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How the concentration of acid affects the rate of reaction

Categories: Concentration

An investigation to find out how the concentration of acid affects the rate of reaction between hydrochloric acid and calcium carbonate (marble chips)


The aim of my investigation is to find out how changing the concentration of acid affects the rate of the reaction between CaCO3 and HCl. To make my experiments fair tests, I will only change one variable: the concentration of the acid. I will have to keep the volume of the acid, the mass and size of the marble chips and the temperature of the acid the same in all my experiments to insure a fair test, because by changing any of the above I will change the conditions and therefore the results will be inaccurate and the rate of reaction will be affected.

The products of the reaction between the hydrochloric acid and the marble chips are calcium chloride, water and carbon dioxide.

CaCO3 (s) + 2HCl(aq) CaCl2 (l) + H2O (l) + CO2 (g)

As CO2 is a gas it will escape as the reaction is taking place, therefore there will be a mass loss.

Following from that I have decided to monitor this mass loss by weighing my components on a set of scales as the reaction takes place, the conical flask with the reactants will be constantly on the scales and I will be reading the mass of the display every 10 seconds. I will use 5 different concentrations of the acid. The strongest concentration will be 2 molar and the weakest will be 0.4 molar. The different concentrations will be obtained by adding a certain volume of water to the acid to dilute it this will reduce the concentration.

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Concentration of the acid / molar

Volume of acid / cm�

Volume of water / cm�
















I predict that the higher concentration of the acid will have a much faster rate of reaction than the weaker solution, i.e. the mass loss will be more rapid. This is because a higher concentration of acid will contain more acid molecules per a set volume than low concentration solution. This means that there will be a higher chance of the calcium carbonate molecules colliding with the hydrochloric acid molecule and reacting.

According to the collision theory, reactant particle have to collide in order for the reaction to form a product in this case (calcium chloride, water and carbon dioxide). However not all the collisions are successful and if there is not enough energy the molecules will simply rebound. When the concentration is high there are more collisions therefore a bigger chance of the collsions being successful.

The rate of the reaction is directly proportional to the concentration of the acid.

E.g. doubling the concentration of the acid should double the rate of the reaction; therefore my results should show that the reaction rate at concentration of acid 0.8 molars is half the rate of the reaction when the concentration of the acid is 1.6 molars.

The reaction rate, however, should theoretically decrease as the reaction between HCl and CaCO3 progresses, this is because as the reaction time increases the number of HCl molecules present will decrease as they react with CaCO3 molecules to produce calcium carbonate, water and carbon dioxide. Although carbon dioxide escapes into the air, the water produced will dilute the hydrochloric acid even further, so the concentration will go down, as there will be less successful collisions, as there will be less HCl molecules. This should therefore make the graph a curve and not a straight line as the mass will not be changing at a constant rate.

This reaction is an exothermic reaction and therefore the energy produced in the reaction will be needed to break old bonds in CaCO3 and HCl, and make new bonds in CO2, H2O and CaCl2. The atoms have to be broken loose from their arrangements and re-arranged with new bonds into new substances. Learn what is the optimum concentration of pectinase

As you can see from the table, earlier on, I will use 100cm� of acid in each of my experiments. However I designed my experiments so that as the reaction takes place the volume of the acid will become a limiting factor where the graph will level off. By doing this I will get 5 graph lines that will level of at different levels. From the work in class I know that you need 1000cm� of 2 molar hydrochloric acid to completely dissolve 100g of marble chips. Therefore 10g of marble chips will dissolve in the volume that I will be using (100cm�). However I want to make sure that I have more marble chips than is required so therefore I decided to use 11g of marble chips.


I will need:


Conical flask

Cotton wool

Small size marble chips (12g x 5)

Hydrochloric acid (100cm� + 80 cm� + 60 cm� + 40 cm� +20 cm� = 300 cm�)

Water (300 cm�)



I will set up my experiment as shown in the diagram below.

Firstly the electrical scales will be switched on and checked if they were working properly. Then 100 cm� of acid will be poured into the conical flask and placed on the scales, along with 11g of marble chips (at that point in time the marble chips will be separate from the acid and the cotton wool. It is vital to weigh everything that will be on the scales when the experiment will take place, as all the extra mass should be taken into account so that it doesn’t affect the mass loss measurements. When all of this is weighed the scales will be programmed to read 0.0g so that it will be easier to record the mass loss, as any mass loss will be negative. The marble chips will then be added to the acid, the cotton wool will be pluged at the top of the conical flask and the stopwatch will be started. The cotton wool is needed so that we can insure that only carbon dioxide is escaping. As the reaction takes place and gas is produced there will be a lot of fizzing and so the acid can spit out of the conical flask, this will affect the final results. The cotton wool stops the acid escaping, however it let’s the carbon dioxide escapes. The reading of the scale’s diplay will be taken every 10 seconds and then recorded into a table. The experiment will be left to run until the mass stops changing, this will be the indication that the acid has run out and that the reaction can no longer take place.

5 different concentrations of the acid will be obtained by diluting the acid with different amounts of water as you can see from the table earlier on. The same experiment will be repeated for all 5 concentrations. Everything will be kept the same to insure fair test. The size of the marble chips, the temperature of the acid and the mass of the chips will be kept the same.

To make it a safe test, I will wear goggles to protect my eyes in case of the acid spilling. I will also have to handle the glassware very carefully when carrying it around so that it doesn’t smash.

Preliminary work

When I did some preliminary work before the actual experiments. I found that the normal scales could not be used for the experiment as the flask with the acid and the marble chips was to heavy, and the scales could only take 200g. To solve this problem I used scales that had a maximum weight limit of about 1000g. When I started the experiment I found that 11g of marble chips was just enough and was actually running out when they were dissolved in 100 cm so therefore I decided to use a mass which had a greater excess. Therefore in my real experiment I will use 15g of small marble chips. As I started to dissolve the marble chips in 2 molar acid, I found that the mass change was quite fast, however I decided not to change the size of the chips, because when the acid was less concentrated the reaction would not be as fast with the small size chips, while with larger chips the mass change would be to slow if the surface area wa decreased. I also found that taking the readings of the mass loss every 10 seconds is to often as sometimes in the period of 10 seconds the mass change is so insignificant that the scales do not detect it. I decided that I would only take readings every 10 seconds for the first 2 minutes, as at that time the mass loss is very rapid.

I used the thermometer in the pilot experiment however I found that the temperature changes my about 3-4 degrees Celsius every time, so I have decided not to use it in my real experiment as it cause a lot of inconvenience while trying to start the clock and putting the thermometer in place.

It also took a bit of time to get used to putting everything back on the scales (that was there when everything was weighed) when the reaction starts. I always forgot the paper, however now that I got used to it, it should not be hard to remember.

Obtaining evidence

During my experiment I tried to do follow the safety procedures while handling the acid and carrying the apparatus around the lab. I kept all the variables as constant as possible except for the concentration which I was changing. However it was hard with the temperature as at the beginning of the reaction it would go up by about 4 degrees and then go back down again. However I found that I needed help, as it was really hard to start the clock at the same time as putting the marble chips into the acid and plugging the conical flask with the cotton wool. So therefore I had to ask someone in the lab to assist me and start the clock as soon as I put the marble chips into the acid. This made it much easier.

I have repeated the experiment for every concentration of the acid; in order t obtain more accurate results. I found that it was useful to repeat the experiments as this would give a more accurate mass loss and it would clearly show any anomalies. The results and the graphs are shown on separate sheets.

I found it quite convenient to read the mass off the scales every 10 seconds for the first two minutes and then every 20 seconds, as sometimes the mass loss was so small in the period of 10 seconds that the balance did not detect it, as the readings were to 2 decimal places.


From my results and my graphs I can clearly see that the higher is the concentration the faster is the rate of the reaction. Also it takes longer for the graph to level off , as there is more hydrochloric acid molecules that had to react with marble chips in the same volume at higher concentration.

I can also see that as the concentration of the acid goes down in same amounts, the mass loss (carbon dioxide produced) goes down proportionally to the decrease in concentration.

Every time the concentration decreases by 0.4M, the total mass loss decreases by about twice that, by 0.8 (on average).

Acid concentration/Molar Total Mass Loss/g Difference in mass loss/g

2.0 3.70


1.6 2.94


1.2 2.06


0.8 1.26


0.4 0.49

These results are not very accurate as some times the difference is bigger than 0.8g and some times it is smaller. However they all seem to be quite near 0.8g. You can see it illustrated on the graph as the distance between the lines at 500 seconds are about equal. Following from this we can say that if the difference between the concentrations is 0.8M, the difference between the total mass losses will be about 1.6g.

2.0M – 3.70g

3.70 – 2.06 =1.64g

1.2M – 2.06g

At every concentration of the acid the rate of reaction is fast at first and then it slows down as the acid is diluted by water produced by the reaction of HCl and marble chips (the curved lines of the mass loss illustrate this).

Concentration – 1.2M

Mass Loss/100 sec – 1.18g Rate of reaction/sec = 1.18/100 = 0.018

Mass Loss/200sec – 1.72g Rate of reaction/sec = 1.72/200 = 0.0086

The rate of the reaction is directly proportional to the concentration of the acid, i.e. doubling the concentration doubles the rate of the reaction

When the concentration is 1.6M the mass loss after 2 minutes (120 sec) is 1.83g.

When the concentration is 0.8M the mass loss after 2 minutes (120 sec) is 0.91g.

1.6/0.8 = 2

0.01525/0.007583 = 2.01

This illustrates that the concentration is directly proportional to the rate of the reaction.

e.g. When the concentration is 2M the mass loss after 6 minutes (360 sec) is 3.57g.

When the concentration is 1.2 M the mass loss after 6 minutes (360 sec) is 2.01g.

2/1.2 = 1.67

0.00991/0.00558 = 1.78

From my results I can conclude that the higher is the concentration of the acid, the higher is the mass loss in a set period of time therefore the higher is the rate of the reaction between HCl and marble chips. This is because at higher concentration there are more reactant molecules per set volume. Therefore there will be more collisions per second, so more particles reacting per second.

However the rate of reaction decreases as the experiment progresses. This is because, although we use an excessive amount of marble chips, to keep this variable more or less constant, it is still used up and so the surface area, where reaction can take place, decreases. Also the products of the reaction between the HCl and the marble chips gradually dilute the acid, especially as the water is produced, therefore the concentration of reactant molecules decreases and the rate of reaction goes down.

The curves of mass loss level of at different levels because the acid runs out at different times and at different amounts of mass loss, as the concentration goes down the mount of HCl molecules decreases and so less carbon dioxide can be produced as less successful collision can take place.

I think that my conclusion and my results fully support my prediction, as I have predicted that the higher is the concentration, the higher is the rate of the reaction, and I also stated that the concentration is directly proportional to the rate of reaction.


My experiments went well to some extent, there were no huge errors that could have affected my results and made a lot of anomalies. As you can see from the graph and form my results table, my results were quite accurate. I got very smooth curves for mass loss on all 5 of the concentrations and they levelled of at different levels as I have expected, i.e. at lower concentration the total mass loss was less than at higher concentrations. The only anomalies that can be identified on the mass loss graph are that the mass loss at the beginning of the experiment for 1.6M and 1.2M acid seem a bit lower than it would be expected, as some of the point e.g. at 20 seconds overlap each other. I can also see that the points at the beginning of line for the acid 0.4M seem to not go in a curve but in a straight line. I think that this is because at the beginning of the reaction you need energy to start the reaction and that is why this reaction is exothermic. However, at lower concentration less energy is produced and so it takes longer for the molecules to start reacting.

Some of the results could have also been slightly affected by the measuring of the scales. The scales only measured to 2 decimal places and so the results are not precise. Also the scale display was affected by the movement near by in the lab, as I have noticed that when some one went by the readings would jump up and down and it would take them some time to settle back down again.

The scatter around best fit of the graphs for mass loss is minimal and the curves seem to be very smooth. The same can be said about graphs for the rates of reaction at different times. There are no points that lay a long way from the best-fit line as it goes through the origin. The only points that seem to be a bit out from the best fit lines are the ones for the 0.4M acid rate of reaction on all 3 times (50 seconds, 100 seconds and 200 seconds). As I said before this might be because the change in the mass loss could have been so slow that the scales did not register it as they rounded the readings to the nearest to decimal points.

I thought the procedure was quite safe and easy to do, as it did not take a lot of time to set it up or stop it. However I would change the way of how I measured the rate of reaction. I would actually measure the volume instead of the mass loss. Carbon dioxide does not have a very big atomic weight:

C = 12

O2 = 16 x 2 = 32

CO2 = 44

However, it has a bigger gas volume that could be measured with much more accuracy. It would take a longer time but I would not need to remember of putting everything back on the scales as there will not be any and this will eliminate any other human error.

I think that my results are sufficient to support my prediction and my conclusion, as they seem to be very accurate without a lot of critical anomalies. The results clearly show that the rate of reaction is directly proportional to the concentration of the acid, and that the rate of the reaction decreases as the reaction of hydrochloric acid with marble chips progresses. All the graphs on the rate of reaction go through the origin and in all my experiment it is evident that the acid runs out at the end as all the lines level off. In all my graphs the points seem to lie very close to the best-fit line, which shows that they are very near to accurate.

The only aspect that I was concerned about was that the mass loss in the first trials was lower than in the second trial. However I think that calculating the average, which gave quite accurate results, solved it. This difference would not have really affected my conclusion as it still showed that the rate of the reaction slowed down as the reaction progressed and that it was faster when the concentration was higher.

If I were to do this investigation again I would firstly measure the volume of the gas instead of the mass loss, as I think that this would give me more accurate results. I would also try different surface areas of the marble chips and investigate how this affects that rate of reaction at different concentrations.

A solid in a solution can only react when particles collide with the surface of this solid, in this case calcium carbonate. The bigger the area of the solid surface, the more particles can collide with it per second therefore the faster is the rate of the reaction between hydrochloric acid and marble chips.

You can increase the surface area of a solid by breaking it up into smaller pieces. A powder has the largest surface area and will have the fastest reaction rate. This is why catalyst is often used as powders. So therefore if we break the marble chips up into powder we get some sort of a catalyst for this reaction.

The reaction rates can be compared using large marble chips, and small marble chips of the same mass. This difference in the reactions can be illustrated by the following graph graph:

The reaction rate is faster, as the slope is steeper, for the reaction where we would use the powder, which has a greater surface area. Using different surface areas will not affect the final mass loss or gas volume collected. In both reactions it should be the same. This is because the same mass of calcium carbonate will give the same mass of carbon dioxide.

I would also investigate the rate of reaction at different temperatures of the acid. Instead of waiting for the acid to run out I might measure out that it would be the marble chips that run out. By doing this I could also see how long it takes for the marble chips to dissolve at different concentrations or temperature.

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How the concentration of acid affects the rate of reaction. (2020, Jun 01). Retrieved from

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