Concentration of Acid-
The higher the concentration of acid, the more particles there are to react with. Here are two diagrams to illustrate this:
An increase in reactant in turn means an increase in reaction rate. As you increase the acid concentration you are increasing the number of particles in that given volume. Therefore there is a greater chance that the acid particles will collide with the surface of the metal. So therefore you increase the rate of reaction. If you double the number of particles you double the rate of reaction so the rate of reaction is directly proportional to the concentration. If for example the concentration of Hydrochloric acid is doubled then there are more Hydrochloric particles present and so there will be more chance of the particles colliding.
Pressure of Gases-
Increasing pressure decreases the volume. Therefore it’s increasing concentration, so the rate increases.
I decided that it was best if I further investigated the affect concentration has on rate of reactions.
I carried out preliminary work to help with the experiment. This was our plan
Plan:
I have chosen to investigate how concentration affects the rate of reaction.
I will make sure our method is safe by wearing safety goggles and follow safety procedures carefully. The table in which the experiment was to be conducted is going to be cleared and I will make sure that nothing except the necessary apparatus will be kept on the table. In order for this to be a fair test several factors needed to be taken under consideration. I will make sure that the temperature of the room was constant. The experiment is to be repeated three times and then an average taken in order to gain a fair set of results. I decided that the independent variables will be the concentration of Hydrochloric Acid. The controlled variables will be the mass of marble chips, which I decided to be as 20 grams, and the volume of the gas produced which was to be 100 cm3. The dependant variables will be the time taken for the reaction to take place and the volume of the gas. I decided that in order for this to be a fair test I will have to measure out the marble chips after every time I did the experiment incase the mass changed. I also decided that I will have to repeat the experiment three times in order to get an average time so that I can calculate the average speed
Here is the list of apparatus I decided to use:
Stop clock
Conical Flask
Delivery Tube connected to a bung
Beaker
Water trough
Clamp Stand
Water
Hydrochloric Acid (2mol)
Balance
Weighing Boat
The next table will show how the concentration I decided to use for our experiment:
I then worked out the mole ratio between Hydrochloric Acid and Calcium Carbonate. In order to do this I looked at the equation:
CaCO3(s) + 2HCl(aq) → CO2(g) + H2O(l) + CaCl2(aq)
The mole ratio is 1:2. This means that one mole of calcium carbonate will react with two moles of hydrochloric acid. 1 mole of CaCO3 = 100 g chips
100g chips → 1000cm³ of 2molar acid
10g chips → 100cm³ of 2molar acid
2g chips → 20cm³ of 2molar acid
Using the two mole ratio I was able to calculate that 2 grams of chips reacts with 20 cm3 of 2 molar acid, but I decided to use excess moles of acid with each concentration so that it would not run out.
Using this information, I can find out the molarity for each concentration that is shown in the table above. Concentration = moles/ total volume of gas given off.
Moles in 50cm³ acid = dm3molarity
= 50/1000 x 2
= 0.1Moles
Molarity = moles / dm3
= 0.1/(50 / 1000)
= 2Moles / dm3
Moles in 40cm³ acid = dm3molarity
= 40/1000 x 2
= 0.08Moles
Molarity = moles / dm3
= 0.08/(50 / 1000)
= 1.6Moles / dm3
Moles in 30cm³ acid = dm3molarity
= 30/1000 x 2
= 0.06
Molarity = moles / dm3
= 0.06/(50 / 1000)
= 1.2Moles / dm3
Moles in 20cm³ acid = dm3molarity
= 20/1000 x 2
= 0.04
Molarity = moles / dm3
= 0.04/(50 / 1000)
= 0.8moles/dm3
Moles in 10cm³ acid = dm3molarity
= 10/1000 x 2
= 0.02
Molarity = moles / dm3
= 0.02/(50/1000)
= 0.4moles/dm3
Hypothesis:
I believe that by increasing the concentration of acid will increase the speed of the reaction.
This is because the more concentrated the acid more particles are present and so there is a greater chance for the particles to successfully collide with each other. In dilute acids the chances of successful collisions are reduced. Therefore if concentration is increased, the
reaction rate also increases.
However the reaction rate, should decrease as the test progresses because as the reaction time increases the number of hydrochloric acid molecules present will decrease as they have been reacted to form water calcium chloride and carbon dioxide. The additional water and calcium chloride present as the experiment progresses should also decrease the rate of reaction because of decrease in concentration. This should make a graph of the reaction, curved, as the reaction rate slows down.
The increase in the concentration should be directly proportional to the increase of the reaction rate at a given time. This is because by doubling the number of hydrochloric acid molecules present the chance of a collision should be doubled, as there is now twice the possibility of a collision-taking place initially.
Method:
Set up the apparatus as shown
Diagram
I measured out the amount of water needed and poured it in a beaker
I then measured out the amount of acid needed and poured it into the same beaker
I then took the weighing boat and using the balance weighed out 20 grams of marble chips
I then put the marble chips in the conical flask
I then took the delivery tube and put the end without the bung underneath the upside down measuring cylinder
Pour the hydrochloric acid and water from beaker into conical flask
Put the bung on the conical flask
Start Stopcock
I set up the apparatus and wanted to see if the hydrochloric acid was the only thing affecting the Calcium Carbonate. I didn’t expect a reaction, as from our knowledge I know that water will not react with the Calcium Carbonate.
The above results show that the water didn’t react with CaCO3 and hence Hydrochloric Acid is the only thing reacting with the Calcium Carbonate.
I carried out the experiment and repeated it three times in order to have a set of results, which are valid.
I calculated the rate of the reaction by the following formula
Rate = Total Volume of Gas/ time taken to collect fixed volume
Analysis
It is evident from these set of results that the rate of reaction increases as the concentration increases. This is because the more concentrated the solvent the more particles are present and so there is a greater chance for the particles to successfully collide with each other. In dilute acids the chances of successful collisions are reduced. Therefore if concentration is increased, the reaction rate also increases. The collision theory states that if a solution is made more concentrated, it means that there are more particles of reactant, which makes collisions between the reacting molecules more likely.
The results agree with my hypothesis. The results seem to be accurate and reliable. As I can see from the graphs as well as the tables, as the concentration increased the time decreased and as that the speed and concentration were directly proportional to each other. For example when the concentration was 20 % the time was 92 seconds and when the concentration was 60% the time was 21 seconds. The graph showing the average speed of the reaction is a straight-line graph, which shows that they are directly proportional to each other. I have marked on the graph where this is evident. I can see this because the rate doubles. I can see from the graph that when the concentration was 80% the average speed of the reaction was 6.6cm3 per second and when the concentration was 40% the average speed of the reaction was 3.25cm3 per second, which shows that the rate of the reaction doubled.
Evaluation:
The results I obtained were reliable and accurate. Our results all seemed to fit in with the general pattern. I feel that these results are good enough to draw a firm conclusion that being that concentration does in fact increase the rate of reaction. The fact that the reaction rate was highest at the highest concentrations shows that the higher the concentration the more particles that are present per set volume which shows increases the rate of reaction. This also proves tat activation energy does not play a part because if this did the gradients of the lines would not increase by the amounts they did. This is due to the fact more/less energy would be needed to start the reaction and there would be a higher or lower gradient of the line so the gradients would not go up down by the same amounts.
In order to perhaps gain a more accurate set of results I would make sure that the size of the bung fitted perfectly because whilst carrying out experiment I found that the size of the bung wasn’t the correct size and so I had to push the bung in every time.
Another method I could have used which would have been a little more accurate in my opinion is to have:
Taken readings at regular time intervals
Put the results in a table and work out the loss in mass for each reading
Repeat with stronger acid solutions but always with the same amount of calcium carbonate
The volume of the acid always kept the same, only concentration increased
To improve the experiment, I could have used more accurate apparatus, for example, a more accurate measuring cylinder. If I were to carry out further investigation on the rates of reaction I would try and measure how temperature would affect the rate of reaction I feel that I would get a set of results, which would be more accurate as when you measure temperature it is easier to obtain an accurate set of results.
