C5 - Monitoring and Controlling Chemical Reactions Flashcards
what is the yield of a product
- the mass of the product made in a chemical process
what is the theoretical yield
- the maximum mass possible to make from a given mass of reactants
how can you calculate the theoretical yield using the law of conservation of mass
- if you know:
> the mass of the limiting reactant
> the relative formula masses of reactants + products
> the balanced equation for the reaction
put into formula:
theoretical yield = mass of limiting reactant / sum of Mr for limiting reactant x sum of Mr for product
what is the equation for finding theoretical yield
theoretical yield =
mass of limiting reactant / sum of Mr of limiting reactant
x sum of Mr of products
what is the actual yield
- the mass of product you actually make in a chemical reaction
what equation is used to calculate percentage yield
percentage yield = actual yield / theoretical yield x 100
what factors affect percentage yield
- reaction may not go to completion - often happens with reversible reactions
- unwanted reactions/other products form than expected
- you may lose some of the product when you separate it from the reaction mixture + purify it
what is atom economy
- a measure of how may atoms in the reactants form a desired product
what equation is used to calculate atom economy
- atom economy = mass of atoms in desired product / mass of atoms in reactants x 100
= desired / reactants x100
> when finding out the Mr remember to use the big numbers as well
what is a reaction pathway
- reaction or series of reactions for making a particular substance
what factors help us decide what reaction pathways to use
- yield of product
- atom economy of reaction
- usefulness of any by-products
- rate of reaction
- equilibrium position, if it’s a reversible reaction
when choosing a reaction pathway which result is the best
- higher yield
- higher atom economy as more desired product is produced
what is a by-product
- a substance formed in a reaction in addition to the desired product
are by-products useful or no
- yes
> some could be useful and could be sold which increases atom economy as by-product becomes a desirable product - no
> some by-products may be toxic or of little use, making a reaction pathway involving them less desirable
> also to dispose of some of the by-products carefully may be too costly
how do you convert from cm3 to dm3 and vice versa
- cm3 to dm3 = divide by 1000
- dm3 to cm3 = multiply by 1000
what is the equation used to calculate concentration
concentration g/dm3 = mass g / volume dm3
+
concentration mol/dm3 = moles / volume dm3
what is titration
- a technique that uses a neutralisation reaction to find the concentration of an acid/alkali
what equipment is used to carry out titration
- burette
- conical flask
- pipette + pipette filler
- white tile
- funnel
- clamp + stand
describe how to carry out a standard titration
- add acid to a burette
- use volumetric pipette to add 25cm3 of alkali solution to a conical flask
- add a few drops of indicator to the conical flask
- read initial burette reading
- slowly add the acid to the flask, swirling the flask constantly
- when the solution in the conical flask changes colour, stop adding acid
- record the final volume of the burette
- calculate the total volume of acid added from the burette which is your titre
why does the conical flask have to be swirled during titration
- to ensure all particles react
why is a white tile used when carrying out titration
- the white tile is placed under the conical flask so the colour change of the indicator is easier to see
what is a titre
- the volume of acid needed to neutralise the alkali
what are concordant titres
- titres that are within 0.10cm3 of each other
how many times do you have to repeat titration + why
- until you get at least 2 concordant titres
> this allows you to calculate a mean titre that has high precision
after carrying out a titration what things will you have obtained
- the two reactants used (acid + alkali)
- volume + concentration of one of the reactants
- the volume, but NOT concentration of other reactant
how can you find the concentration of a reactant after titration having been given everything else
- balance your equation
- work out moles of solution you know vol + conc abt (multiply the two)
- find molar ratio
- find unknown concentration using mols + vol
what is the molar volume of a gas
- the volume occupied by one mole of any gas at the same room temperature and pressure
what is the molar volume of gas at room temp + pressure
24dm3 or 24000cm3
what is the equation for finding moles of gas
moles of gas = volume of gas dm3 / 24dm3/mol
how do you measure the volume of gas (experiment)
- fill measuring cylinder with water + while keeping its mouth underwater, turn it upside down
- clamp measuring cylinder securely
- mix the reactants in a conical flask and attach a delivery tube
- measure the volume of gas produces in the reaction by recording the end reading on the measuring cylinder
what is the rate of reaction
- a measure of how quickly reactants are used or products are formed
how can you calculate the rate of reaction
rate of reaction = amount of reactant used / time taken
rate of reaction = amount of product formed / time taken
how can you measure the volume of gas using a syringe
- in the experiment, if gas is produced the syringe fills and the plunger moves outwards
> you can record the volume of gas it contains
> then you can calculate the rate of reaction
a reaction can only happen if:
- the reactant particles collide with each other
- the colliding particles have enough energy to react
what is a successful collision
- a collision that leads to a reaction
> the greater the rate of successful collisions, the greater the rate of reaction
when will a collision not be successful
- if the particles have less energy than the activation energy
what ways can we change the rate of a chemical rection
- concentration
- pressure
- temperature
- surface area
- (catalysts)
the rate of reaction is ____ to the reaction time
- inversely proportional
1/reaction time is inversely proportional to rate of reaction
> reaction time decreases as temp inc
> rate of reaction inc as temp inc
how can you measure how fast hydrogen is produced
- place dilute hydrochloric acid in a conical flask connected to a gas syringe
- add piece of magnesium ribbon + stopper the flask + start a stop clock
- record the time + volume of hydrogen at regular intervals
- calculate rate of reaction
- analyse your results by drawing a line graph
how can the rate of reaction be measured using a digital mass balance
- when a gas is produces it will escape the reaction vessel / site causing mass to decrease
> record initial mass of reactants - start timer when reactants are combined
- record mass at regular intervals
- calculate rate of reaction
how can the rate of reaction be measured if a precipitate is formed
- disappearing cross experiment
> place conical flask over black cross - start timer as soon as reactants are combined in the conical flask
- time how long it takes for the black cross to disappear
why is the disappearing cross rate of reaction experiment not very accurate
- very subjective
> difference of opinions on when cross disappears
how can the rate of reaction be calculated at a specific time in a reaction
- plot results on graph
- draw a tangent at a specific time
- rate of reaction at this time is equal to gradient of tangent (change in y / change in x)
how do concentrations of reactants + products change during a reaction
- conc of products increase rapidly at start before slowing down and staying constant when reaction is complete
> products = 0% –> 100% - conc of reactants decrease rapidly at start then slow down before staying constant when one / all of reactants have been used up
> reactants = 100% –> 0%
describe + explain the gradient of the curve on a rate of reaction graph
- initially the gradient is very steep because rate of reaction is fastest at the start (there are more reacting particles so more frequent successful collisions)
- the gradient decreases over time as reactants are used up
- the curve eventually levels off when reaction is complete (one or all the reactants have been completely used up)
how does temperature affect the rate of reaction
- increasing temperature increases the rate of reaction
> because the particles gain more kinetic energy and move around faster, resulting in more frequent collisions
> a greater proportion of the colliding particles will have the activation energy or more
> as a result, the rate of successful collisions increases and ultimately the rate of reaction increases
how does concentration affect the rate of reaction
- increasing concentration increases rate of reaction
> because the particles become more crowded and so will collide more often in a given time
> this results in a greater rate of successful collisions and therefore a greater rate of reaction
how does pressure affect the rate of a gaseous reaction
- increasing the pressure of a gaseous reaction increases the rate of reaction
> because as pressure increases the volume decreases making particles more crowded
> this makes the particles collide more often at a given time which increases the rate of successful collisions and so increases the rate of reaction
how does surface area affect the rate of reaction
- increasing the surface area increases the rate of reaction
> because more reactant particles are available for collisions
> this means collisions are more likely
> as a result particles collide more often at a given time and so the rate of successful collision increases and therefore the rate of reaction will increase
the energy stored in _____ doesn’t change but…
- the energy stored in particles doesn’t change but the rate of collisions increase, which means that the rate of successful collisions increase
what happens to surface area to volume ratio as size of lumps decrease
- the surface area to volume increases
> which also increases rate of reaction
what is a catalyst
- a substance that increases the rate of reaction but is unchanged / not used up at the end of the reaaction
how do catalysts work to increase the rate of reaction
- catalysts work by providing an alternative reaction pathway with a lower activation energy
> the amount of energy in particles don’t change BUT a greater proportion of colliding particles have required activation energy
> the rate of successful collision increases compared to the rate in an uncatalysed reaction
what do catalysts not affect during a reaction
- frequency of collisions
- energy stored in particles
why is only a small amount of catalyst used to catalyse a reaction between large amounts of reactants
- because they are unchanged and not used up so you don’t need a lot
> helpful as catalysts often contain expensive metals
how can you investigate a catalyst
- place hydrogen peroxide in conical flask connected to gas syringe
- add manganese oxide into the flask + stopper it + start stop watch
> only small amounts added as gas can be produced very quickly and violently - draw graph and calculate rate of reaction
> you could vary the mass of catalysts and repeat
what are reversible reactions
- when the products and react together to re-form the original reactants
> a double arrow is used in equations to represent the reversible reactions
> it combines the forward and backward reaction
what is a closed system
- when no substances can enter / leave
what is dynamic equilibrium
- dynamic equilibrium occurs in a closed system, when the forward and backward reactions are occurring at the same rate and the concentrations of all reacting substances remain constant
what 3 things can change the position of equilibrium
- temperature
- pressure (gases)
- concentration
what is Le Chatelier’s principle
- for a closed system at equilibrium, if a change is made, the position of equilibrium will shift in the direction which will oppose the change
what happens to the position of equilibrium if the concentration of reactants increase
- the equilibrium shifts to the right to reduce concentration of reactants
> more product formed
what happens to the position of equilibrium if the concentration of the products increase
- the position of equilibrium will shift to the left to decrease the concentration
> less product formed
how does a change in pressure affect the position of equilibrium
- increasing pressure causes the position of equilibrium to shift towards the side with less moles of gas
what happens to the position of equilibrium if the temperature of a reaction is increased
- equilibrium shifts in the endothermic direction
what happens to the position of equilibrium if the temperature of a reaction is decreased
- equilibrium shifts in the exothermic direction
how do you know if the forward reaction is exo or endothermic
- the energy change is negative for exothermic reaction as energy is lost to surroundings
> if forward reaction is negative then its exothermic
what is a compromise pressure
- a pressure high enough to achieve a reasonable equilibrium yield but not so high as to be expensive or hazrdous
- what is a compromise temperature
- a temperature low enough to achieve a reasonable equilibrium yield, but high enough to achieve a reasonable rate of reaction
