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C6 - The Rate and Extent of Chemical Change (Le Chatelier's Principle,…
C6 - The Rate and Extent of Chemical Change
Rates of Reaction
Reactions Can Go at All Sorts of Different Rates
The rate is how fast the reactants turn into products
One of the slowest is the rusting of iron
Other slow ones are chemical weathering
Moderate speed is magnesium reacting with acid
A fast reaction is burning
You Need to Understand Graphs for the Rate of Reaction
You can find the speed of a reaction by noting the amount of products formed
The steeper the line, the faster the reaction
The quickest reactions have the steepest lines and level off quicker
Particles Must Collide with Enough Energy in Order to React
The rate depends on the collision frequency of the reacting particles
Also depends on the energy transferred during a collision
Factors Affecting Rates of Reaction
The Rate of Reaction Depends on Four Things
Temperature
Concentration or pressure
Surface area
Catalyst
More collisions Increases the Rate of Reaction
All four methods increase the amount of collisions
Increasing the Temperature Increases the Rate
The particles all move faster
They then collide more frequently
More collisions means more energy, so there's enough energy for the reaction to take place
Increasing the Concentration or Pressure Increases the Rate
More particles are knocking about in the same volume
The same number of particles are in a smaller space when the pressure increases
This makes collisions more frequent
Increasing the Surface Area Increases the Rate
Breaking it into pieces increasing its surface area to volume ratio
For the same volume, the particles have more area to work on
Using a Catalyst Increases the Rate
A catalyst speeds up a reaction without being used up
Different catalysts are needed for different reactions
Enzymes are biological catalysts
Measuring Rates of Reaction :star:
There Are Three Ways to Measure the Rate of Reaction
Precipitation and Colour Change
You can record the time if the initial solution is transparent and the end product is a precipitate
You can observe a mark and time how long it takes to disappear
If the reactants are coloured and the products are colourless, you can measure the time it takes to lose its colour
The results are subjective
Change in Mass
You can do this using a mass balance
The mass disappearing is measured on the balance
The quicker the reading drops, the faster the reaction
If you take regular measurements, you can plot a graph
This is the most accurate method because the balance is accurate
The Volume of Gas Given Off
This involves using a gas syringe
The more gas given off, the faster the reaction
They're quite accurate
Two Rates Experiments :star:
Magnesium and HCl React to Produce H2 Gas
Start by putting dilute hydrochloric acid into a conical flask on a mass balance
Add a piece of magnesium ribbon and plug with cotton wool
Start the stop watch and record the mass at regular intervals
Plot the results and work out mass lost. Then plot a graph
Repeat with more concentrated acid solutions
The higher the concentration, the faster the rate of reaction
Sodium Thiosulfate and HCl Produce a Cloudy Precipitate
They start off clear but form a yellow precipitate
Add sodium thiosulfate to a conical flask
Put the flask on a black cross. Add some dilute HCl and start the stop watch
Watch the black cross disappear and time how long it takes
The reaction can be repeated with different concentrations
Shows the effects of increasing the concentration of HCl
The higher the concentration, the quicker the reaction
It doesn't give a set of graphs
Reversible Reactions
Reversible Reactions Will Reach Equilibrium
As reactants react, their concentrations fall so the forward reaction slows down
After a while both reactions will be going at the same rate - equilibrium
Both reactions are still happening, but there's no overall effect
Equilibrium is only reached in a closed system
The Position of Equilibrium Can be on the Right or the Left
It doesn't mean the amounts of reactants and products are equal
Equilibrium lies to the right if the concentration of products is greater
Equilibrium lies to the left if the concentration of reactants is greater
The position of equilibrium depends on these conditions
Temperature
Concentration
Pressure
Reversible Reactions Can Be Endothermic and Exothermic
If the reaction is endothermic in one direction, it's exo in the other
The energy transferred to the surroundings is equal to the energy taken in from the surroundings
Le Chatelier's Principle
Reversible Reactions Try to Counteract Changes
If you change the conditions in the system, it will try to counteract that change
Can be used to predict the effects of the changes you make
Changes to Temperature
All reactions are exo one way and endo the other
If you decrease temperature, the equilibrium moves in the exothermic direction
If you raise the temperature, it moves in the endothermic direction
Changes to Pressure
Changing the pressure only affects gases
If you increase pressure, the equilibrium shifts to the side with fewer molecules
If pressure is decreased, the equilibrium shifts to the side with more molecules of gas
Changes in Concentration
If you change the concentration, the system isn't at equilibrium
The system responds to bring itself to equilibrium
If you increase the concentration of reactants, it will shift to produce more products
If you increase the concentration of products, equilibrium will shift to produce more reactants