3 Rates of reaction Flashcards
What is the rate of a reaction?
The speed at which the amount of reactants decreases or the amount of products increases. It is measured as a change in the concentration, or amount, of reactants or products per unit time.
How can we measure the rate of reaction?
Rate of reaction =
Change in conc, vol, mass
————————————
time
How could we investigate the rate of reaction?
By using marble chips and dilute hydrochloric acid.
What are marble chips made of?
Calcium carbonate,
What do the marble chips react with hydrochloric acid to produce?
Calcium carbonate + hydrochloric acid —> Calcium chloride + water + carbon dioxide
What is the apparatus used in this experiment?
What is step 1 of the procedure?
Use a measuring cylinder to measure 25cm^3 of 2.00mol/dm^3 dilute hydrochloric acid.
What is step 2 of the procedure?
Add 5.00g of large marble chips to a conical flask and place a piece of cotton wool at the opening of the flask. The cotton wool is there to allow the carbon dioxide to escape during the reaction, but to stop any acid spitting out. The marble is in excess - some of it will be left over when the acid is all used up.
What is step 3 of the procedure?
Place everything on a balance and reset it to zero.
What is step 4 of the procedure?
Add the acid to the marble chips and record the reading on the balance every 30 seconds.
What is a picture of the apparatus during the reaction?
Why is the mass decreasing?
Once the reaction starts, the balance shows a negative mass. The mass goes down because the carbon dioxide escapes through the cotton wool.
What would a graph of mass of carbon dioxide lost against time look like?
What is an explanation of the graph?
About 0.47g of carbon dioxide is produced in the first minute. Only about 0.20g of extra carbon dioxide is produced in the second minute, the reaction is slowing down.
What does the slope show?
The rate of reaction.
What happens if there is a steeper slope?
The faster the reaction.
What is the equation to find the rate?
time
How would we find the average rate of the reaction in the first minute?
0.47/1 = 0.47
How would we find the average rate of reaction in the second minute?
0.20/1 = 0.20
How would we find the average rate of reaction over the first two minutes?
0.67/2 = 0.34g/min.
What is happening in the beginning of the reaction is relation to the speed?
We can see that the reaction is fastest at the beginning.
Why does the reaction slow down and eventually stop?
Because all of the hydrochloric acid has been used up.
How can we measure how fast a reaction is going at any time point?
By finding the slope of the line at that point. This is the rate of reaction at that point, rather than the average.
How do you measure the rate of reaction at a specific point?
This is done by drawing a tangent to the line at the time you are interested in and finding its slope.
What is an example of finding the rate at a specific point on the graph?
At 5 minutes the carbon dioxide is being lost at the rate of about 0.05g per minute.
What happens to the marble chips at the end of the reaction?
They were in excess. So there will still be unreacted marble chips in the flask at the end.
What is another way of following the rate of this reaction?
By measuring the volume of carbon dioxide given off.
What is a example of the apparatus that could be used to measure the volume of carbon dioxide given off?
What is another apparatus that could be used to measure the volume of carbon dioxide given off?
How can we explain the shape of the curve in the graph?
By thinking about the particles present and how they interact. This is called collision theory.
What is collision theory?
It states that for a reaction to occur, the reactant particles must collide with each other and the collision needs too have sufficient energy and the correct orientation.
When can reactions only happen?
Only when particles collide.
Does every collision between a particle result in a reaction?
No, not all collisions end up in a reaction. Many particles just bounce off each other.
What is needed for a reaction to happen during a collision?
The particles have to collide with a minimum amount of energy called the activation energy.
What is the activation energy?
It is the minimum amount of energy required for a collision to be sucessful.
In relation to the activation energy, what makes a successful collision?
The collisions with energy greater than or equal to the activation energy are usually called successful collisions.
What is a successful collision?
It is a collision with energy greater than or equal to the activation energy. These collisions result in reactions.
How can collision theory be applied to the rate of reaction between marble chips and dilute hydrochloric acid?
In the reaction between calcium carbonate and hydrochloric acid, particles in the acid have to collide with the particles at the surface of the marble chips. As the acid particles are getting used up, the collision rate decreases, and so the reaction slows down. The marble is in a large excess so that is shape doesn’t change very much during the reaction.
What is a diagram that shows the hydrochloric acid particles and the marble chips early in the reaction with lots of acid particles and lots of collisions?
What is a diagram that shows the hydrochloric acid particles and the marble chips later in the reaction with fewer hydrochloric acid particles left?
What are graphs that would usually be presented to us in the exam?
A graph that shows the mass or volume of a product formed during a reaction.
What is a rarer case of the graph that we may be able to observe?
It is possible that you will see the graphs showing the fall in the concentration of one of the reactants - in this case the concentration of the dilute hydrochloric acid.
What is an image of the graph that shows the fall in concentration of hydrochloric acid over time?
What is shown where the graph is falling most quickly?
Where the graph is falling most quickly (its steepest), shows that the reaction is fastest.
Why does the graph eventually become horizontal?
Because the reaction had stopped when all the acid has been consumed.
In what way can we alter the experiment to do with marble chips and dilute hydrochloric acid?
We could use exactly the same quantities of everything, but using smaller marble chips.
What results should we obtain when we investigate the effect of changing the surface area of marble chips on the rate of reaction between marble chips and dilute hydrochloric acid?
The reaction with the smaller chips happens faster.
What would the graph of both sizes of marble chips look like?
What is the main difference between the two lines?
The reaction with the smaller chips starts off much faster and finishes sooner.
How does changing the surface area affect the line in the graph?
- Much steeper slope at the beginning, showing a faster reaction.
- Graph becomes horizontal more quickly, showing that the reaction stops sooner.
- The lines will both stop at the same place as although the reactions occur at different speeds, the same amount of carbon dioxide is produced.
When are reactions between solids and liquids, or solids and gases faster?
If the solids are present in a lot of small pieces rather than a few big ones.
What could we do to the solid to make the reaction happen quicker?
The more finely divided the solid, the quicker the reaction.