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.