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Chemistry Paper 2 > Topic 6a- Rates of Reaction > Flashcards

Topic 6a- Rates of Reaction Flashcards

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1
Q

What is the rate of reaction?

A

The rate of a chemical reaction is how fast the REACTANTS are CHANGED into PRODUCTS.

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2
Q

Give examples of a slow chemical reactions.

A

-The rusting of iron is one of the SLOWEST reactions. Other low reactions include CHEMICAL WEATHERING-like acid rain damage to limestone buildings.

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3
Q

Give examples of a moderate chemical reaction.

A

-A MODERATE SPEED reaction would be the metal MAGNESIUM reacting with an ACID to produce a gentle stream of bubbles.

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4
Q

Give examples of a fast chemical reaction.

A

-BURNING is a FAST reaction.

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5
Q

Give examples of a very fast chemical reaction.

A

-EXPLOSIONS are even FASTER and release a lot of gas. Explosive reactions are all over in a FRACTION OF A SECOND.

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6
Q

What are the four main factors that affect the rate of a reaction?

A

1) Temperature- the higher the temp, the faster the reaction.
2) The CONCENTRATION of a solution or the PRESSURE of a gas- the more concentrated or the higher the pressure, the faster the reaction.
3) SURFACE AREA- this changes depending on the size of the lumps of solid. The larger the surface area to volume, the faster the reaction.
4) The presence of a CATALYST- reactions go faster than reactions without a catalyst.

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7
Q

What does collision theory tell us?

A

Reaction rates are explained by COLLISION THEORY.

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8
Q

What is collision theory?

A

Collision theory just says that the rate of reaction depends on two things:

  1. The COLLISION FREQUENCY of reacting particles (how OFTEN they collide). The MORE collisions there are the FASTER the reaction is.
    E.g. doubling the frequency of collisions doubles the rate.
  2. The energy TRANSFERRED during a collision. Particles have to collide with ENOUGH ENERGY for the collision to be successful.
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9
Q

How can you change the rate of reaction?

A

Changing either of these factors will change the rate of a reaction, meaning there are 2 ways to increase the rate of reaction.

One way is to increase the frequency of collisions, so that the probability of a SUCCESSFUL COLLISION (a collision that results in a reaction) increases.

Or increase the energy of the collisions, so that more collisions are successful.

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10
Q

Define collision theory./ What must happen for a reaction to occur between two particles?

A

The theory that in order for a reaction to occur, particles must collide with sufficient energy.

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11
Q

What is activation energy?

A

The MINIMUM amount of energy that particles must have to react is called the ACTIVATION ENERGY.

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12
Q

Why do particles need energy in order to react?

A

Particles need this much energy to BREAK THE BONDS in the reactants to start the reaction.

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13
Q

How does increasing the temperature increase the rate of reaction?

A

1) When the temperature is INCREASED, the particles all move FASTER.
2) If they’re moving faster, they’re going to collide more FREQUENTLY.
3) Also the faster they move the MORE ENERGY they have, so MORE of the COLLISIONS will have ENOUGH ENERGY to make the reaction happen.

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14
Q

How does increasing the concentration increase the rate of reaction?

A

If a solution is made more CONCENTRATED, it means there are MORE PARTICLES of reactant knocking about between the SAME VOLUME of water (or other solvent).

This makes COLLISIONS between the reactant particles MORE FREQUENT.

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15
Q

How does increasing the pressure increase the rate of reaction?

Tip-this makes sense-you’re much more likely to bump into someone when you’re in a crowd of people than when there aren’t many people in the room.

A

Similarly, when the PRESSURE of a gas is INCREASED, it means that the SAME NUMBER of particles occupies a SMALLER SPACE. So they’re squashed closer together.

This makes COLLISIONS between the reactant particles MORE FREQUENT.

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16
Q

Explain how increasing the surface area increases the rate of reaction.

A

1) If one of the reactants is a SOLID, then breaking it up into SMALLER PIECES will increase its SURFACE AREA TO VOLUME RATIO.
2) This means that for the SAME VOLUME of the solid, the particles around it will have MORE AREA to work on- so there will be collisions MORE FREQUENTLY.

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17
Q

What is a catalyst?

A

A catalyst is a substance that SPEEDS UP a reaction WITHOUT being CHANGED or USED UP in the reaction. This means its’s NOT part of the overall EQUATION.

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18
Q

How can using a catalyst increase the rate of reaction?

A

DIFFERENT catalysts are needed for different reactions, but they all work 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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19
Q

What are enzymes?

A

Enzymes are BIOLOGICAL CATALYSTS- they catalyse reactions in LIVING THINGS.

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20
Q

What do catalysts do?

A

Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a LOWER ACTIVATION ENERGY.

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21
Q

What is rate and proportionality of a chemical reaction?

A

The RATE of a reaction is DIRECTLY PROPORTIONAL to the FREQUENCY of successful COLLISIONS.

This means that, if the frequency of successful collisions doubles, the rate will also double. If the frequency of successful collisions triples, the rate triples and so on.

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22
Q

How can you find the speed of the reaction?

A

You can find the speed of the reaction by recording the amount of PRODUCT FORMED, or the amount of REACTANT USED UP over TIME.

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23
Q

How can you understand the rate of reaction from a graph?

A

The STEEPER the line on the graph, the FASTER the rate of reaction. OVER TIME the line becomes LESS STEEP as the reactants are USED UP.

24
Q

How can you find the quickest reaction from a graph?

A

The QUICKEST REACTIONS have the STEEPEST lines and become FLAT in the LEAST TIME.

25
Q

How can you increase the amount of product formed and get a faster reaction time?

A

This can only happen if you add MORE REACTANT at the start.

26
Q

How can you calculate the rate of reaction?

A

You can you find the rate of reaction by measuring how quickly the reactants are used up or how quickly the products are formed.

Once you’ve taken these measurements, you can work out the mean reaction time using the formula:

mean rate of reaction =
quantity of reactant used ÷ time taken

mean rate of reaction =
quantity of product formed ÷ time taken



27
Q

What are the units in the formula for rate of reaction?

A

When the product or reactant is a GAS you usually measure the amount in ㎤.
If it’s a SOLID, they you use GRAMS (g).
Time is often measured in SECONDS (s).

28
Q

What are the units for rate?

A

The units for rate nay be in ㎤/s or g/s.

You can also measure the amount of product in MOLES-so the units of rate could be mol/s.

29
Q

How can you measure the rate of reaction?

A

There are 3 different ways that you can measure the formation of products during a reaction, and so calculate the rate.

1) PRECIPITATION and COLOUR CHANGE
2) Change in MASS (usually given off)
3) The VOLUME of gas given off

30
Q

What is a precipitate?

A

A precipitate is an insoluble solid that is formed in a solution during a chemical reaction.

31
Q

How can you calculate the rate of reaction by precipitation and colour change?

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 log 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, you can time how long it takes for the solution to LOSE or GAIN its COLOUR.

32
Q

What are the problems of measuring rates of reaction by precipitation and colour change?

A

1) It only works for reactions where there’s a visual change in the solution as the reaction occurs.
2) The results are very SUBJECTIVE-DIFFERENT PEOPLE might not agree over the EXACT point when the mark ‘disappears’ or the solution changes colour.
3) If you use this method, you can’t plot a rate of reaction GRAPH from the results.

33
Q

What is turbidity?

A

The posh way of saying that the cloudiness of a solution changes is to say that its ‘TURBIDITY’ changes.

34
Q

Describe the method to calculate the rate of reaction by the change in mass.

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 DISSAPEARING 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.

35
Q

What are the advantages and disadvantages of finding the change of mass to calculate rate of reaction?

A

Advantages:
-This is the MOST ACCURATE of the three methods described on this page because the mass balance is very accurate.

Disadvantages:
-But it has the disadvantage of releasing gas straight into the room, which isn’t very good if the gas is dangerous.

36
Q

How can you increase the accuracy of the experiment to find the change of mass?

A

Some gases don’t weigh very much so the change in mass can be quite small.
The trick is to use a mass balance with a high RESOLUTION, so that very small changes in mass can be detected.

37
Q

Describe the method to calculate the rate of reaction by 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 ㎤, 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.

38
Q

Why is it important that you can plot a rate of reaction graph from your results?

A

The mean rate of reaction tells you what the average rate was as a certain amount of product was formed.

But rate isn’t constant during a reaction, it slows down. So drawing a graph can let you calculate the rate of reaction at a particular point in time during the reaction.

39
Q

How can you calculate the mean rates from graphs?

A

To find the mean rate for the whole reaction from a graph of the amount of a substance against time, you just work out the overall change in the y-value (the amount of substance) and then divide this by the total time taken for the reaction. You can also use the graph to find the mean rate of reaction between any two points in time.(E.g. on pg 204 B CGP).

40
Q

What is a tangent?

A

A tangent is a straight line that touches the curve at a particular point without crossing it.

41
Q

How can you find how fast the rate is at a particular point in a reaction from a graph?

A

To get an idea of how fast the rate is at a particular point in the reaction you need to find out how steep the curve is at that point. And to do that you need to be able to draw a TANGENT to the curve.

By drawing tangents at various points along the curve of a reaction, you can see how the rate changes over time.

E.g. If the slope of the tangent is steep, the reaction is fast. If the tangent is less steep, the reaction is slowing down. If the tangent is a flat line, it shows the reaction has stopped.

42
Q

How can you calculate rates from tangents?

A

As well as drawing the tangent of a curve, you can also calculate the gradient of the tangent. The value you calculate will be equal to the rate of the reaction at that particular point in time.

43
Q

What is the formula for gradient?

A

gradient= change in y ÷ change in x

44
Q

What is the formula for the mean rate of reaction?

A

Mean rate of reaction= change in y ÷ change in x

45
Q

What is a hypothesis?

A

A hypothesis is a possible explanation for a scientific observation.

In science a hypothesis must be testable.

46
Q

What chemicals are used for the experiment to investigate the effect of increased concentration on the rate of the reaction?

A

You can use the reaction of MAGNESIUM metal with DILUTE HYDROCHLORIC ACID to investigate the effect of increased reactant concentration on the rate of a reaction.

47
Q

How can measure the rate of reaction of magnesium and dilute HCl?

A

The reaction of magnesium and dilute HCl gives off hydrogen gas, so you can follow the RATE of reaction by MEASURING the VOLUME of GAS produced.

48
Q

Explain the method used to investigate the effect of concentration on the rate of reaction.

A
  1. Using a measuring cylinder place 50㎤ of HYDROCHLORIC ACID into a beaker.
  2. Attach the conical flask to a BUNG and a DELIVERY TUBE.
  3. Now place the deliver tube into a CONTAINER FILLED with WATER.
  4. Then place an UPTURNED MEASURING CYLINDER also filled with water over the delivery tube.

Ensure that the measuring cylinder stays filled with water when turned upside down.

  1. Add a 3cm strip of MAGNESIUM to the hydrochloric acid and start a stopwatch.
  2. The reaction produces HYDROGEN GAS which is trapped in the measuring cylinder.
  3. Every ten seconds, measure the VOLUME OF HYDROGEN GAS in the measuring cylinder.

Continue until NO MORE hydrogen is given off.

  1. Repeat the experiment using DIFFERENT CONCENTRATIONS of hydrochloric acid, e.g. 1.0 M HCl.
  2. Plot a graph with:
  • ‘Volume of gas produced in cm3 (for 2.0 M hydrochloric acid)’ on the y-axis
  • ‘Time in seconds’ on the x-axis

Draw a smooth curved line of best fit.

Plot a curve for 1.0 M hydrochloric acid on the same graph.

  1. Use this graph to compare the rates of reaction of 1.0 M and 2.0 M hydrochloric acid with magnesium.
49
Q

When investigating the affect of concentration on the rate of reaction by observing a colour change, what reactants are used?

A

In this reaction the reactants used are:

  1. Sodium Thiosulfate
  2. Hydrochloric acid
50
Q

What state are the reactants in?

What do the reactants Sodium Thiosulfate and Hydrochloric acid produce?

A
  • Sodium thiosulfate and hydrochloric acid are both clear solutions.

-They react together to form a yellow precipitate of sulfur.
Sulfur is a SOLID.
The sulfur makes the solution CLOUDY. Scientists call this cloudiness TURBIDITY.

We can use this to see HOW LONG the reaction takes to FINISH.

51
Q

Explain the method used to investigate the effect of concentration on the rate of reaction by observing a precipitate forming or a colour change.

A
  1. Use a measuring cylinder to put 10㎤ of SODIUM THIOSULFATE solution into a conical flask.
  2. Place the conical flask onto a printed black CROSS.
  3. Next add 10 ㎤ of HYDROCHLORIC ACID into the conical flask.
    At the same time SWIRL the solution and start a STOPWATCH.
  4. LOOK down through the top of the flask.

After a certain time, the solution will turn CLOUDY.

  1. Stop the clock when we can NO LONGER SEE the cross.
  2. Record the time in seconds.
  3. Repeat the experiment again using LOWER CONCENTRATIONS of sodium thiosulfate solution.
  4. Repeat the whole experiment TWICE more and calculate MEAN VALUES for each concentration of sodium thiosulfate solution.
    REMEMBER- We do NOT include any ANOMALOUS RESULTS when calculating a mean.
52
Q

Why is it important to compare your results with others in the class?

A

To check if the results are REPRODUCIBLE.

53
Q

What is reproducibility?

A

A measurement is REPRODUCIBLE if it can be repeated by ANOTHER PERSON or using a DIFFERENT TECHNIQUE or EQUIPMENT and still get the SAME RESULT.

54
Q

What are the risk factors involved in this experiment, and how can we remain safe?

A

Take care not to breathe in any sulfur dioxide fumes.

55
Q

What is the problem with this experiment?

A

The problem with the disappearing cross experiment is that DIFFERENT PEOPLE have different EYESIGHTS.

That means that some people can see the cross for longer than others so they MAY NOT GET THE SAME RESULTS.

56
Q

How can we reduce the problems with accuracy of the cross experiment?

A

However, because all the students use the SAME SIZE PRINTED CROSS, this problem may not be too great.

57
Q

What do the results from the experiment between magnesium and HCl to produce hydrogen gas show?

A

Both of these experiments show us that the GREATER THE CONCENTRATION of a chemical reaction, the FASTER the reaction takes place.

Because this result is shown by TWO DIFFERENT EXPERIMENTS we can say that this finding is REPRODUCIBLE.

Chemistry Paper 2 (4 decks)

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