C6 Flashcards
The rate of a reaction:
-is how fast the reactants are changed into products.
Slow examples
- the slowest is iron rusting.
- chemical weathering (e.g. acid rain damage to a building)
Moderate speed reactions
-metal magnesium reacting with an acid to produce a gentle stream of bubbles.
Fast reactions
- burning
- explosions are faster, releases a lot of gas.
- explosive reactions are all over in a fraction of a second.
Rates of reaction graphs
- steeper the line, faster the rate of reaction.
- over time the line becomes less steep as the reactants are used up.
- quickest reactions have the steepest lines and become flat in the least time.
Collision theory
The rate of a chemical reaction depends on:
- how often particles collide. The more collisions, the faster the reaction.
-the energy transferred during a collision (the more the better!)
Particles need the minimum amount of energy (activation energy) to break the bonds in the reactants and start the reaction.
Factors affecting rates of reaction: temperature
When the temperature is increased, the particles move faster.
If moving faster, more collisions occur.
+ the faster they move, the more energy they have so more of the collisions will have enough energy to make a reaction happen.
Factors affecting rates of reaction: concentration of solution/pressure of gas
A concentrated solution means more particles are moving in the same volume of solvent.
When gas pressure is increased, the same no. of particles take up a smaller space.
So collisions between the reactant particles are more frequent.
Factors affecting rates of reaction: surface area
Changes depending on the size of the lumps in the solid.
If one of the reactants is a solid, then breaking it will increase its surface area to volume ratio.
So for the same volume of the solid, the particles will have more area to work on- so more frequent collisions.
Factors affecting rates of reaction: presence of a catalyst
It’s a substance that speeds up a reaction without being used up.
They all work by decreasing the activation energy needed.
They do this by providing an alternative reaction pathway with a lower Ae.
Enzymes catalyse reactions in living things.
rates of reaction equation
Amount of reactant used or product formed divided by time. Gas = cm cubed Solid = grams Time = seconds Units for rate = cm cubed/s or in g/s Or if it’s measured in moles then mol/s.
measure the rate of a reaction: precipitation and colour change (a)
-Record visual change if initial solution is transparent and the product is a precipitate, clouds solution (becomes opaque).
-Observe a mark through solution. Measure how long it takes to disappear
Faster it disappears, the quicker the reaction.
measure the rate of reaction: precipitation and colour change (b)
- If reactants are coloured and products are colourless, you can time how long it takes for the solution to lose (or gain) it’s colour.
- The results are v subjective - different people might not agree on the exact point the mark disappears. With this method you can’t plot a graph.
measure the rate of a reaction: change in mass (a)
- measuring the speed of a reaction that produces a gas can be carried out using a mass balance.
- as the gas is released, the mass disappearing is measured on the balance.
- quicker the reader on the balance drops, the faster the reaction.
measure the rate of a reaction: change in mass (b)
- If you take measurements at regular intervals, you can plot a graph and find the rate easily.
- this is the most accurate method because the mass balance is v accurate. But it has the disadvantage of releasing the gas straight into the room.
measure the rate of reaction: the volume of gas given off
- involves 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.
- they’re quite accurate. You can take measurements at regular points and plot a graph with this method. Careful:vigorous reaction blows plunger out of end of syringe.
Magnesium and HCl practical
1
- Add dilute hydrochloric acid to a conical flask. Carefully place on a mass balance.
- Add magnesium ribbon to acid and quickly plug the flask with cotton wool.
- Start stopwatch, record mass on the balance. Take readings of the mass at regular intervals.
Magnesium and HCl practical
2
- Plot results, work out the mass lost for each reading. Plot graph: time on x axis and loss of mass on y axis.
- Repeat with more concentrated acid solutions. Amount of magnesium ribbon and volume of acid should be the same each time. Only change acids concentration.
- The 3 graphs show that a higher conc. of acid gives a faster rate of reaction.
Sodium Thiosulfate and HCl practical
1
Both are clear solutions. React together to form a yellow precipitation of sulfur.
- Add dilute sodium thiosulfate to a conical flask.
- Place flask on piece of paper with a black cross drawn on. Add dilute HCl and start stopwatch.
- Black cross will disappear through cloudy sulfur. Time how long it takes.
Sodium and Thiosulfate HCl practical
2
4.Repeat reaction with either reactant at different concentrations (only change conc. of one reactant at a time.
Depth of liquid must be kept the same.
5.Higher the conc, quicker the reaction, so takes less time for mark to disappear.
6.Cant make a graph. Do get readings of how long it takes to disappear.
Calculating mean reaction rate from a graph
- Mean rate for whole reaction= overall change in the y value divided by total time taken for the reaction.
- You can also use graph to find mean rate of reaction between any two points in time.
Tangents on the graph : what does it do.
-Finds the rate of the reaction at a particular point in time.
Drawing a tangent : what to do.
- Position ruler on graph at point where you want to know the rate.
- Adjust ruler until space between ruler and curve is equal on both sides of the point.
- Draw a Line along ruler to make the tangent. Extend line right along graph.
- Pick 2 points on line that are easy to read. Use them to calculate gradient of tangent to find the rate.
Reversible reactions
1
- As reactants react, their concentrations fall so the forward reaction will slow down. But as more and more products are made and their concentrations rise, the backward reaction will speed up.
- After a while the forward reaction will be going at exactly the same rate as the backward one - system is at equilibrium.
Dynamic equilibrium
Both reactions are still happening, but there’s no overall effect.
Reversible reactions
2
It’s a dynamic equilibrium
So the concentrations of reactants and products have reached a balance and won’t change.
Equilibrium is only reached if the reversible reaction takes place in a closed system.
‘Closed reaction’
None of the reactants or products can escape and nothing else can get in.
Position of equilibrium
- If the equilibrium lies to the right, the conc. of products is greater than that of the reactants.
- If it lies to the left, the conc. of reactants is greater than products.
- Position of equilibrium depends on certain conditions: temperature, pressure (if it’s gas) and concentration of reactants and products.
Endothermic and exothermic
In reversible reactions, if it’s endothermic in one direction, it will be exothermic in the other.
The energy transferred from surroundings by endothermic reaction = energy transferred to surroundings during exothermic reaction.
E.g. of reaction: thermal decomposition of hydrated copper sulfate.
Hydrated copper sulfate - anhydrous copper sulfate + water.
If you heat blue hydrated copper (II) sulfate crystals, it drives the water off and leaves white anhydrous copper (III) sulfate powder. This is endothermic.
If you then add a couple of drops of water to the white powder you get the blue crystals back again. This is exothermic.
Le Chateliers Principle
- The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change.
- It can be used to predict the effect of any changes you make to a reaction system.
Changing conditions: temperature
If you decrease temp, equilibrium will move in the exo direction to make more heat. So you’ll get more products for the exo reaction and fewer products for the endo reaction.
If you raise the temp, the equilibrium will move in the endo direction to try and decrease it. So you’ll get more products for the endo reaction and fewer for the exo reaction.
Changing conditions: pressure
Only affects equilibrium involving gases.
If you increase pressure, the equilibrium tries to reduce it, it moves in the direction where there are fewer molecules of gas.
If you decrease, equilibrium tries to increase it, it moves in the direction where there’s more molecules of gas.
Use a balanced symbol equation for a reaction to see which side has more gas molecules.
Changing conditions: concentration
If you change the conc. of reactants or products, the system won’t be at equilibrium any longer.
So system responds to bring itself back to equilibrium.
Increase conc of reactants system tries to decrease it by making products.
Decrease conc of products system tries increase it again by reducing the amount of reactants.
