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Chemistry > C6 - the rate and extent of chemical change > Flashcards

C6 - the rate and extent of chemical change Flashcards

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1
Q

what is the rate of a chemical reaction?

A

how fast the reactants are changed into products

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2
Q

what are two examples of chemical reactions with slow rates?

A

iron rusting and chemical weathering (like acid rain damage to limestone buildings)

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3
Q

what is an example of a moderate speed reaction?

A

magnesium reacting with an acid to form a gentle stream of bubbles

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4
Q

what are two examples of fast reactions?

A

burning and explosions (but explosions are faster)

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5
Q

what are two ways you can find the speed of a reaction?

A

recording the amount of product formed, or recording the amount of reactant used up over time

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6
Q

on a rate of reaction graph, does a steeper line mean a faster or slower reaction?

A

faster

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7
Q

why does the line become less steep over time on a rate of reaction graph?

A

as the reactants are used up

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8
Q

what must happen in order for a reaction to occur?

A

particles must collide with enough energy

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9
Q

what two things does the rate of a chemical reaction depend on?

A
  1. the collision frequency

2. the energy transferred during a collision

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10
Q

what are five things the rate of reaction depends on?

A
  1. temperature
  2. the concentration of a solution
  3. the pressure of a gas
  4. surface area
  5. the presence of a catalyst
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11
Q

explain the effect changing temperature has on the rate of a reaction?

A

in order for a reaction to occur, the particles have to collide. increasing the temperature increases the speed the particles move at, and so they collide more frequently, so an increase in temperature increases the rate of a reaction

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12
Q

explain the effect concentration has on rate of reaction?

A

in order for a reaction to occur, particles have to collide. the more concentrated a solution is, the more particles there are per unit volume, so there are more frequent collisions. this means that an increase in concentration increases the rate of a reaction

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13
Q

explain the effect pressure has on the rate of a reaction?

A

in order for a reaction to take place, particles must collide. the higher the pressure, the more particles there are per unit volume, so there are more frequent collisions. this means that an increase in pressure increases the rate of reaction

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14
Q

explain the effect surface area has on the rate of a reaction

A

in order for a reaction to occur, particles have to collide. increasing the surface area to volume ratio of a solid will increase the amount of the solid exposed to the other reactants, so more frequent collisions will occur. this means that an increase in surface area to volume ratio increases the rate of a reaction

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15
Q

what is a catalyst?

A

a substance that speeds up the rate of a reaction, without being used up in the reaction itself

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16
Q

are catalysts part of the overall reaction equation?

A

no

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17
Q

how do catalysts work?

A

by decreasing the activation energy needed for the reaction to occur. they do this by providing an alternative reaction pathway with a lower activation energy.

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18
Q

give an example of a biological catalyst

A

enzymes - they catalyse reactions in living things

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19
Q

what is the formula for the mean rate of reaction?

A

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

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20
Q

how do you find the rate of a reaction at one particular point in time?

A

you have to plot a graph and find the gradient at that time

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21
Q

what are the three methods to measure the rate of a reaction?

A
  1. precipitation and colour change
  2. change in mass (usually gas given off)
  3. the volume of gas given off
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22
Q

what units would you use to measure a product or reactant if it’s a gas?

A

cm^3

23
Q

what units would you use to measure a product or reactant if it’s a solid?

A

grams

24
Q

what is the most common measurement of time when calculating rate of reaction?

A

seconds

25
Q

give three common examples of the units used for the rate of a reaction

A

cm^3/s, g/s, or mol/s

26
Q

how do you measure the rate of a reaction using precipitation and colour change? (4 points)

A
  1. you can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)
  2. you can observe a mark through the solution and measure how long it takes for it to disappear - the faster the mark disappears, the quicker the reaction
  3. if the reactants are coloured and the products are colourless (or vice versa), you can time how long it takes for the solution to lose (or gain) its colour
  4. the results are very subjective - different people might not agree over the exact point when the mark ‘disappears’ or the solution changes colour. Also, if you use this method, you can’t plot a rate of reaction graph from the results
27
Q

how do you measure the rate of a reaction using a change in mass? (5 steps)

A
  1. measuring the speed of a reaction that produces a gas can be carried out using a mass balance
  2. as the gas is released, the mass lost is measured on the balance
  3. the quicker the reading on the balance drops, the faster the reaction
  4. if you take measurements at regular intervals, you can plot a rate of reaction graph and find the rate quite easily
  5. this is the most accurate of the three methods because the mass balance is very accurate, but it has the disadvantage of releasing gas straight into the room
28
Q

how do you measure the rate of a reaction using the volume of gas given off?

A
  1. this involves the use of a gas syringe to measure the volume of gas given off
  2. the more gas given off during a given time interval, the faster the reaction
  3. gas syringes usually give volumes accurate to the nearest cm^3, so they’re quite accurate. you can take measurements at regular intervals and plot a rate of reaction graph using this method too. you have to be quite careful though - if the reaction is too vigorous, you can easily blow the plunger out of the end of the syringe
29
Q

what do magnesium and HCl react to produce?

A

Hydrogen gas

30
Q

what are the 6 steps to using the reaction between magnesium and HCl to investigate the effect of concentration on the rate of reaction?

A
  1. start by adding a set volume of dilute hydrochloric acid to a conical flask and carefully place on a mass balance
  2. now add two magnesium ribbons to the acid and quickly plug the flask with cotton wool
  3. start the stopwatch and record the mass on the balance. take readings of the mass at regular intervals
  4. plot the results in a table and work out the mass lost for each reading. now you can plot a graph with time on the x-axis and loss of mass on the y-axis
  5. repeat with more concentrated acid solutions. variables such as the amount of magnesium ribbon and the volume of acid used should be kept the same each time - only change the acid’s concentration. This is to make your experiment a fair test
  6. the three graphs show that a higher concentration of acid gives a faster rate of reaction
31
Q

what are the 6 steps to using the reaction between sodium thiosulfate and HCl to investigate the effect of concentration on the rate of reaction?

A
  1. Sodium Thiosulfate and HCl are both clear solutions. They react together to form a yellow precipitate of sulfur
  2. start by adding a set volume of dilute sodium thiosulfate to a conical flask
  3. place the flask on a piece of paper with a black cross drawn on it. add some dilute HCl to the flask and start the stopwatch.
  4. now watch the black cross disappear through the cloudy sulfur and time how long it takes to fully disappear
  5. the reaction can be repeated with solutions of either reactant at different concentrations (but only change the concentration of one reactant at a time). the depth of the liquid must be kept the same each time
  6. The higher the concentration, the quicker the reaction and therefore the less time it takes for the mark to disappear
32
Q

why should the reaction between sodium thiosulfate and HCl be carried out in a well-ventilated place?

A

because it releases sulfur dioxide

33
Q

what do sodium thiosulfate and HCl react together to produce?

A

sulfur - a yellow, cloudy precipitate

34
Q

what does a rate of reaction graph show on its X and Y axis?

A

it shows the amount of product formed or reactant used up on the y-axis and time on the x-axis

35
Q

how do you find the mean rate for a whole reaction?

A

you work out the overall change in the y-axis of the graph (amount of product formed or reactant used up), and divide this by the total time taken for the reaction

36
Q

how do you use a graph to find the rate of a reaction at a particular point in time?

A

you draw a tangent and work out the gradient of the tangent:

  1. position a ruler on the graph at the point where you want to know the rate
  2. adjust the ruler until the space between the ruler and the curve is equal on both sides of the point
  3. draw a line along the ruler to make the tangent. extend the line right across the graph
  4. pick to points on the line that are easy to read. use them to calculate the gradient of the tangent in order to find the rate
37
Q

how does a reversible reaction reach equilibrium?

A
  1. as the reactants react, their concentrations fall, so the forward reaction will slow down. But as more and more products are made and their concentrations rise, the backward reaction will speed up
  2. after a while the forward reaction will be going at exactly the same rate as the backward one - the system is at equilibrium
  3. at equilibrium, both reactions are still happening, but there’s no overall effect (it’s a dynamic equilibrium). This means the concentrations of reactants and products have reached a balance and won’t change
  4. equilibrium is only reached in a closed system
38
Q

can a reaction be at equilibrium with unequal amounts of reactants and products?

A

yes - equilibrium doesn’t mean that the amounts of reactants and products are equal

39
Q

what does it mean if the position of equilibrium lies to the right?

A

the concentration of the products is greater than that of the reactants

40
Q

what does it mean if the position of equilibrium lies to the left?

A

the concentration of reactants is greater than that of the products

41
Q

what conditions does the position of equilibrium depend on?

A
  1. the temperature
  2. the pressure (in gases)
  3. the concentration of the reactants and products
42
Q

is the reaction from hydrated copper sulfate to anhydrous copper sulfate + water endothermic or exothermic?

A

endothermic

43
Q

is the reaction from anhydrous copper sulfate + water to hydrated copper sulfate endothermic or exothermic?

A

exothermic

44
Q

what is le chatalier’s principle?

A

the idea that if you change the conditions of a reversible reaction at equilibrium, the system will react to counteract that change

45
Q

what can le chatalier’s principle be used for?

A

it can be used to predict the effect of any changes you make to a reaction system

46
Q

what happens if you lower the temperature of a reversible reaction at equilibrium?

A

the equilibrium will move in the exothermic direction to produce more heat. this means you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction

47
Q

what happens if you raise the temperature of a reversible reaction at equilibrium?

A

the equilibrium will move in the endothermic direction to try and decrease the temperature. You’ll get more products for the endothermic reaction and fewer products for the exothermic reaction

48
Q

which reactions at equilibrium are affected by pressure?

A

those involving gases

49
Q

what happens if you increase the pressure of a reversible reaction at equilibrium?

A

the position of equilibrium shifts towards the side where there are fewer molecules of gas - it tries to reduce the pressure

50
Q

what happens if you decrease the pressure of a system at equilibrium?

A

the position of equilibrium shifts towards the side where there are more molecules of gas in an attempt to increase the pressure again

51
Q

what happens if you change the concentration of either the reactants or the products of a system at equilibrium?

A

the system will no longer be at equilibrium, so it responds to bring itself back to equilibrium

52
Q

what happens if you increase the concentration of the reactants in a system at equilibrium?

A

the system makes more products to counteract the change

53
Q

what happens if you decrease the concentration of products in a system at equilibrium?

A

the system reduces the amount of reactants to counteract the change

Chemistry (16 decks)

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