Reactions can go at all sorts of different
Rates
What is the rate of a chemical reaction?
The rate of a chemical reaction is how fast the reactants are changed into products.
Examples of slow reactions
The rusting of iron Chemical weathering (acid rain damage to limestone buildings)
Example of a moderate speed reaction
Metal magnesium reacting with an acid to produce a gentle stream of bubbles
Burning is a ____ reaction.
Fast reaction. But explosions are even faster and release a lot of gas. Explosive reactions are all over in a fraction of a second.
How can you find the speed of a reaction
Recording the amount of product formed or the amount of reactant used over time
Rate of reaction - the steeper the line on the graph
The faster the rate of reaction. Over time the line becomes less steep as the reactants are used up.
The quickest reactions have the steepest lines and
Become flat in the least time as the reactants are used up
Particles must collide with ________ in order to react
Enough energy
Reaction rates are explained perfectly by the
Collision theory
Collision theory
The rate of chemical reaction depends on- The collision frequency of reacting particles (how often they collide) More collisions, faster reaction
The energy transferred during a collision. Particles need to collide with enough energy for the collision to be successful
Activation energy
The minimum amount of energy particles need to react. Particles need this energy to break the bonds in the reactants and start the reaction.
What will increase the rate of reaction
Factors that increase the number of collisions (So a greater proportion of reaction particles collide) or the amount of energy particles collide with will increase the rate of the reaction.
Which 4 things does the rate of reaction depend on?
Temperature
Concentration of solution or pressure of gas
Surface area (changes depending on the size of the lumps of a solid)
Presence of a catalyst
More collisions increase the
Rate of reaction
All four methods of increasing the rate of a reaction can be explain in terms of
Increasing the number of successful collisions between reacting particles
Increasing the temp
Increases the rate
When temperature is increased
The particles all move faster
If the particles are moving faster
They are going to collide more frequently
Also the faster the particles move the more energy they have
So more of the collisions will have enough energy to make the reaction happen
Increasing the conc or pressure
Increases the rate
If a solution is more concentrated
It means there are more particles in the same volume of water (or other solvent)
When the pressure of a gas is increased
It means that the same number of particles occupies a smaller space. This makes collisions between the reactant particles more frequent
Increasing the SA
Increases the rate
If one of the reactants is a solid
Then breaking it up into smaller pieces will increase its SA: V RATIO
For the same volume of the solid
The particles around it will have more area to work on so collisions are more frequent
Using a catalyst
(You guesses it) increases the rate
A catalyst is a substance that speeds up a reaction without being used up in the reaction itself which means
It’s not part of the overall reaction equation
Different catalysts are needed for different reactions
But they all work by decreasing the activation energy needed for the reaction to occur.
Catalysts decrease the activation energy needed by
Providing an alternative reaction pathway with a lower activation energy
Enzymes are biological catalysts
They catalyse reactions in living things.
Three ways of measuring rate of reaction
Precipitation and colour change
Change in mass (usually given off)
The volume of gas given off
Mean Rate of reaction equation
ROR = amount of reactant used or amount of product formed / time
How to find the rate of reaction at a specific time
Plot a graph and find the gradient at that time
When the product is a gas
You usually measure the amount in cm3
If the product or reactant is a solid
You measure in grams
Time is often measure in
Seconds
The units for rate may be in
Cm3/s or in g/s
You can also measure the amount of product or reactant in moles
Measurement could be mol/s
You can record the visual change in a reaction if
The initial solution is transparent and the product is a precipitate which clouds the solution (it becomes opaque)
You can observe a mark through the solution and measure how long it takes for it to disappear
The faster the mark disappears the faster the reaction
If the precipitation products are colourless or vice versa,
You can time how long it takes for the solution to lose or gain its colour
In precipitation why are the results very subjective?
Different people might not agree over the exact point when the mark disappears or the solution changes colour.
What can you not do if you use the precipitation method
Can’t plot the rate of reaction graph from the results
What is a mass balance used for?
To measure the speed of the reaction that produces a gas.
How is the mass balance used?
As the gas is released the mass disappearing is measured
The quicker the reading on the mass balance drops…
The faster the reaction
When measuring the change in mass, how would you plot the rate of reaction graph?
if you take measurements at regular intervals You can plot the rate of reaction graph and find the rate quite easily
Change in mass method is the most…
Accurate method of three
Why is the change in mass method the most accurate?
Because the mass balance is very accurate
Disadvantage of change in mass method
Releasing gas straight into the room
The volume of gas given off involves
The use of a gas syringe to measure the volume of gas given off
The more gas given off during a given time interval
The faster the reaction
Gas syringes usually give volumes accurate to the
Nearest cm3 so they are quite accurate
How to plot a graph using the volume of gas given off method
Taking measurements at regular time intervals
Why do you have to be careful while using the volume of gas given off method
Because if the reaction is too vigorous you can easily blow the plunger out the end of the syringe
2 examples of measuring the effect of concentration on the rate of reaction
Mg and HCL react to produce H2 gas
Sodium thiosulfate and HCL produce a cloudy precipitate
Step 1 of Mg + HCL —
Ass a set volume of dilute HCL to a conical flask and carefully place it on a mass balance
Step 2 of Mg + HCL —
Add some Mg ribbon to the acid and quickly plug the flask with cotton wool
Step 3 of Mg + HCL —
Start the stopwatch and record the mass on the balance. Take readings of the mass at regular intervals
Step 4 of Mg + HCL —
Plot the results in a table and work out the mass lost for each reading. Plot a graph with time on the x axis and loss of mass on the y axis
Step 5 of Mg + HCL —
Repeat with more conc acid solutions. Variables such as the amount of Mg ribbon and the vol of acid should be kept the same each time - only change the acids conc.
Why do we keep the amount of mg ribbon and vol of acid the same each time in step 5?
To make the experiment a fair test
Step 6 of Mg + HCL —
The three graphs will show that a higher conc of acid gives a faster rate of reaction.
On the Mg + HCL —
Most conc on top, least conc on bottom
Step 1 of Na thiosulfate + HCL —
Add a set volume of dilute sodium thiosulfate to a conical flask
Na thiosulfate + HCL are both
Clear solutions. They react together to form a yellow precipitate of sulfur
Step 2 of Na thiosulfate + HCL —
Place the flask on a piece of paper with a black cross drawn on it. Add some dilute HCL and start the stopwatch
Step 3 of Na thiosulfate + HCL —
Black cross will disappear through the cloudy silver and time how long it takes to go
Step 4 of Na thiosulfate + HCL —
The reaction can be repeated with solutions of either reactant at different concentrations. The depth of the liquid must be kept the same each time.
What do the results show of Na thiosulfate + HCL —
The effect of increasing the conc of HCL on the rate of reaction, when added to an excess of sodium thiosulfate
When should the Na thiosulfate + HCL —
Releases sulfer dioxide so it needs to take place in a well ventilated place
Why does it take less time for the mark to disappear in Na thiosulfate + HCL —
Because the higher the conc, the faster the reaction
Disadvantage of Na thiosulfate + HCL —
Doesn’t give a set of graphs. Gives a set of readings of how long it took until the mark disappeared for each conc
What can you calculate from a reaction graph?
Mean reaction rate (and mean reaction rate between any two points in time)
What does a rate of reaction graph show?
The amount of product formed or amount of reactant used up over time
How to find the mean rate for the whole reaction using a graph
Work out the overall change in the y value and then divide this by the total time taken for the reaction
Mean ROR =
Change in y / change in x
How to find the rate of reaction at a PARTICULAR POINT on a graph
Draw a tangent
How to do a tangent
Draw a line which touches the point on the graph you want and make sure the curve is equal on both sides of the point
Extend it right across the graph
Calculate the gradient of tangent (do the triangle thing)
Gradient =
Change in y / change in x
Some reactions can go…
Backwards
Reversible reactions will reach
Equilibrium
As the reactants react
The forward reaction will slow down
As more products we made and their concentrations rise
The backward reaction will speed up
After a while of the forward reaction slowing and the backward reaction getting faster
Both reactions will be going at exactly the same rate
When both reactions are going at exactly the same rate the system is at
Equilibrium
At equilibrium both reactions are still happening but
There’s no overall effect (dynamic equilibrium) so the conc of the r and p have reached balance and won’t change
Equilibrium is only reached if the reversible reaction takes place in a
Closed system
What is a closed system?
None of the reactants or products can escape and nothing can get in
Position of the equilibrium can be on the
Right or left
When reactions at equilibrium it doesn’t mean…
The amounts of reactants and products are equal
If the equilibrium right lies to the right
The concentration of products is greater than that of the reactants
If the equilibrium lies to the left
The conc of the reactants is greater than that of the products
The position of the equilibrium relies on the following conditions
The temp
The pressure (only effect equilibria involving gases)
The conc of the reactants and products
Reversible reactions can be e…
Endothermic and exothermic
In reversible reactions if the reaction is endothermic in one direction
It will be exothermic in the other
The energy transferred from the surroundings by the endothermic reaction is equal to
The surroundings during the exothermic reaction
A good example of exo and endo reversible reactions is
Thermal decomposition of hydrated copper sulfate
Heat blue hydrated copper sulfate crystals it leaves white anhydrous copper sulfate powder which is endo. Reverse thing and it’s exo
What is Le Chateliers principle?
If you change to conditions of a reversible reaction at equilibrium the system will try to counteract the change
What can Le Chateliers principle be used to do
To predict the effect of any changes you make to a reaction system
All reactions are…
Exothermic in one direction and endothermic in the other.
What will happen if you decrease the temperature?
The equilibrium will move in the exothermic direction to produce more heat.
When the equilibrium shifts what will you get?
More products in one reaction and fewer products in the other (either exo or endo)
If you raise the temperature…
The equilibrium will move in the endothermic direction to try and decrease it. More products for endo and less products for the exo
Changing the pressure…
Only affects an equilibrium involving gases.
If you increase the pressure,
The equilibrium tries to reduce it - it moves in the direction where there are fewer molecules of gas.
If you decrease the pressure
The equilibrium tries to increase it - it moves in the direction where there are more molecules of gas.
What equation can you use for a reaction to see which side has more molecules of gas?
Balanced symbol equation.
If you change the conc of either of the reactant or the products…
The system will no longer be at equilibrium
If you increase the conc of the reactants the system tries to decrease it by making more
system tries to decrease it by making more products
If you decrease the concentration of products
The system tries to increase it again by reducing the amount of reactants.
