Download Revision C3

C3 revision
Easter holidays
Rates of reaction
Rates of reaction
• The rate of a reaction is the speed in which reactants
are turned into products
• There are 4 ways you need to know about – what are
they?
1.
2.
3.
4.
Temperature
Surface area
Concentration
Using a catalyst
Surface area example
Giant statue
4 mini statues
They are made out of the same amount of limestone
Concentration example
Which box has the highest concentration of acid particles?
= water particle
= acid particle
A
B
C
Match the correct boxes
High
concentration
0.2 M
acid
Medium
concentration
0.5 M
acid
Low
concentration
1M
acid
Extension – write word definitions of high and low concentration
Which 3 boxes would you choose to
make the rate of reaction the fastest?
High
temperature
Low surface
area
Medium
concentration
Medium
temperature
High surface
area
Low
concentration
Low
temperature
Medium
surface area
High
concentration
Helpful video
• http://www.youtube.com/watch?v=OttRV5yk
P7A
Rate of reaction
The particles of the
reactants need to
get together so
that they can react
How does that work then?
Reactant particles
collide
REACTION
Product particles
formed
Some reactants have collision
but with no effect
Reactant particles
collide
Product particles
not formed as there is
not enough energy
Activation energy
• Reactions only
happen if the
particles have
enough energy
• The minimum amount
of energy needed to
start a reaction is
called the Activation
energy
• The amount of
activation energy
needed is different
for each reaction
But
• Every reaction has
activation energy, they
all need a little push to
get started.
More than just activation
energy
O
O
O
C
C
O
More frequent successful collisions per second 
more product
The Collision Theory
• Particles are constantly moving
• For a chemical reaction to take place the
reactant particles must collide first
• For the collision to be effective the particles
must have the right amount of activation energy
• More collisions per second means that more
product is made so the reaction is faster
Concentration
• More particles in the same space means more collisions
• More successful collisions means more effective
collisions
• If we double the concentration we double the collisions
per second
Temperature
• Particles turn heat energy into kinetic energy
• When they get hotter they move faster
• When they move faster they collide more often
• More collisions per second means faster rate of reaction
Catalysts
Activation energy
with a catalyst
• Catalysts reduce the activation energy needed for a
reaction
• They do this by offering an alternate route for the
reaction to take
• Less activation energy means more effective collisions
• More frequent successful collisions per second means
faster rate
Surface Area/Particle Size
• Using smaller particles increases rate
• Increase in surface area allows more collisions at
surface
• More frequent successful collisions per second means
faster rate
What is the effect of reducing the concentration?
What is the effect of increasing concentration?
What is the effect increasing surface area?
What is the effect increasing the temperature?
What is the effect increasing the concentration?
Things to remember with rate
graphs
• When graph plateaus the reaction is finished
• Faster reaction will be the line at a steeper gradient
• Reactions will ALWAYS stop because they will run out of
reactants (limiting factor): could be one reactant or both
• We draw line of best fits on graphs so that we can
– Extrapolate – make an estimate outside out results
– Interpolate – make an estimate that is between our results
Calculating rate of reaction from graphs
How can the rate of reaction be calculated from a graph?
70
hydrogen produced (cm3)
60
x
50
rate of reaction =
40
y
x
30
y
20
10
0
0
10
20
30
40
50
time (seconds)
The gradient of the graph is equal to the initial rate of reaction at that time
rate of reaction =
45 cm3
20 s
rate of reaction =
2.25 cm3/s
Calculations
Balancing equations
• Balance the below equations:
1. Mg + O2  MgO
2. AgNO3 + MgCl2  AgCl + Mg(NO3)2
ANSWERS
1. 2Mg + O2  2MgO
2. 2AgNO3 + MgCl2  2AgCl + Mg(NO3)2
Balanced Equations
• They tell us how much of each substance is involved
in a chemical reaction
2H2 + O2  2H2O
2 parts of hydrogen react with 1 part of oxygen to make
2 part of water
• This is useful because now we can use it to work out
what mass of hydrogen and oxygen we need and how
much water is made
2H2 + O2  2H2O
Masses in the equation
• To find out the masses in the equation, all we
need to do is add up the masses of each
individual compound in the equation…
• 2 parts of hydrogen = 2x 2g = 4g
• 1 part of oxygen = 1 x 16g = 16g
• 2 part of water
= (2 x 1g)+ (2 x 16g) = 36g
Parts to moles
• When we have mentioned ‘parts’, in chemistry what we
actually call it is moles
• 1 mole is the relative molecular mass of the compound
• Using your periodic table work out the RMM of the
following:
H2
CH4
O2
H2O
MgSO4
CuS
Activity
• Balance the equations below
• Write the number of moles of each we have
• Work out the RMM of each component
1. Mg + HCl  MgCl2 + H2
2. NaOH + Cl2  NaOCl + NaCl + H2O
SUPPORT
Grab a mini whiteboard
EXTENSION
Write what amount of MgCl2
we would make from 10g of
Mg
Answers
1. Mg + 2HCl  MgCl2 + H2
1 mole of magnesium reacts with 2 moles of hydrochloric acid to produce 1
mole of magnesium chloride and 1 mole of hydrogen.
1 mole of Magnesium = 24g
2 moles of hydrochloric acid = 73g
1 mole of magnesium chloride = 95g
1 mole of hydrogen = 2g
2. 2NaOH + Cl2  NaOCl + NaCl + H2O
2 moles of sodium hydroxide reacts with 1 mole of chlorine to produce 1 mole
of sodium hypochlorite, 1 mole of sodium chloride and 1 mole of water
2 moles of sodium hydroxide = 80g
1 mole of chlorine= 71g
1 mole of sodium hypochloite = 74.5g
1 mole of sodium chloride = 58.5g
1 mole of water = 18g
Conservation of mass
• What ever is used at the start of a reaction
(reactant), has to be there at the end of the reaction
(product)
• This is why we have to balance equation and why we
can work out reacting masses
Some examples of questions
1. Zinc oxide (ZnO) is heated with carbon (C) to make zinc
(Zn) and carbon monoxide (CO). How much zinc oxide do
you need to make 130 tonnes of zinc?
162 tonnes
2. A student adds 4.8g of magnesium (Mg) to excess dilute
hydrochloric acid (HCl). Hydrogen is made but what mass
of magnesium chloride (MgCl2) would be made?
19g
3. If you add 5.5g of sodium carbonate (Na2CO3) to excess
dilute sulphuric acid(H2SO4), what mass of sodium
sulphate (NaSO4) would be made? Water and carbon
dioxide is also made.
6.1g (Does not specify rounding/SF)
Worked answers
Percentage yield and atom
economy
Percentage Yield
• Percentage yield is a way of comparing the amount
of product made (actual yield) to the amount of
product expected to be made (predicted yield).
• You can calculate percentage yield by using this
formula:
Actual yield
Predicted yield
x 100
Percentage Yield Example
• A company making sulphuric acid gets an actual
yield of 74 tonnes. They predicted a yield of 85
tonnes. What is the percentage yield?
Actual yield
Predicted yield
74
85
x 100
= 87%
Percentage yield
Fred reacted potassium iodide with lead nitrate. He filtered
the solution to collect the products. He expected to make
20g but only made 18g.
1.
2.
3.
4.
5.
What was his expected yield?
What was his actual yield?
What was his percentage yield?
Suggest a reason for this loss of product
What mass of product would he have made if his
percentage yield was 72%?
Percentage Yield in Industry
• There are several reasons why the percentage yield is
less than the expected yield.
• The products could be lost in evaporation, filtration or
during the transfer of liquids.
• Not all reactants may have been used to make the
products.
• In industrial processes a high percentage yield is desired
• This reduces waste and therefore cost.
Atom economy
• Way of measuring the amount of waste in a reaction
– measures loss of atoms
• Inefficient (wasteful) products have a low atom
economy
• Efficient processes have a high atom economy
Atom Economy (%) = mass of desired product
x 100
total mass of products
Example
CH4(g) + H2O(g)  CO(g) + 3H2(g)
Hydrogen is needed for the hydrogenation of oils to make
margarine. Without the hydrogen, the margarine would not be solid.
What is the atom economy for producing hydrogen from reacting
natural gas (methane) with steam?
Step 1 – work out the
Ar – 12 + (4 x 1)
relative atomic mass and
Mr –
16
relative molecular mass.
Step 2 – work out the
mass of products and
mass of desired products.
Step 3 – calculate the
atom economy using the
formula.
(2 x 1) + 16 12 + 16 3 x (2 x 1)
18
28
Total mass of products = 34
Mass of desired product (H2) = 6
6
x 100 = 17.6%
34
6
Atom economy
• Every industry wants a high atom economy and a
high percentage yield
• They want to make as much of the desired product
as possibly and get as little waste as possible
• Reactions with low atom economy and % yield are
inefficient and cost money
Processes to make chemicals
• Chemicals are made through either batch or
continuous processes
• Batch – made to a certain recipe and many
different types. Chemical/drug is made in batches
and in small amounts
• Continuous – always the same recipe, huge
amounts made on a continuous production 24/7
Exothermic and endothermic
reactions
Endo and exothermic reactions
• Exothermic – heat is given out from the reaction
(exit)
• Endothermic – heat is taken in to the reaction (enter)
• Both reactions involve a temperature change which
can be measure with a thermometer
Endo and exothermic reactions
• When bonds break – energy is taken in (endo)
• When bonds are made – energy is given out (exo)
• What would a graph look like to show this? (think back
to activation energy graphs when we were discussing
catalysts)
Energy level diagrams
What type of reaction is shown on this graph?
Energy level diagrams
This is an endothermic reaction, energy is taken in from the
environment around the reaction (reactants lower than products)
Reaction graph
Macromolecules
Graphite and diamond
• Graphite and diamond are macromolecules
• You need to know about their structure and their
bonding
Graphite
Lustrous/black/opaque
Soft and slippery
Very high melting point
Insoluble in water
Conducts electricity
Diamond
Lustrous/shiny/transparent/co
lourless
Very hard
Very high melting point
Insoluble in water
Does not conduct electricity
Diamond vs Graphite
• Both are made only from carbon
• Carbon is very good at bonding to itself and ideally
each carbon atom wants to make 4 bonds (how do
we know this?)
• Both are allotropes of carbon – elements in different
forms
Diamond
• Diamond is formed when
each carbon forms 4 bonds
each to another carbon
• The structure around each
atom is tetrahedral (4
sided) like a triangular
based pyramid
Giant covalent
structure
• In 3D the bonds are as far
apart from each other as
possible
• This gives diamonds a regular continuous structure
and because each bond within it is very strong
• Diamond is the hardest naturally occurring
substance, and used in drills and cutting tools
• It also means that the melting point is extremely
high (about 38000C) as all the bonds need to be
broken for the solid to melt
• There are NO free electrons in diamond so it does
not conduct electricity
Graphite
• Graphite differs from
carbon because each
carbon atom only bonds
to three others and not
the usual four
•The hexagons form into
flat sheets and there are
weak interactions
between each sheet
• This means that there is
an unbonded electron so
graphite conducts
electricity
Giant covalent
structure
• Because of the strong bonds between the atoms,
graphite also has a high melting point but it is not
just as high as diamond’s
• It is a relatively hard structure (because of the
strong C-C bonds) but the interactions between
each sheet are weak and so the layers can slide
over each other
• In addition, the extra electron is free to move and
so graphite can conduct electricity
Buckminster Fullerene
• Buckminster Fullerene –
the bucky ball - is C60
• It is a sphere of carbon
atoms bonded like the
patches on a football
Giant covalent
structure
• Each carbon has three
bonds, and some are in
hexagons and others in
pentagons
• These have free electrons like graphite so conduct
electricity
• Scientists have managed to join many bucky balls
together to make a bucky tube – used in sport
equipment to reinforce
• They are also used to trap/carry molecule.
Researchers are looking into the being used to
deliver drugs around the body
• Also used as industrial catalysts