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OCR Gateway A GCSE Chemistry > C5 - Monitoring and Controlling Chemical Reactions > Flashcards

C5 - Monitoring and Controlling Chemical Reactions Flashcards

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1
Q

what is the yield of a product

A
  • the mass of the product made in a chemical process
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2
Q

what is the theoretical yield

A
  • the maximum mass possible to make from a given mass of reactants
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3
Q

how can you calculate the theoretical yield using the law of conservation of mass

A
  • 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
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4
Q

what is the equation for finding theoretical yield

A

theoretical yield =
mass of limiting reactant / sum of Mr of limiting reactant
x sum of Mr of products

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5
Q

what is the actual yield

A
  • the mass of product you actually make in a chemical reaction
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6
Q

what equation is used to calculate percentage yield

A

percentage yield = actual yield / theoretical yield x 100

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7
Q

what factors affect percentage yield

A
  • 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
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8
Q

what is atom economy

A
  • a measure of how may atoms in the reactants form a desired product
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9
Q

what equation is used to calculate atom economy

A
  • 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

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10
Q

what is a reaction pathway

A
  • reaction or series of reactions for making a particular substance
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11
Q

what factors help us decide what reaction pathways to use

A
  • yield of product
  • atom economy of reaction
  • usefulness of any by-products
  • rate of reaction
  • equilibrium position, if it’s a reversible reaction
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12
Q

when choosing a reaction pathway which result is the best

A
  • higher yield
  • higher atom economy as more desired product is produced
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13
Q

what is a by-product

A
  • a substance formed in a reaction in addition to the desired product
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14
Q

are by-products useful or no

A
  • 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
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15
Q

how do you convert from cm3 to dm3 and vice versa

A
  • cm3 to dm3 = divide by 1000
  • dm3 to cm3 = multiply by 1000
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16
Q

what is the equation used to calculate concentration

A

concentration g/dm3 = mass g / volume dm3
+
concentration mol/dm3 = moles / volume dm3

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17
Q

what is titration

A
  • a technique that uses a neutralisation reaction to find the concentration of an acid/alkali
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18
Q

what equipment is used to carry out titration

A
  • burette
  • conical flask
  • pipette + pipette filler
  • white tile
  • funnel
  • clamp + stand
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19
Q

describe how to carry out a standard titration

A
  • 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
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20
Q

why does the conical flask have to be swirled during titration

A
  • to ensure all particles react
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21
Q

why is a white tile used when carrying out titration

A
  • the white tile is placed under the conical flask so the colour change of the indicator is easier to see
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22
Q

what is a titre

A
  • the volume of acid needed to neutralise the alkali
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23
Q

what are concordant titres

A
  • titres that are within 0.10cm3 of each other
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24
Q

how many times do you have to repeat titration + why

A
  • until you get at least 2 concordant titres
    > this allows you to calculate a mean titre that has high precision
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25
Q

after carrying out a titration what things will you have obtained

A
  • the two reactants used (acid + alkali)
  • volume + concentration of one of the reactants
  • the volume, but NOT concentration of other reactant
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26
Q

how can you find the concentration of a reactant after titration having been given everything else

A
  1. balance your equation
  2. work out moles of solution you know vol + conc abt (multiply the two)
  3. find molar ratio
  4. find unknown concentration using mols + vol
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27
Q

what is the molar volume of a gas

A
  • the volume occupied by one mole of any gas at the same room temperature and pressure
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28
Q

what is the molar volume of gas at room temp + pressure

A

24dm3 or 24000cm3

29
Q

what is the equation for finding moles of gas

A

moles of gas = volume of gas dm3 / 24dm3/mol

30
Q

how do you measure the volume of gas (experiment)

A
  • 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
31
Q

what is the rate of reaction

A
  • a measure of how quickly reactants are used or products are formed
32
Q

how can you calculate the rate of reaction

A

rate of reaction = amount of reactant used / time taken
rate of reaction = amount of product formed / time taken

33
Q

how can you measure the volume of gas using a syringe

A
  • 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
34
Q

a reaction can only happen if:

A
  • the reactant particles collide with each other
  • the colliding particles have enough energy to react
35
Q

what is a successful collision

A
  • a collision that leads to a reaction
    > the greater the rate of successful collisions, the greater the rate of reaction
36
Q

when will a collision not be successful

A
  • if the particles have less energy than the activation energy
37
Q

what ways can we change the rate of a chemical rection

A
  • concentration
  • pressure
  • temperature
  • surface area
  • (catalysts)
38
Q

the rate of reaction is ____ to the reaction time

A
  • 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
39
Q

how can you measure how fast hydrogen is produced

A
  • 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
40
Q

how can the rate of reaction be measured using a digital mass balance

A
  • 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
41
Q

how can the rate of reaction be measured if a precipitate is formed

A
  • 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
42
Q

why is the disappearing cross rate of reaction experiment not very accurate

A
  • very subjective
    > difference of opinions on when cross disappears
43
Q

how can the rate of reaction be calculated at a specific time in a reaction

A
  • 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)
44
Q

how do concentrations of reactants + products change during a reaction

A
  • 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%
45
Q

describe + explain the gradient of the curve on a rate of reaction graph

A
  • 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)
46
Q

how does temperature affect the rate of reaction

A
  • 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
47
Q

how does concentration affect the rate of reaction

A
  • 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
48
Q

how does pressure affect the rate of a gaseous reaction

A
  • 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
49
Q

how does surface area affect the rate of reaction

A
  • 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
50
Q

the energy stored in _____ doesn’t change but…

A
  • the energy stored in particles doesn’t change but the rate of collisions increase, which means that the rate of successful collisions increase
51
Q

what happens to surface area to volume ratio as size of lumps decrease

A
  • the surface area to volume increases
    > which also increases rate of reaction
52
Q

what is a catalyst

A
  • a substance that increases the rate of reaction but is unchanged / not used up at the end of the reaaction
53
Q

how do catalysts work to increase the rate of reaction

A
  • 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
54
Q

what do catalysts not affect during a reaction

A
  • frequency of collisions
  • energy stored in particles
55
Q

why is only a small amount of catalyst used to catalyse a reaction between large amounts of reactants

A
  • because they are unchanged and not used up so you don’t need a lot
    > helpful as catalysts often contain expensive metals
56
Q

how can you investigate a catalyst

A
  • 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
57
Q

what are reversible reactions

A
  • 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
58
Q

what is a closed system

A
  • when no substances can enter / leave
59
Q

what is dynamic equilibrium

A
  • 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
60
Q

what 3 things can change the position of equilibrium

A
  • temperature
  • pressure (gases)
  • concentration
61
Q

what is Le Chatelier’s principle

A
  • 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
62
Q

what happens to the position of equilibrium if the concentration of reactants increase

A
  • the equilibrium shifts to the right to reduce concentration of reactants
    > more product formed
63
Q

what happens to the position of equilibrium if the concentration of the products increase

A
  • the position of equilibrium will shift to the left to decrease the concentration
    > less product formed
64
Q

how does a change in pressure affect the position of equilibrium

A
  • increasing pressure causes the position of equilibrium to shift towards the side with less moles of gas
65
Q

what happens to the position of equilibrium if the temperature of a reaction is increased

A
  • equilibrium shifts in the endothermic direction
66
Q

what happens to the position of equilibrium if the temperature of a reaction is decreased

A
  • equilibrium shifts in the exothermic direction
67
Q

how do you know if the forward reaction is exo or endothermic

A
  • the energy change is negative for exothermic reaction as energy is lost to surroundings
    > if forward reaction is negative then its exothermic
68
Q

what is a compromise pressure

A
  • a pressure high enough to achieve a reasonable equilibrium yield but not so high as to be expensive or hazrdous
69
Q
  • what is a compromise temperature
A
  • a temperature low enough to achieve a reasonable equilibrium yield, but high enough to achieve a reasonable rate of reaction

OCR Gateway A GCSE Chemistry (7 decks)

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