C14 rates of reactions

LEARNING OUTCOMES

Explain what is meant by “rate of reaction”; Interpret graphical diagrammatic presentation of

data obtained in studying rates of reactions Identify the factors which affect the rate of

reaction Predict the effect of factors on rates of reaction

from given data

RATES OF REACTIONSChapter 14

Measuring the Speed of Reaction Different kinds of reactions take place at different speeds. Some reactions are very fast e.g. explosion of gases and

chemicals. On the other hand, some reactions are slow e.g. rusting of

iron and fermentation of sugar to form ethanol.


Rate of reaction

The rate of a reaction tells us how fast or slow a reaction is taking place.

We can measure the rate of a reaction in 3 ways:

2. Measuring the amount of product formed per unit time

3. Measuring the amount of reactant used up or remaining per unit time

1. Measuring the time taken for a reaction to complete


Measuring the Rate of ReactionAn experiment was set up to measure the rate of reaction between magnesium and two different solutions of dilute hydrochloric acid.

hydrochloric acid(2 mol/dm3)


magnesium ribbon magnesium ribbon

Experiment I Experiment II


Measuring the time taken for a reaction to complete

If the time taken in Experiment 1 for the magnesium to completely dissolve in the acid was 60 s, and the time taken in Experiment II was 30 s, then the speed of the reaction in Experiment II was two times as fast as in Experiment I.



magnesium ribbon magnesium ribbon



It can be seen that the shorter the time taken for a reaction to complete, the faster the speed of the reaction. Thus the speed of a reaction is inversely

proportional to the time taken:

Reaction Rate (speed ) = ___1____ Time taken

Measuring the time taken for a reaction to be complete


Measuring the amount of product formed in a reaction

An experiment was set up to measure the rate of reaction between calcium carbonate and dilute hydrochloric acid. The CO2 produced is collected in a gas syringe.

The speed of the reaction can be determined by measuring the volume of carbon dioxide produced at regular time intervals during the reaction.

A graph of the volume of gas formed is plotted against time taken.


The gradient becomes zero at 2.5 minutes, showing that no more gas is produced and the reaction has stopped.

The gradient of the graph is greatest at the start of the experiment, showing that the rate of the reaction is fastest at the start of the experiment.


The gradient decreases with time, showing that the rate of the reaction is decreasing over time.


Rate (Speed) of reaction = Quantity of product formed Time taken

The rate of reaction at a particular point P on the graph is given by the gradient of the graph at P. Rate of reaction at P = Gradient = y x = 26 cm3/min

The average rate of reaction over a time interval is given by the formula:

For e.g. average rate for the first 2.5 minutes of the reaction = (70 – 0) cm3

2.5 min = 28 cm3/min



Measuring the amount of reactant left

The change in mass of the reaction mixture can be read off from the electronic top pan balance and a graph of mass of the flask with its contents is plotted against time.

The speed of reaction between calcium carbonate and hydrochloric acid can also be determined by measuring the loss of mass of the flask as carbon dioxide escapes from the reaction mixture.


The gradient of the graph is greatest at the start of the experiment, hence the speed of the reaction is greatest at the start of the experiment.

The gradient decreases with time, showing that the speed of the reaction decreases as time proceeds.

The gradient is zero after about 4.2 min, showing that the reaction has stopped.

The reaction has stopped because one of the reactants (either HCl or CaCO3 ) has been used up in the reaction.

Measuring the amount of reactant left


Quick check 11. Explain what is meant by the “rate of reaction”. How is the reaction

rate related to the time taken for a reaction to complete?

2. How may the speed of chemical reactions be measured experimentally? Give two examples to illustrate your answer.

3. The graph shows the total volume of hydrogen produced plotted against time in a reaction.Calculate the average rate of the production of hydrogen.

Solution


1. The “rate of reaction” tells us how fast or slow a reaction is taking place. The reaction rate is inversely proportional to the time taken.

2. The speed of a chemical reaction can be measured by:(i) determining the quantity of product formed per unit time;E.g. to find the speed of reaction between magnesium and hydrochloric acid, we can measure the volume of hydrogen produced over a period of time and determine from the gradient of the volume-time graph, the speed of the reaction at any particular time interval.(ii) determining the quantity of reactant used up per unit time.E.g. to find the speed of reaction between dilute hydrochloric acid and calcium carbonate, we can measure the loss of mass form the reacting mixture over a period of time. From the gradient of the mass-time graph, the speed of reaction can be obtained at any particular time interval.

3. Average rate of the production of hydrogen = 32 cm3

80 s = 0.4 cm3/s

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Solution to Quick check 1


Effect of Temperature on the Speed of Reactions We know that food cooks faster when the temperature is

higher. For this reason, a pressure cooker is able to cook red beans in 30 minutes compared to an ordinary cooker which may take more than 2 hours. The temperature in a pressure cooker is about 120 ºC compared to 100 ºC in an ordinary cooker.

Temperature is a very important factor in the speed of reaction. In general, the rate of reaction increases two times for about every 10 ºC rise in temperature.


Factors Affecting the Speed of Reactions

How temperature affects the Speed of Reactions At higher temperature, the reacting particles move at higher

speeds as they have more kinetic energy. At higher speeds, the particles collide more often and with

greater force. This leads to more successful collisions and hence increases the rate of reaction.


Effect of particle size on the speed of reactions We know that meat and vegetables can be cooked more quickly

by cutting them into smaller pieces. This is because the smaller the size of the particles, the faster

the rate of a chemical reaction. When a solid is broken into smaller sizes, the surface area of

the solid is increased, thus exposing more particles of the solid to the reactant, and more reactions can occur.


Effect of Concentration on Speed of Reactions We all know that concentrated acids react more vigorously and faster than dilute acids with metals and other reactants.


The speed of reaction in Experiment II was about two times as fast as in Experiment I. This is because the concentration of the hydrochloric acid in Experiment II was higher than that of Experiment I.


Effect of concentration on the speed of reaction In general, the rate of reaction increases when the

concentration of one or more of the reactants is increased.

This is because a more concentrated solution contains more particles per unit volume, so there will be more particles to react with one another.


Effect of pressure on the speed of reactions

Pressure has very little effect on the rate of reactions in solids and liquids, because they cannot be compressed.

Pressure is important in gases because it has a great effect on the volume of gases. At higher pressure, gas particles are compressed closer together so there are more

particles per unit volume. This is equivalent to increasing its concentration thus increasing the rate of reaction.


The Collision Theory

A chemical reaction only occurs when two particles (atoms or molecules) collide into each other and bond together by chemical forces.

In order for the particles to be bonded together, the force of collision must be great enough to overcome the initial repulsive forces (the activation energy of the reaction).

We can use the collision theory to explain the effect of temperature and concentration on the rate of reaction.

Pow!Pow! CompoundCompoundformedformed


How concentration affects the speed of reactions At higher concentration, the number of reacting particles

increases. The reacting particles are more crowded and there will be

a greater chance for them to meet, therefore resulting in more collisions.


Some everyday applications of the speed of reactions

When cooking food, we cut them into smaller pieces and use a higher temperature to make the food cook faster.

To slow down the process of decay, food is kept at a low temperature in a refrigerator.

To make certain medicines work faster, they are often taken in powder form and with warm water.

Precaution must be taken in coal mines and flour mills to prevent explosions due to the fine coal or flour dust particles.


Quick check 21. State 3 factors which affect the rate of reactions.2. The graph below shows the results of an experiment done to compare the

rate of reaction between marble chips and marble powder with dilute hydrochloric acid.

Graph A

Graph B

Vol o

f CO

2

(a) Which graph shows the reaction between the acid and (i) marble chips, (ii) marble powder?

(b) Which graph shows that the rate of reaction is faster? Explain why.

(c) At what time does the reaction between the marble chips and the acid stop?

(d) State one variable that must be kept constant when carrying out the experiment.

0 1 2 3 4 5 6 7 8 Time/ min

Solution


3. The following table shows the results of an experiment done to compare the effect of concentration of sulphuric acid on magnesium.

Test tube No. 1 2 3 4 5

Volume of HCl/ cm3 50 40 30 20 10

Volume of H2O/ cm3 0 10 20 30 40

Total volume/ cm3 50 50 50 50 50

Time taken/ s 10 12 18 25 50

(a) Why are different volumes of water added to each test tube of acid?

(b) In which test tube is the concentration of the acid most concentrated?

(c) In which test tube is the concentration of the acid least concentrated?

(d) Plot a graph of the time taken for the magnesium to dissolve with the volume of the acid used.

(e) What conclusion can you get from your graph? Solution


1. Concentration of reactants, temperature and particle size.

2. (a) (i) Marble chips: graph B, (ii) Marble powder: graph A

(b) Graph A is faster because it has a steeper gradient.

(c) 8 minutes after the start of the reaction.

(d) Concentration of the acid/ Temperature/ Mass of the calcium carbonate.

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(a) To make the total volume of each acid solution equal to 50 cm3.

(b) Most concentrated - Test tube 1

(c) Least concentrated - Test tube 5

(d) A curve with decreasing gradient is obtained.

(e) The speed of the reaction decreases as the concentration of the acid is decreased.

Graph of Vol. of acid vs Time taken

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Time/s

Vo

l. o

f A

cid

/cm

3

3.

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What is a catalyst? A catalyst is a substance which changes the speed of

a chemical reaction, but is itself chemically unchanged at the end of the reaction.

Catalysts are very important for making slow chemical reactions go faster.


A catalyst works by one or both ways:

1. It provides an alternative reaction pathway with lower activation energy. More particles are able to react because of the lower activation energy required.

2. A catalyst (often in finely divided form) provides a large surface area for the reactants to adsorb and brings them into close contact with one another.

How does a catalyst work?


The chemical industry depends on catalysts for many of the industrial processes.

Examples are: Manufacture of ammonia: iron catalyst; Manufacture of sulphuric acid: vanadium(V) oxide; Manufacture of margarine: nickel catalyst; Catalytic converter in motorcars: platinum.

Importance of catalysts


Enzymes Enzymes are biological catalysts found in plants and

animals. They are mainly made up of proteins. The enzymes in our bodies enable us to carry out our

bodily functions such as digestion of food and absorption of nutrients.

E.g. the enzyme amylase catalyses the conversion of starch that we eat into sugars;

Enzymes present in yeast are used in the making of bread and wine.


Catalytic decomposition of hydrogen peroxide

We can show the effect of a catalyston the speed of a chemical reactionby carrying out the experiment asshown in the diagram.

Hydrogen peroxide decomposes rapidly when a little manganese(IV) oxide is added as a catalyst. 2H2O2 2H2O + O2

This process is usually used in the preparation of oxygen in the laboratory.

H2O2 + MnO2

oxygen


Quick check 31. What is a catalyst? Give an example

of the use of a catalyst in a particular chemical reaction.

2. The graph shows the catalytic decomposition of hydrogen peroxide.

2H2O2 2H2O + O2 (a) Which reaction is faster? State two ways how you can make the reaction faster.

(b) What is the total volume of oxygen produced? Calculate the mass of hydrogen peroxide decomposed.

Solution


1. A catalyst is a substance which changes the speed of a chemical reaction, but is itself unchanged after the reaction. E.g. iron in the manufacture of ammonia in the Haber process.

2.

(a) Reaction A is faster. (i) Use manganese(IV) oxide as catalyst, (ii) Heat the reacting mixture.

(b) Total volume of oxygen produced = 48 cm3 (0.002 mol)

Mass of hydrogen peroxide = 0.002 mol x 2 x 34 g mol-1 = 0.136 g



1. http://www.nelsonthornes.com/secondary/science/scinet/scinet/reaction/rates/content.htm

2. http://www.jghs.edin.sch.uk/mathscience/chemistrynotes/topic2.html

To Learn more about Speed of Chemical Reactions, click on the links below!


References

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver

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C14 rates of reactions

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Transcript of C14 rates of reactions