Download Speed of reactions

Speed of reactions
Chemical reactions in the lab do not all occur at the same speed. Some are over in
a fraction of a second (fast), some take a few minutes (medium) and others take
days or even years (slow). Everyday reactions are no different.
Carry out these experiments to find out about the speed of
different reactions.
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Copy and complete the table to show your observations and conclusions.
Substances reacting
Observations
Reaction speed
magnesium/water
barium chloride solution/
sodium sulphate solution
magnesium/
dilute hydrochloric acid
Copy and complete the following table.
Use a ! to show the rate of the reaction.
Reaction rate
Chemical reaction
Slow
food rotting
a firework exploding
bread toasting
formation of oil
iron rusting
a cake baking
a gas explosion
coal burning
Medium
Fast
Changing the speed: the effect of particle size
As the particle size decreases, the surface area of reactant particles increases.
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What is the effect of decreasing particle size on the speed of a
chemical reaction? You will have planned an investigation to find this
out. Now carry out the experiment.
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What happens to the surface area as the particle size decreases?
Write a brief report about the investigation that you planned and carried out.
What was done to make the investigation fair?
In what way does decreasing the particle size of a reactant change the reaction
speed?
Which reacts faster with dilute acid?
chalk powder
or
chalk lumps
Changing the speed: the effect of concentration
A solution is formed when a substance dissolves in water.
A concentrated solution has a lot of the dissolved substance in a certain volume
of water
. think of concentrated orange juice. Adding water dilutes the solution.
dilution
concentrated
solution
dilute solution
What is the effect of decreasing the concentration on the speed of
a chemical reaction? You will have planned an investigation to find
this out. Now carry out the experiment.
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What is the difference between a concentrated and a dilute solution?
Write a brief report about the investigation that you planned and carried out.
What was done to make the investigation fair?
In what way does decreasing the concentration of a reactant change the
reaction speed?
Which reacts faster with magnesium ribbon?
dilute acid
or
concentrated acid
Changing the speed: the effect of temperature
What is the effect of increasing the temperature on the speed of
a chemical reaction? You will have planned an investigation to find
this out. Now carry out the experiment.
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Write a brief report about the investigation that you planned and carried out.
What was done to make the investigation fair?
In what way does increasing the temperature of a reactant change the reaction
speed?
Which reacts faster with oxygen?
magnesium at room temperature
or
magnesium at 400 oC
Changing the speed: everyday examples
Decreasing the particle size of a reactant increases the speed of a chemical
reaction.
Decreasing the concentration of a reactant decreases the speed of a chemical
reaction.
Increasing the temperature of a reactant increases the speed of a chemical
reaction.
Give the answer with a reason for each of the following questions.
The first one has been done for you.
(a) If you had stomach ache which would act faster?
indigestion powder
or
indigestion tablets
Indigestion powder because the particle size is smaller.
(b) Which would cook slower?
small pieces of carrot
or
large pieces of carrot
(c) Which would cook faster?
potatoes in water at 75 oC
or
in water at 100 oC
or
logs
or
plants outside in the open air
(d)
Which burn slower?
small sticks
(e) Which grow more quickly?
plants in a green-house
(f)
Which kills germs more slowly?
bleach straight from the bottle or
bleach diluted with water
Making reactions go faster
A catalyst is a substance that speeds up a chemical reaction.
Catalysts have everyday uses, e.g. in a catalytic converter a catalyst is used to
speed up the conversion of harmful gases to harmless gases in a car exhaust.
Catalysts are widely used in the chemical industry.
reaction makes the production more profitable.
The increased speed of
What is the purpose of a catalyst in a chemical reaction?
Carry out this experiment to find out.
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What is a catalyst?!
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Give an everyday use of a catalyst.
*
Why are catalysts used in industry?
Write a brief report about the experiment that you carried out.
In your report mention the name of the catalyst that you used and what you
found out.
More about catalysts
In order to speed up a reaction a catalyst has to take part in the reaction; they
provide an easy route from reactants to products.
However, we can show that the amount of catalyst at the end of the reaction is the
same as at the start, i.e. the catalyst is not used up in the reaction.
If need be, the catalyst can be recovered chemically unchanged at the end of
reaction.
Your teacher will demonstrate an experiment to find out more
about catalysts.
*
Does the catalyst take part in the reaction?
*
Is the catalyst used up in the reaction?
*
Is the catalyst chemically changed in the reaction?
*
Can the catalyst be recovered at the end of the reaction?
What is the name of the catalyst?
What is the colour?
What is the first colour change?
What does this show?
What is the second colour change?
What does this show?
Enzymes
Enzymes are biological catalysts.
They speed up the reactions that take place in the living cells of plants and animals.
What is the purpose of an enzyme in a chemical reaction?
Carry out this experiment to find out.
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What is an enzyme and what does it do?
Write a brief report about the experiment that you carried out.
In your report include what you found out.
Catalase: an enzyme
Vegetables contain an enzyme called catalase.
Hydrogen peroxide solution breaks up slowly to give off a gas. Catalase speeds up
this reaction.
You will have planned an investigation to find out about catalase in
vegetables. Now carry out the experiment.
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Write a brief report of the experiment that you carried out.
Why were four test tubes used but only three vegetables?
Which vegetable contained the most catalase?
How were you able to tell?
Suggest why this experiment only gives a rough comparison of the catalase in
vegetables.
Everyday uses of enzymes
As well as being involved in reactions in the human body, enzymes are used in
industry.
Read the following information.
Draw a diagram to show the information.
The proteins in the food we eat contain large
molecules and before we can make use of them, we
must break them down into much smaller molecules
called amino acids.
The breakdown starts in the stomach where the
enzymes, pepsin and trypsin convert the proteins into
peptides. The peptides move into the small intestine
and using enzymes called peptidases break down into
small amino acids.
Read the information in the following passage.
Draw a table with TWO headings to show the uses of enzymes.
Cellulose is used to make jeans look more faded.
Yeast contains zymase which catalyses the
fermentation of sugars to alcohol.
Protease is added to detergent powders to assist the
removal of stains from clothes.
To make meat more tender papain is added.
Measuring speed of reactions
As a chemical reaction proceeds, reactants are being used up while products are
being formed. The rate at which this happens can be followed by measuring the
change in a property of a substance involved in the reaction over a period of time,
e.g. the decrease in mass (due to gaseous product being given off) can be
measured using a balance; the volume of gas produced can be collected using a
measuring cylinder or a syringe.
What happens to the speed of a reaction as the reaction proceeds?
Follow the instructions of your teacher to find out.
Draw a table to show how the mass of reactants or the volume of gas produced
changes with time for the reaction studied.
Use the results to plot a graph.
What happens to the speed of a reaction as the reaction proceeds?
What are elements made up of?
Everything that exists is made up of atoms (from the Greek word atmos meaning
indivisible). An element is a substance that is made up of atoms of only one kind.
A silver ring contains millions of silver atoms which are all the same. Every atom in
an iron nail is an atom of iron, and all atoms of iron are the same
but an iron
atom is different in size and mass from a silver atom.
iron nails
silver ring
When scientists make new elements they make new kinds of atoms.
Each different atom has a number . This is called the atomic number. All atoms of
the one element have the same atomic number. Atoms of different elements
cannot have the same atomic number. An element can be defined as a collection of
atoms all with the same atomic number.
Elements are arranged in the Periodic Table in order of increasing atomic numbers.
A new period begins when an element has similar chemical properties to the other
elements in Group 1.
*
What is meant by an element?
Copy and complete the following table?
Element
Atomic number
magnesium
chlorine
iron
19
53
82
The mass of atoms
A single atom is so small that it cannot be weighed on a balance. There has to be
a special scale to measure the mass of something so light. The mass of an atom is
measured on the relative atomic mass scale. Since this is a relative scale, it
has no units. The relative atomic mass for some elements is shown on page
of
the Data Booklet.
*
What is the scale used to measure the mass of an atom?
*
Why does this scale not have units?
Copy and complete the following table?
Element
Relative atomic mass
2 versions of this
page!!
carbon
sulphur
copper
iodine
magnesium
plutonium
Copy and complete the following table?
Element
Which table
Relative atomic mass
magnesium
chlorine
iron
19
53
82
What are atoms made up of?
Atoms themselves consist of even smaller particles (sub-atomic particles). The way
in which the sub-atomic particles are arranged is referred to as the structure of the
atom.
At the centre of the atom is a very small core called the
nucleus. The nucleus is very small compared to the
size of the rest of the atom; if atoms were magnified to
the size of a football park, then the nucleus would be
about the size of a pin-head.
Protons are found in the nucleus. Protons have a
positive charge. This gives a positive charge to the nucleus.
Electrons move outside of the nucleus.
Electrons have a negative charge, equal and opposite to that of protons.
Atoms are overall neutral, i.e. they are neither positive nor negative.
This means that the total positive charge of the protons in the nucleus is equal to
the total negative charge of the electrons, i.e. the positive and negative charges
cancel out.
Neutrons are also found in the nucleus. Neutrons do not have a charge, i.e. they
are neutral. The neutrons allow the positive protons to pack together in a very small
space.
Compared to protons and neutrons, electrons have a negligible mass.
*
What is meant by the nucleus of an atom?
*
Copy and complete the following table.
Charge
Where found
Proton
Neutron
Electron
*
Why are atoms electrically uncharged?
*
Which of the three sub-atomic particles has negligible mass?
More about the atomic number
Each element in the Periodic Table has its own atomic number
this gives the
number of protons in an atom of an element. Since atoms are neutral and the
charge on an electron is equal and opposite to the charge on a proton, the atomic
number also gives the number of electrons in an atom.
An element can be defined as a collection of atoms all with the same atomic
number or the same number of protons.
*
What information is given by the atomic number of an element?
Copy and complete the following table.
Element
Atomic
number
Number of
protons
Number of
electrons
carbon
hydrogen
magnesium
7
20
11
Electron arrangement
Most of the atom is empty space. Electrons move through this space. The
electrons do not however move I n a haphazard fashion; they are arranged in a
particular way.
Electrons are arranged in shells (energy levels). There is a limit to the
number of electrons each shell can hold.
The first shell (nearest the nucleus) can hold 2 electrons.
The second shell can hold 8 electrons.
The third shell can hold 8 electrons
(for the first twenty elements).
Electrons always enter the shell in which there is space. This is the shell nearest
the nucleus, e.g.
lithium (atomic number 3)
has 3 electrons arranged 2, 1
chlorine (atomic number 17)
has 17 electrons arranged 2, 8, 7
All the atoms of elements in the one group have the same number of electrons in
the outer shell, e.g. 1 electron for all the atoms of the alkali metals.
The chemical reactions of an element depend on the number of electrons in the
outer shell. This is the reason for all the elements in the one group having similar
chemical reactions, e.g. all the alkali metals are stored under oil because they are
very reactive.
The electron arrangement of atoms of all the elements is given on page 6 of the
Data Booklet.
Your teacher will demonstrate the reaction of the alkali metals
with water.
Briefly describe the reaction of the alkali metals with water.
Why do these elements show similar chemical reactions?
Draw diagrams to show how the electrons are arranged in shells (energy
levels) in each of the following atoms.
(a)
hydrogen (atomic number 1)
(b)
boron (atomic number 5)
(c)
sulphur (atomic number 16)
(d)
potassium (atomic number 19)
Copy and complete the table below.
Element
Atomic number
Electron arrangement
carbon
silicon
helium
7
15
2, 7
2, 8, 8