The Effect of Concentration on the Rate of Reaction


The purpose of this report is to determine the factors that affect reaction rates through experiments. The theory of rate of reaction is the speed at which a chemical reaction occurs (Britannica, 2018) – also known as the “collision theory”. According to this theory, the frequency of reactant molecules colliding and how often they react with one another control how fast the reaction rate is (Lumen Learning, 2019).  This indicates a necessity for atoms and molecules to bump and collide with one another for a chemical reaction to occur. In addition, the reaction rate may proceed at vastly different speeds depending on certain factors that decrease or increase a molecules energy, like the reactant concentration, the physical state of the reactants and surface area, the temperature, and the presence of a catalyst (Key, 2014).

In this experiment, the effect of changing concentration on the rate of reaction was investigated, through the reactions of 50mL of 2M hydrochloric acid with two different concentrations (1M and 0.5M) of 1g calcium carbonate. The reaction rate can be determined by measuring the volume of gas produced, as gaseous CO2 is formed when CaCO3 reacts with HCl acid over a series of minutes.  This means the acid concentration was chosen as the independent variable as the volume of CO2 gas being produced every 30 seconds would be strongly affected by the amount of the hydrochloric acid can react to in this time frame. The control variables are how long experiment was conducted for, the temperature of HCl acid, the amount and form of CaCO3 used. When the control variables are remained constant, it is expected that the reaction rate at which CO2 is produced will raise or drop proportionally to the HCl concentration at its maximum or minimum. This is because when raising concentration of reactants, there will be more molecules with the minimum required energy which make the collisions more frequent and come up with a higher rate of reactions. Therefore, increasing concentration of reactants will increase the rate of reaction (Nuffield foundation,2013).

Research Question: How will an increase of Hydrochloric acid concentration affect the chemical reaction rate in the set time frame of seven minutes?

Hypothesis: As the concentration of acid (HCl) is increased, there will be an increase in chemical rate of reaction when the control variables are kept constant.

Chemical reaction:

Hydrochloric acid + Calcium Carbonate   -> Calcium Chloride + Carbon Dioxide + Water

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



In this experiment, the concentration of HCl is decreased to 1.5M and 0.5M, which are smaller than 2M acid. This affects to the rate of reaction, which is lowered the rate of reaction in order to conform the hypothesis that by increasing the concentration, the rate of reaction will increase or the higher the concentration, the faster the rate of reaction.

Original Methodology:

Experiment 1. The effect of changing concentration on the rate of reaction


  1. Set up apparatus a shown in the diagram or according to your teacher’s instructions.
  2. Weigh out 1 g of marble chips and add them to the conical flask. Make sure the cork gives a good seal.
  3. Measure the temperature of 50 mL of 2M HCl and add to the conical flask and quickly put the cork on the flask. Start the timer.
  4. Measure and record the volume of gas accumulating in the measuring cylinder every 0.5 min
  5. Repeat steps 1. to twice more.
  6. Repeat the experiment using your two alternative independent variables.

Management of risks and ethical considerations:


Hazard/Ethical consideration

Risk Management Strategy

Glass conical flask


·       Broken conical flask, might touch fragments glass around rim and get injuries/cuts.

·       Reported to the teacher

·       Swept up the fragments glass with dustpan or brush.

·       Discarded any chipped or cracker

·       Did not use finger to clean up broken glass.

Hydrochloric acid


·       Spillage HCl

·       Skin and eyes might be irritated, (get burn).


·       Avoided direct skin contact.

·       Washed off skin under cool water immediately if got burn.

·       Wore safety goggle while doing experiment.



·       Water spilled onto the floor could lead to slippering hazard.

·        Avoided walking through the water area.

·       Used mop to clean the floor

·       Report to the teacher



DATA 1:  Noticable change in this data was the first 1.5 minutes, the 2M acid produced less amount of CO2 gas than 1M acid, which also meant 2M acid had the lower rate of reaction over this time than the 1M acid. However, the rest of the data was normal as it was expected, which support the hypothesis that as the control variables are kept constant, the higher the concentration, the faster the rate of reaction.

GRAPH:  Illustrate the volume of CO2 gas produced of CaCO3 marble chips in different concentrations of HCl acid as time progresses with linear trendline. 

Note: the volume shown in the table are averages taken from 3 trials (see calculation in “Sample calculation”.

Sample Calculations:

2M concentration of HCl (1 minutes):

  • Average of volume: =
  • Rate of reaction:
  1. Concentration HCl 2M:
  • Rate of reaction = 212/7 = 30.29 mL/min
  1. Concentration HCl 1M:
  • Rate of reaction = 170/7 = 24.29 mL/min
  1. Concentration of HCl 0.5M:
  • Rate of reaction = 52/7 = 7.43 mL/min

Note: all calculations were rounded to 2 decimal places

Interpretation of Data:

 The graph represents the volume of CO2 gas (Dependent Variable) produced by changing concentrations of HCl (Independent Variable) dropped onto the CaCO3 over time (Control Variable). It means the rate of reaction could be determine by the volume of CO2 gas produced. The volume of CO2 gas (mL) from 0 to 250 mL was plotted on the Y-axis; whereas, the X-axis shows the time (from 1 to 7 minutes). There were three concentrations of HCl used (Independent Variable): 2M-blue line, 1M-orange line and 0.5M-grey line, which led to the changing rate of reactions. The other control variables are temperature and surface area which were kept constant in order to make the concentration the important aspect of the study.

It was expected that the volume of CO2 gas produced for the first 1.5 minutes would be significantly larger for the 2M HCl than for 1M HCl; but it was lower by 8 mL over this time. However, the trendline was matched as it was expected, the 2M acid (32.67mL/min) had a steeper gradient than the 1M acid (26mL/min) so that the rate of reaction was higher for the 2M HCl by 6.67mL/min. According to collision theory, by increasing the concentration, the number of particles in a given volume is increased so they collide more frequently, which leads to the reaction rate increasing (Cognito,2019), although, the initial average volume for 2M HCl (58 mL) was a notable exception being lower than 1M HCl (66 mL). The value of 58 mL for the 2M HCl concentration was an abnormality as it was substantially lower than expected probably due to the smaller surface area of marble chips(As they wear away, the surface area became larger).The trendline illustrates that the steeper the gradient, the faster the rate of reaction, which also support the hypothesis that if the concentration increases, the reaction rate will also increase.                                                                                                                                                              

For the next 5.5 minutes, the 2M acid produced a dramatically increase amount of CO2 (170.33 mL). As well as the 2M concentration’s gradient moved farther away (the farther the gradient, the faster the rate of reaction) up to 30.73mL/min. Likewise, the reaction rate of 1M acid (24.55 mL/min) was significantly lower than 2M acid by 6.18 mL/min over the time frame. The trendline of 0.5M illustrates the lowest rate of reaction as it was dropped to 7.27mL/min and the smallest amount of gas was produced over this time (42 mL).  Similarly, from 6 minutes to 7 minutes, the linear was getting wiser as the reaction rate of 2M acid is the highest (30.29 mL/min). 1M acid had the second highest rate of reaction and the 0.5M, the smallest rate of reaction (24,29 mL/min and 7.43 mL/min), respectively.       

The graph illustrates approximately linear relationships (first-order relationship) for the three concentration (independent variable) through the corresponding of CO2 gas produced and the concentrations. It demonstrates that if other control variables were kept constant and the concentration of HCl increases then the reaction rate increases proportionally.                                 

 Overall, the data’s trend is the higher the concentration of acid, the faster the reaction rate as predicted by the Collision theory (Cognito,2019). In this graph, the blue line – 2M concentration was steeper than the orange line – 1M concentration over the time frame after 7 minutes, which showed a faster rate of reaction. This support the hypothesis as well as the research question that if the surface area or temperature are kept constantly, increasing acid concentration will increase the reaction rate (Nuffield Foundation,2013).


Table 2: Limitations of the evidence and reliability and validity of the experimental process:



Impact on results (reliability or validity?)

Systematic error: Difference in surface area of Calcium Carbonate (control variable)

Difference in surface area will affect the reaction rate of concentration because a high surface area produces a faster reaction rate and low surface area produces a slower reaction rate, which affect the accuracy of the results and decreased the validity.

Systematic error: Measuring cylinder occurred uncertainty ±0.5 mL

For example: 40 mL of HCl would be 40.5 or 39.5 mL.

Affect the volume of HCl which further affect (increase/decrease) the gas produced. Decreasing in validity and reliability.

Systematic error: The time put the cork on the flask may be different each trial which could increase the amount of bubble of gas CO2 released and strongly effect to the volume of CO2 at the end.

As the CO2 released, the rate of reaction will decrease so that may weakness the validity of experiment and reliability of the result at the end.

Random error: Using contaminated instruments, which means using the same flask from last experiment without washing it with distilled water or had not be rinsed yet.

Caused a difference in the number of products that were formed. Water also reacts with HCl and CaCO3, which probably decreased the concentration of acid and the reaction rate will decrease as well.

 The accurate of result decreases validity.

Random error: Meniscus levels, as the HCl was measured by different individuals in each trial allowing different amount of HCl present in the reaction.

Caused difference in the excepted amount of HCl present in the reaction, which led to a higher or lower reaction rate. Effected on the reliability of the experiment, especially the accuracy of reaction rate.

Suggested improvements and extensions:

  • Difference in surface area: To control the surface area, CaCO3 can be grinding down into a powder instead of using large lump marble chips of CaCO3 as powder indicates the same surface area, which will not affect the concentration of HCl.
  • Measuring cylinder: try to put at the same height or eyes level for every trial so that the results are read at the same angle, which decrease the uncertainties of HCl used.
  • Time waste: it might take approximately 10 to 20s to close and open the flask so try to do it quicker and pin the flask at the same time as pouring HCl into the flask before timing in order to decrease the amount of gas escape through without counting time.
  • Contaminated instruments: flask should be washed carefully with distilled water and rinse or residue wiped well before starting a new trial in order to limit the old reagents get into the new one which decrease the accuracy of the final rate of reaction.
  • Meniscus level: HCl acid should be measured by one individual for all trials to limit the error that affect the accuracy of reaction rate.


 In conclusion, the rate of reaction can be affected by changing in concentration if other control variables are kept constant. According to Collision theory, the greater the frequency of successful collisions, the greater the rate of reactions (Nuffield foundation,2013). In this experiment,the highest concentration of HCl (2M) had the highest average rate of reaction (30.624 mL/min) after 7 minutes. While other two smaller concentrations (1M and 0.5M) produced lower average reaction rates were 24.656 mL/min and 7.732 mL/min, respectively (from graph). This demonstrates that the higher the concentration, the higher the rate of reactions. The error while doing experiment can also be reduced by minimised the mistakes while performing experiment (details in Limitation), especially random errors. However, there were some systematic error hard to detect and avoid because it is the uncertainties of measurements from instruments. Thus, Eliminates the amount of random error by repeating the measurements would improve the accuracy for the experiment and the final outcomes.

Reference list:

Fiona Dang

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Nathan Nguyen
2 years ago

Amazing article! Great job Faoni!

1 year ago

a lot of information

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