalyst
- provides a large surface area for reactions
- allows exhaust gases through
Investigating the catalyst Manganese(IV) oxide with hydrogen peroxide
- Place hydrogen peroxide in a conical flask connected to a gas syringe.
- Put a little Manganese(IV) oxide into the flask, stopper it, and start a stopwatch.
- Record the time and volume of oxygen at regular intervals.
- Draw a graph.
What independent variables can be changed when investigating a catalyst?
The mass of the catalyst and type
How to be safe when investigating a catalyst:
Only add small amounts of catalyst as gas can be produced very quickly and violently
What are the control variables when investigating a catalyst?
Volume, temperature and concentration of hydrogen peroxide should be kept the same
The volume of oxygen produced is affected by this so they have to be kept constant to keep it a fair test
What are biological catalysts?
Enzymes are proteins that act as catalyst in biological systems
An example of biological catalyst:
Liver cells have the enzyme catalase -> which catalyses the decomposition of hydrogen peroxide
How do catalysts work?
They provide an alternative reaction pathway with lower activation energy
Why are catalysts good?
- A greater proportion of colliding particles have the activation energy or more
- The frequency of successful collisions increase compared to that of an uncatalysed reaction
Reaction profile for a catalysed and uncatalysed reaction
With a catalyst, the activation energy curve is smaller and below the uncatalysed reaction’s activation curve, since less energy is required
What are reversible reactions?
Reactions where the products can react together to form the original reactants
What does “hydrated” mean?
Containing water or having ‘water of crystallisation’
