Transcript Measuring rate of reaction

Rates of
Reaction and
Collision Theory
Objectives:
• To explain factors that affect the rate of
chemical reactions.
• To explain the collision theory and what
happens to cause a chemical reaction.
• To carry out practicals and calculate the rate
of reactions.
Chemical Reactions
A reaction occurs when particles
collide with each other.
We can tell that a reaction has
occurred by looking for the following:
• a change in mass
• a colour change
• energy released (exothermic reaction)
• bubbles or a gas being released
• a new compound produced
Rates of reaction
Chemical reactions occur when particles of reactant
collide with enough energy to react.
Rates of Reaction
Chemical reactions occur when different
atoms or molecules collide:
For the reaction to happen the particles must have a
certain amount of energy – this is called the
ACTIVATION ENERGY.
The rate (speed) of a reaction happens depends on four things:
1) The temperature of the reactants,
2) Their concentration
3) Their surface area
4) Whether or not a catalyst is used
Activation energy
Chemical reactions involve the formation of bonds
between atoms but often before new bonds can be
Activation
formed oldEnergy
ones have
is to be broken.
energy
given out as
needed to
new
bonds
This means that there has to be enough
energy
break
form
(activation energy) to start breakingexisting
the old bonds
bonds
before a reaction can occur.
Reactants
Old bonds
start to
break
New bonds
form
Changing the rate of a reaction..
4 factors that affect the rate of reaction are:
• Surface area
• Concentration
• Temperature
• Catalyst
Chemical reactions can be speeded up by
increasing the temperature, surface area or
the concentration of the the reactants.
Catalysts also speed up chemical reactions. However
at the end of a reaction the catalyst has not
chemically changed, it just speeds up the reaction.
Surface area
The reactions of solids can clearly only take place at the
surface of the solid.
If we break a solid into smaller pieces we get more area
and a faster reaction.
Molecules collide with the
surface of the solid
Extra surface for molecules
to collide with.
Surface area
If we grind up a solid to a powder we massively increase
the surface area.
We therefore massively increase the rate of any reaction
Slow
Very fast
Concentration
Reactions in solution involve dissolved particles that must
collide before reaction is possible.
The more crowded (concentrated) the solution, the faster
the reaction.
Collisions infrequent
Collisions frequent
Temperature
At higher temperatures molecules move faster.
As a result there are more collisions per second and so a
faster reaction occurs.
Slow molecules are also less likely to lead to a reaction than
fast ones.
More collisions per second
Fewer collisions per second
Catalysts
A catalyst is a substance that speeds up a
reaction without getting used up in the process.
For example:
In the present of Nickel
catalyst, vegetable oil and
hydrogen bond together to
make margarine !!
• Catalysts are used over and over again, they may
need cleaning but are never used up.
• Different reactions need different catalysts.
Measuring the rate of reaction?
There are several ways to measure the rate of
reaction, these include:
• collect the amount of gas released over a
period of time.
• record the colour change or precipitate
produced over a period of time.
• record a change in mass over a period of time
So, you have to measure the amount of reactant used up
or the amount of product produced per unit of time.
Measuring rate of reaction
Two common ways:
1) Measure how fast the
products are formed
2) Measure how fast the
reactants are used up
Amount of
product formed
Rate of reaction graph
Fast rate
of reaction
here
Slower rate of reaction here
due to reactants being used up
Slower reaction
Time
Rates and Graphs
Amount of product
These show the increasing amount of product or
the decreasing amount of reactant.
Shallow gradient
Slow reaction
Steep gradient
Fast reaction
Time
Acid and metal
Reactive metals (e.g. Magnesium) react with acid
to make hydrogen gas.
magnesium + hydrochloric
acid
Mg(s)
 magnesium
chloride
+ 2HCl(aq)  MgCl2(aq)
+ hydrogen
+ H2(g)
As the gas given off leaves the flask
the total mass of the flask and its
contents decreases slightly.
Readings of the mass (g) can be
taken. Usually at 1 minute intervals.
HCl
Mg
11.71
11.72
11.73
11.74
11.77
11.80
Acid and metal
1.
Measure the agreed
volume of acid / water into
the conical flask.
Cotton
wool
HCl
2. Have a loose plug of cotton
wool to prevent “spitting”
of droplets of liquid.
3. Have a piece of magnesium
of known mass ready.
4. Add the magnesium, place
the cotton wool in the neck
and start taking mass
readings immediately.
Mg
11.71
11.72
11.73
11.74
11.77
11.80
Time (s)
Reading (cm3)
0
0
60
120
Acid and marble
Marble chips are calcium carbonate.
They react with acid to make a gas (carbon dioxide)
calcium
+ hydrochloric
carbonate
acid
 calcium + water
chloride
+ carbon
dioxide
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
Glass tube
The gas given off can be
collected in a syringe and
readings taken every 30
seconds or so.
Gas syringe
Hydrochloric acid
Marble chips
Acid and marble
1.
Measure the agreed mass
of marble chips.
2. Set up the syringe, flask
and connector
3. Measure the acid /water.
4. Add the marble chips and
quickly insert the bung
and start stop clock.
5. Take syringe readings at
30 second intervals.
Time (s)
0
Reading (cm3)
0
Science Coursework
Investigating how
concentration affects the
rate of reaction between
Hydrochloric Acid and
Marble Chips.
PLANNING
Aim – To find out how changing the concentration of acid
affects the rate of reaction with Calcium Carbonate.
Preliminary Work – Write up any extra work you have done
that has helped you plan your work. You must include a
results table and graph to show how it helped your plan.
HINT:
The data collected needs to be spread out to allow you to
spot patterns. Reactions that are too quick will be no good.
Preliminary work
Volume of gas released (cm3)
Time (s)
0.5mol
T1
T2
1.0mol
T1
T2
1.5mol
T1
T2
2.0mol
T1
T2
0
20
40
60
80
100
120
Plot a graph, decide which concentrations you can use to
make a range and be able to explain why ...
Volume of Gas (ml)
PLOTTING YOUR GRAPH
Time (seconds)
Apparatus and Diagram
• conical flask
• 25ml of Hydrochloric Acid (quote
what concentrations you will use)
• goggles
• delivery tube
• tub of water
• bung
• 2 or 3g small marble chips YOU
DECIDE
• measuring cylinder
• digital balance
• stop watch
The important parts of your “Strategy” planning
marks are now for justifying and explaining your
choice of apparatus.
Make any relevant comments here about how your
preliminary work helped you make any decisions.
Fair Test
There are 4 factors that can affect the rate of reaction.
These are : 1)
You must explain how and why
2)
you are controlling 3 of these
3)
factors. Detailed explanations
required!!
4)
In this investigation I am only going to change the
____________________. I must control the other factors.
I will do this by using the __________ marble chips and using
the same equipment for each test.
I will also ________________ and work out the averages.
Safety – How will you make sure the work is done safely ?
I have to make sure that I work safely, I will .........
Method
Write a fully detailed list of instructions.
You must include:
• Instructions how to do the work
• How long each test will last for
• How often will you take your results
• What concentrations of acid to use and how
many repeats to do
RESULTS
Test 1 with 0.2m Hydrochloric Acid
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
Test 1
Test 2
Test 3
Average
RESULTS
Test 2 with 0.4m Hydrochloric Acid
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
Test 1
Test 2
Test 3
Average
RESULTS
Test 3 with 0.6m Hydrochloric Acid
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
Test 1
Test 2
Test 3
Average
RESULTS
Test 4 with 0.8m Hydrochloric Acid
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
Test 1
Test 2
Test 3
Average
RESULTS
Test 5 with 1.0m Hydrochloric Acid
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
Test 1
Test 2
Test 3
Average
AVERAGE RESULTS
Volume of gas released (ml)
Time (s)
0
20
40
60
80
100
0.2m
0.4m
0.6m
0.8m
1.0m
Volume of Gas (ml)
PLOTTING YOUR GRAPH
Time (seconds)
ANALYSIS
By looking at my graph I can see that as concentration
increased the line got steeper. This means the
reaction was happening quicker. The quickest reaction
was with the 1.25m acid, this had the steepest line.
The concentrated acid reacted the quickest because it
has more acid particles in the same volume. This meant
there were more successful collisions with the marble
chips in the flask.
Acid Particles
Marble Chips
I can use my graph to work out the rate of reaction for
each acid I used.
Rate of reaction = Amount of gas
(ml/s)
(ml)
Time taken
(s)
Work out the rate of reaction for each concentration of acid
for the first 60 seconds. Show your working out.
Is there any relationship between your results?
Explain if your results support the collision theory, if
you double the concentration does the rate of
reaction soluble too ?
Was your prediction correct ? Explain what you said and why it is
right or wrong.
EVALUATION
In this work I have found out that increasing the
concentration of the acid increases the rate of a reaction.
On my graph there are _______ anomalous incorrect
results. I have circled them and not included them in line of
best fit.
My results can be made more reliable by finding a way of
controlling the surface area of the marble chips. Because
they were not exactly the same the test was not 100% fair
and accurate. I also need to do all my results in the same
lesson or day to ensure that the working room temperature
is the same.