3 Rates of reaction Flashcards
(132 cards)
1
Q
What is the rate of a reaction?
A
The speed at which the amount of reactants decreases or the amount of products increases. It is measured as a change in the concentration, or amount, of reactants or products per unit time.
2
Q
How can we measure the rate of reaction?
A
Rate of reaction =
Change in conc, vol, mass
————————————
time
3
Q
How could we investigate the rate of reaction?
A
By using marble chips and dilute hydrochloric acid.
4
Q
What are marble chips made of?
A
Calcium carbonate,
5
Q
What do the marble chips react with hydrochloric acid to produce?
A
Calcium carbonate + hydrochloric acid —> Calcium chloride + water + carbon dioxide
6
Q
What is the apparatus used in this experiment?
A
7
Q
What is step 1 of the procedure?
A
Use a measuring cylinder to measure 25cm^3 of 2.00mol/dm^3 dilute hydrochloric acid.
8
Q
What is step 2 of the procedure?
A
Add 5.00g of large marble chips to a conical flask and place a piece of cotton wool at the opening of the flask. The cotton wool is there to allow the carbon dioxide to escape during the reaction, but to stop any acid spitting out. The marble is in excess - some of it will be left over when the acid is all used up.
9
Q
What is step 3 of the procedure?
A
Place everything on a balance and reset it to zero.
10
Q
What is step 4 of the procedure?
A
Add the acid to the marble chips and record the reading on the balance every 30 seconds.
11
Q
What is a picture of the apparatus during the reaction?
A
12
Q
Why is the mass decreasing?
A
Once the reaction starts, the balance shows a negative mass. The mass goes down because the carbon dioxide escapes through the cotton wool.
13
Q
What would a graph of mass of carbon dioxide lost against time look like?
A
14
Q
What is an explanation of the graph?
A
About 0.47g of carbon dioxide is produced in the first minute. Only about 0.20g of extra carbon dioxide is produced in the second minute, the reaction is slowing down.
15
Q
What does the slope show?
A
The rate of reaction.
16
Q
What happens if there is a steeper slope?
A
The faster the reaction.
17
Q
What is the equation to find the rate?
A
time
18
Q
How would we find the average rate of the reaction in the first minute?
A
0.47/1 = 0.47
19
Q
How would we find the average rate of reaction in the second minute?
A
0.20/1 = 0.20
20
Q
How would we find the average rate of reaction over the first two minutes?
A
0.67/2 = 0.34g/min.
21
Q
What is happening in the beginning of the reaction is relation to the speed?
A
We can see that the reaction is fastest at the beginning.
22
Q
Why does the reaction slow down and eventually stop?
A
Because all of the hydrochloric acid has been used up.
23
Q
How can we measure how fast a reaction is going at any time point?
A
By finding the slope of the line at that point. This is the rate of reaction at that point, rather than the average.
24
Q
How do you measure the rate of reaction at a specific point?
A
This is done by drawing a tangent to the line at the time you are interested in and finding its slope.
25
What is an example of finding the rate at a specific point on the graph?
At 5 minutes the carbon dioxide is being lost at the rate of about 0.05g per minute.
26
What happens to the marble chips at the end of the reaction?
They were in excess. So there will still be unreacted marble chips in the flask at the end.
27
What is another way of following the rate of this reaction?
By measuring the volume of carbon dioxide given off.
28
What is a example of the apparatus that could be used to measure the volume of carbon dioxide given off?
29
What is another apparatus that could be used to measure the volume of carbon dioxide given off?
30
How can we explain the shape of the curve in the graph?
By thinking about the particles present and how they interact. This is called collision theory.
31
What is collision theory?
It states that for a reaction to occur, the reactant particles must collide with each other and the collision needs too have sufficient energy and the correct orientation.
32
When can reactions only happen?
Only when particles collide.
33
Does every collision between a particle result in a reaction?
No, not all collisions end up in a reaction. Many particles just bounce off each other.
34
What is needed for a reaction to happen during a collision?
The particles have to collide with a minimum amount of energy called the activation energy.
35
What is the activation energy?
It is the minimum amount of energy required for a collision to be sucessful.
36
In relation to the activation energy, what makes a successful collision?
The collisions with energy greater than or equal to the activation energy are usually called successful collisions.
37
What is a successful collision?
It is a collision with energy greater than or equal to the activation energy. These collisions result in reactions.
38
How can collision theory be applied to the rate of reaction between marble chips and dilute hydrochloric acid?
In the reaction between calcium carbonate and hydrochloric acid, particles in the acid have to collide with the particles at the surface of the marble chips. As the acid particles are getting used up, the collision rate decreases, and so the reaction slows down. The marble is in a large excess so that is shape doesn't change very much during the reaction.
39
What is a diagram that shows the hydrochloric acid particles and the marble chips early in the reaction with lots of acid particles and lots of collisions?
40
What is a diagram that shows the hydrochloric acid particles and the marble chips later in the reaction with fewer hydrochloric acid particles left?
41
What are graphs that would usually be presented to us in the exam?
A graph that shows the mass or volume of a product formed during a reaction.
42
What is a rarer case of the graph that we may be able to observe?
It is possible that you will see the graphs showing the fall in the concentration of one of the reactants - in this case the concentration of the dilute hydrochloric acid.
43
What is an image of the graph that shows the fall in concentration of hydrochloric acid over time?
44
What is shown where the graph is falling most quickly?
Where the graph is falling most quickly (its steepest), shows that the reaction is fastest.
45
Why does the graph eventually become horizontal?
Because the reaction had stopped when all the acid has been consumed.
46
In what way can we alter the experiment to do with marble chips and dilute hydrochloric acid?
We could use exactly the same quantities of everything, but using smaller marble chips.
47
What results should we obtain when we investigate the effect of changing the surface area of marble chips on the rate of reaction between marble chips and dilute hydrochloric acid?
The reaction with the smaller chips happens faster.
48
What would the graph of both sizes of marble chips look like?
49
What is the main difference between the two lines?
The reaction with the smaller chips starts off much faster and finishes sooner.
50
How does changing the surface area affect the line in the graph?
- Much steeper slope at the beginning, showing a faster reaction.
- Graph becomes horizontal more quickly, showing that the reaction stops sooner.
- The lines will both stop at the same place as although the reactions occur at different speeds, the same amount of carbon dioxide is produced.
51
When are reactions between solids and liquids, or solids and gases faster?
If the solids are present in a lot of small pieces rather than a few big ones.
52
What could we do to the solid to make the reaction happen quicker?
The more finely divided the solid, the quicker the reaction.
53
Why does the reaction happen faster if the solid is in a higher quantity?
This is because the surface area in contact with the gas or liquid is much greater and there are more particles of the solid exposed on the surface.
54
Why is it important that there are more particles exposed and readily available on the surface?
Only the particles on the surface are available for collisions.
55
What is the relationship between the frequency of successful collisions and the surface area?
It is directly proportional.
56
What is an image that demonstrates the smaller the surface area, the slower the rate of reaction?
57
What is an image that demonstrates the larger the surface area, the faster the rate of reaction?
58
How are large surface areas used?
They are used to speed up reactions outside the lab.
59
What is an example of large surface areas used to speed up reactions outside the lab?
A catalytic converter for a car uses expensive metals such as platinum, palladium and rhodium coated onto a honeycomb structure in a very thin layer to give the maximum possible surface area.
60
Why is it important for this reaction to be rapid?
In the presence of these metals, harmful substances such as carbon monoxide and nitrogen oxides are converted into relatively harmless carbon dioxide and nitrogen. The large surface area means the reaction is very rapid. This is important because the gases in the exhaust system are in contact with the catalytic converter for only a very short time.
61
What is a catalytic converter?
A device used in cars to convert oxides of nitrogen and carbon monoxide into harmless nitrogen gas and carbon dioxide. It uses platinum, palladium and rhodium as catalysts.
62
How can we change the experiment of marble chips and hydrochloric acid to observe concentration changes?
We can repeat our original experiment with large marble chips and hydrochloric acid. Everything is kept the same except we use hydrochloric acid of half the concentration.
63
What is an image of a graph that shows the effect of changing the concentration of the acid?
64
What does half-ing the concentration of the acid to do the rate of reaction?
Reducing the concentration of the acid makes the reaction slower. We can see this on our graph because the red line is less steep than out original experiment, the blue line.
65
What is an explanation of our graph in reference to the experiment?
In our original experiment we used 25cm3 of 2.00mol/dm3 hydrochloric acid. In this experiment we used 25cm3 of 1.00mol/dm3 hydrochloric acid. In the second experiment we have started with half the number of hydrochloric acid particles (half as many moles) and so we will produce half as much carbon dioxide.
66
What happens to the rate if you increase the concentration of the reactants?
The reaction becomes faster.
67
Why does this happen?
Increasing the concentration increases the number of acid particles within a fixed volume, therefore the particles are closer together and collide more frequently. There are more successful collisions between the acid particles and the marble chips every second.
68
What happens at a higher concentration in reference to particles?
More particles collide per second.
69
What must I do when i describe the changing concentration on the reactants?
You always have to include the idea of time. You should say things like
' the particles collide more frequently'
or
'there are more collisions per second'
70
What is an image that shows particles at a lower concentration?
71
What is an image that shows particles at a higher concentration?
72
What is the first step when investigating the effect of changing the concentration of the acid on the rate of reaction between marble chips and dilute hydrochloric acid?
We can repeat the experiment using marble chips, but using hydrochloric acid of different concentrations. Everything else would be the same, that is, the mass of the marble chips, 5.00g, and the total volume of the acid, 25cm3. We can dilute the acid by adding distilled water to the original 2.00mol/dm3 solution but making sure the total volume of water and acid remains at 25cm3.
73
What is the second step when investigating the effect of changing the concentration of the acid on the rate of reaction between marble chips and dilute hydrochloric acid?
For example, if 12.5cm3 of the original HCI is mixed with 12.5cm3 of distilled water, we get an acid solution of half of the original concentration (1.00mol/dm3). If only 5cm3 of the original HCI is mixed with 20cm3 of distilled water, the acid solution is now one-fifth (5 out of 25) of the original concentration (0.40mol/dm3).
74
What is the third step when investigating the effect of changing the concentration of the acid on the rate of reaction between marble chips and dilute hydrochloric acid?
In this experiment, we are going to calculate the average rate during the first 30 seconds, so we record the mass of carbon dioxide lost in 30 seconds.
75
How could we calculate the average rate of reaction within the first 30 seconds, if we obtain a loss of 0.32g?
0.32/30 = 0.011g/s
76
How can we identify the effect of changing concentration on rate?
We can plot the results for the different concentrations on a piece of graph paper with the rate on the y-axis and concentration on the x-axis.
77
Where should the line of best-fit go through?
The origin (0,0).
78
What happens if there is no acid?
There should be no rate
79
What does a straight line throughout the graph show?
That the rate of reaction is directly proportional to the concentration of the hydrochloric acid.
80
So, what is the relationship between the concentration and the rate?
Directly proportional.
81
What happens if the concentration doubles?
The rate should double.
82
What happens if the concentration triples?
The rate should triple.
83
What is a graph that shows the relationship between the concentration and the rate?
84
How do you calculate the concentration of hydrochloric acid?
Vol of HCI (cm3)
-------------- x original conc (2mol/dm3)
T vol of solution (25cm3)
85
Why do you need an equation to find the concentration of hydrochloric acid?
You need to look at the proportions of acid and water.
86
How else can we alter the experiment with marble chips and hydrochloric acid?
By keeping everything the same, but changing the temperature.
87
What happens to reactions as the temperature is increased?
Reactions get faster as the temperature is increased.
88
What is an image of the graph that shows the effect of changing the temperature of the reaction?
89
What is an explanation of the graph?
- The graph is steeper and finishes sooner.
- The same mass of gas is given off because we have used the same quantities of everything in the mixture.
90
How many factors need to be considered when explaining why increasing temperature increases the rate of reaction?
2
91
What is the first factor?
Increasing the temperature means that the particles are moving faster, and so collide more frequently. That will make the reaction go faster, but it only accounts for a small part of the increase in rate.
92
What is the second factor?
We learned above that in order for a collision to cause a reaction (a successful collision), the particles have to collide with a minimum amount of energy, called the activation energy. A relatively small increase in temperature produces a very large increase in the number of particles with energy greater than or equal to the activation energy. This means that it is going to be much more probable that two particles which have sufficient energy to react collide with each other. So the frequency of successful collisions increases.
93
What is an image that shows particles at a lower temperature with some of them having less activation energy?
94
What is an image that shows particles at a higher temperature with most of them having energy greater than or equal to the activation energy?
95
What difference does changing the pressure of a reaction in which the reactants are only solids or liquids have on the rate?
Almost no difference, as only solids and liquids are involved.
96
How about increasing the pressure of a reaction where gases are involved?
It does speed up the reaction.
97
How do we increase the pressure if we have a fixed mass of gas?
If we have a fixed mass of gas, we increase the pressure by squeezing it into a smaller volume.
98
What effect does squeezing the gas into a smaller volume have, in relation to particles?
The smaller space forces the particles closer together, so they collide together more frequently. This is exactly the same as increasing the concentration of the gas.
99
What is an image of particles at a lower pressure?
100
What is an image of the same particles at a higher pressure?
101
What are catalysts?
It is a substance that speeds up a chemical reaction by providing an alternative pathway of lower activation energy. Catalysts are not used up and remain chemically unchanged at the end of the reaction.
102
Because catalysts aren't consumed, what is useful about them?
Small amounts of catalyst can be used to process lots and lots of reactant particles.
103
Do all reactions use the same catalysts?
Different reactions need different catalysts.
104
How do bombardier beetles protect themselves?
By spreading a liquid, which in that liquid there is a reaction that involves splitting hydrogen peroxide into water and oxygen, using the enzyme catalase.
105
What is an enzyme?
A biological catalyst.
106
What other things catalyse the decomposition of hydrogen peroxide?
- Manganese (IV) oxide.
- Manganese dioxide
- Lead oxide.
107
What is usually. used in the lab to speed up the decomposition of hydrogen peroxide?
Manganese (IV) oxide.
108
How do we represent the reaction that is happening (decomposition of hydrogen peroxide)?
Hydrogen peroxide ---> Water + Oxygen
2H202 ---> 2h2O + O2
109
What is interesting about this equation?
We don't write catalysts into the equation because they are chemically unchanged at the end of the reaction.
110
If you wanted to include the name of the catalyst, how would you do so?
You can write their name or formula over the top of the arrow.
111
How could you show that a substance is a catalyst?
You could show that manganese oxide speeds up the decomposition of hydrogen peroxide to produce oxygen.
112
What was an image that shows manganese oxide speeding up the decomposition of hydrogen peroxide to produce oxygen?
113
How can you find out that manganese oxide has been used but is still chemically unchanged?
You can only find out by weighing it before you add it to the hydrogen peroxide solution and then re-weighing it at the end.
114
How can you extract the manganese oxide from the liquid?
By filtering it through a weighed filter paper, allowing the paper and residue to dry, and then reweighing to calculate the mass of the remaining manganese oxide.
115
What should you find out upon weighing the manganese oxide both times?
You should find that the mass hasn't changed.
116
What is the apparatus needed to investigate the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
117
What is a disadvantage of using this apparatus?
Some oxygen will escape at the beginning of the reaction when the magnesium oxide is added to hydrogen peroxide before the bung can be put back on the conical flask.
118
What is an alternative apparatus that we could use?
(Modified apparatus to avoid the loss of oxygen at the beginning of the experiment).
119
How do you use this apparatus?
When you are ready to start the reaction, shake the flask so that the weighing bottle falls over and the manganese oxide comes into contact with the hydrogen peroxide. You need to keep shaking so that an even mixture is formed.
120
What is the first step in the practical when investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
Measure 100cm3 of 2 vol hydrogen peroxide and transfer to a 250cm3 conical flask.
121
What is the second step in the practical when investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
Weigh out 0.20g of manganese oxide on a balance.
122
What is the third step in the practical when investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
Add the manganese oxide to the hydrogen peroxide and quickly replace the bung with the gas syringe already attached. Swirl the reaction mixture at a constant speed.
123
What is the fourth step in the practical when investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
Record the amount of oxygen produced every 20 seconds for 3 minutes and plot a graph of volume of oxygen versus time.
124
What is the fifth step in the practical when investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide solution?
Repeat the reaction with 0.20g of lead (IV) oxide and copper (II) oxide but keep everything else the same.
125
What is an image of a graph that shows the catalytic effect of different solids on the decomposition of H202?
126
What is an explanation of the graph?
The sample data below show manganese (IV) oxide is a very effective catalyst for the decomposition of hydrogen peroxide. In comparison, lead (IV) oxide is less effective as the rate of the reaction is much slower and copper (II) oxide does not act as a catalyst for this reaction at all.
127
Why do not all collisions result in a reaction happening?
Collisions have to involve at least a certain minimum amount of energy, called the activation energy.
128
What does it mean in terms of particles if a reaction is slow?
If a reaction is slow, it means that very few collisions have this amount of energy; when most collisions happen, the particles simply bounce off each other.
129
How do catalysts work?
Catalysts work by providing an alternative route for the reaction, involving a lower activation energy.
130
What happens if the activation energy is lower?
Many more collisions are likely to be succesful.
131
Why does the reaction happen faster with catalysts?
The reaction happens faster because many more particles have energy greater than or equal to the activation energy for the alternative route. The energy of the particles has not changed but it has been made easier for them to react.
132
What do catalysts NOT DO?
They do not 'lower the activation energy'. The original route is still there, and if the particles collide with enough energy they will still use it.
