Rate of Reaction
Rate of reaction
Rate of reaction measures
How quickly a reactant is used up in a reaction
How quickly a product forms in a reaction
OR
Factors and Collision Theory
Collision theory
There are four factors that can affect the speed of a reaction
temperature
concentration
surface area of a solid
addition of a catalyst
You need to be able to explain why these factors affect the rate of a reaction using COLLISION THEORY
COLLISION THEORY
FOR A REACTION TO TAKE PLACE BETWEEN MOLECULES, THE MOLECULES MUST COLLIDE WITH SUFFICIENT ENERGY FOR THE BONDS OF THE REACTANT MOLECULES TO BE BROKEN SO THAT THE NEW BONDS OF THE PRODUCT MOLECULES CAN FORM
The next few slides explore this
Concentration/Pressure
Low concentration of
High concentration of
PARTICLES PER UNIT VOLUME
Collisions per second
Temperature�
Raising temperature will always increase the rate of a chemical reaction
IT MUST INCREASE THE RATE OF COLLISIONS BETWEEN REACTANT MOLECULES
When temperature increases the particles gain more kinetic energy meaning the reactant particles move faster, resulting in more collisions per second
“Increasing temperature increases the kinetic energy of the reactant molecules resulting in an increase in the number of collisions between them”
Successful or Unsuccessful Collision
Surface area
The red particle will only be able to react with particles at the surface of the solid
The particles in the red circle below will not be able to react as they are hidden below the surface of the solid, the red particles will be unable to collide with them.
To increase the surface area we need to break the solid up
Now every particle in the solid can be collided with, including the ones in the red circle
The rate increases
By the way, chopping things into smaller pieces increases their surface area. Powders have the biggest surface area of all!!
Catalysts
First of all what is a catalyst?
A SUBSTANCE THAT SPEEDS UP A CHEMICAL REACTION WITHOUT UNDERGOING ANY CHEMICAL CHANGE ITSELF
THIS MEANS CATALYSTS DO NOT GET CONSUMED (USED UP IN CHEMICAL REACTIONS)
How do catalysts speed up reactions
HIGHER ONLY
Unlike with temperature/surface area/concentration, they DO NOT increase the number of collisions
What catalysts do is lower the energy needed to break the bonds in the reactants molecules. Which means that collisions between reactant molecules are more likely to be successful
Industry
Studying Rate
Looking at a reaction over time
There are various ways of studying chemical reaction. Remember, you will be measuring how quickly something gets used up or how quickly something is made
Take the reaction between magnesium and hydrochloric acid
Magnesium + hydrochloric 🡪 magnesium + hydrogen
acid chloride
Mg(s) + 2HCl(aq) 🡪 MgCl2(aq) + H2(g)
You don’t need to know this equation. We are using it to see how the rate of this reaction could be monitored
1) Notice that hydrogen gas is made. To monitor the rate we could collect the gas over time using a gas syringe
Gas collected from reaction
or the cheaper way
(without using a gas syringe)
The graph you’ll get
From the graph you should see that as time goes on, we collect more and more gas. (DUH!)
BUT you should also see that we don’t keep collecting the same amount of gas with each minute. Instead we begin to collect less and less with each minute, this results in the curve becoming less steep, till eventually we reach a time where no extra gas collects and the curve goes FLAT
By finding where the curve goes flat we can see at what time the reaction finished
In the first couple of minutes, the curve is steepest.. More than half the gas produced in the reaction is collected in this period.
In the next couple of minutes we collect more gas, but not near as much as was collected in the first minute, this results in the curve being less steep
The reaction is still going but now you collect only a small amount of gas compared to before.
From now on the line is flat, no EXTRA gas is collected. The reaction is over!
KEY POINTS
THE STEEPER THE CURVE THE FASTER THE REACTION IS GOING
OVER TIME THE CURVE BECOMES LESS STEEP SHOWING THE REACTION IS SLOWING DOWN ( THIS IS BECAUSE THE REACTANTS ARE BEING USED UP SO THEIR CONCENTRATION BECOMES LESS AND LESS
THE POINT AT WHICH THE GRAPH GOES FLAT TELL US WHEN THE REACTION HAS FINISHED
GOING FAST
GOING SLOWER
GONE FLAT REACTION HAS FINISHED
By looking at time where the curve goes flat, we can see how long the reaction took to finish
Reactions which produce a gas, can also be monitored using a set of weighing scales.
If a reaction produces a gas it loses particles from the reaction flask to the air, so its weight decreases over time
Procedure:
The flask is placed on some scales and the reaction is started. As time proceeds you record the weight of the flask every minute. Then you work out how much gas has been lost by subtracting the weight recorded every minute from the weight right at the start (0 seconds)
Time (min) | Mass (g) | Mass of gas produced (g) |
0 | 156 | |
1 | 142 | |
2 | 132 | |
3 | 124 | |
4 | 118 | |
5 | 114 | |
6 | 112 | |
156-156 = 0
156-142 = 18
156-132= 24
156-124 = 32
156-118 = 38
156-114 = 42
156-112 = 44
Or you might get a graph like this
In this graph, they haven’t bothered to work out how much gas has been lost. They have just plotted the weight of the flask over time. Now that is JUST LAZY!!
The graph you get would be the same as when you collected the gas produced in a gas syringe but with a different label on the Y-axis
Typical question with a rate vs time graph:
5g of magnesium were reacted to an excess of hydrochloric acid (excess means lots more acid than the magnesium needs to completely react). The gas was collected over 10 minutes and the volume collected in the gas syringe was recorded every minute. The results were plotted to produce the graph below
How long did the reaction take to finish?
6 ½ minutes
Sketch a line on the graph that would be produced if the reaction were repeated at a higher temp?
The blue line could also be drawn,if you were told that the surface area of the magnesium was increased, a higher concentration of acid was used or a suitable catalyst was added to the reaction.
Thiosulphate and hydrochloric acid
A special reaction to study:
Sodium + hydrochloric 🡪 sodium + sulphur + sulphur + water
Thiosulphate acid chloride dioxide
Na2S2O3(aq) + 2HCl(aq) 🡪 2NaCl(aq) + SO2(g) + S(s) + H2O(l)
The good news is that you don’t need to learn this equation. So why have I written it?
Well, look at the state symbols
You should see that both reactants are in the form of solutions
Now look at the state symbol of the sulphur
The (s) tells us that a solid forms in the reaction, this causes the solutions to go cloudy. We call the cloudiness a precipitate.
Here is a picture of the reaction taking place. In the first container you can see the reaction at the start. No sulphur has been produced yet.
The following containers become cloudier and cloudier with time as more and more sulphur is produced in the reaction.
It is the reaction going cloudy that allows us to study the speed of this reaction. The next few slides explain how.
Light Sensor
As the reaction proceeds, more and more precipitate forms, making the reaction cloudier and cloudier. This means less light can pass through the reaction mixture to the light sensor.
This results in a graph like the one below
What would the curve look like if the reaction were repeated using
a higher concentration of the acid/thiosulphate
A higher temperature
catalyst
Another way of following the reaction is VERY HIGH TECH!
It uses a
X
Lets say you want to see the effect of the temperature on the rate of the reaction of sodium thiosulphate and acid
Using the X method you would carry out the reaction 5 times, in each reaction you would keep all the conditions that could affect the rate (the control variables) the same, changing only the temperature of the reaction each time.
Once the reactants have been added together, the beaker is placed over the xross and you time how long it takes for the xross to disappear due to the reaction becoming cloudier (formation of the sulphur precipitate).
Start of reaction, mixture clear as no sulphur formed yet, X clearly visible
Enough sulphur has now formed in the solution, making it cloudy enough for the x to disappear
The less time it takes for the cross to disappear, the faster the ppt is forming, so the faster the reaction
1 minute 23 seconds after mixing (don’t click WAIT FOR THE MAGIC!
The results would be plotted in a graph like the one below
x
x
x
x
x
x
x
Where do you think the points will go? Consider what you know about temp and rate and how we are measuring the rate
The points show us that as temp increased less time was needed for the cross to disappear
This graph tells us that as temperature increases so does the rate of the reaction.
Control variables that would need controlling in this method
As with all experiments that study rate you always need to control variables of
You would only want to change one of these variables at a time keeping all the others constant
In this experiment you would also need to have the same depth of solution, which can be obtained by using the same volume of solutions in the same container
This is necessary as the deeper the solution, the quicker the cross would disappear
The cross used should also be the same, for obvious reasons
Which method is best
Well the datalogger is more reliable
This is because the cross method relies on someone JUDGING when the cross has disappeared
To improve the reliablity of the cross method you would need to have the SAME person judging when the cross has disappeared.
However, the datalogger is expensive equipment and takes time to set up