Question:

What effect does the concentration of a substance have on the overall rate of a reaction?

Hypothesis

A higher concentration of a given substance may cause a reaction to proceed at a faster rate when compared to a reaction with a lower concentration of the same given substance.  This would be because there would be more acid molecules to collide with the magnesium atoms. Therefore, an increase in concentration would cause an increase in possible collisions, thus there may be a faster reaction rate.

Reaction

2HCl(l) + Mg(s) -> MgCl2(s) + H2(g)

The reaction between magnesium metal and hydrochloric acid is suitable for this particular experiment. The concentration of hydrochloric acid can easily and safely be adjusted by adding water to the solution. The reaction produces a gas, so the reaction rate can easily be determined by measuring the amount of gas released as the reaction proceeds. Furthermore, the progression of the reaction can be seen, without difficulty, as the amount of magnesium metal decreases visibly until completely diminished.

Materials

  • Electronic balance
  • Weighing boats
  • Timer
  • 40mL 8.9 mol hydrochloric acid
  • 1.5g magnesium metal
  • Distilled water
  • 100mL Erlenmeyer flask
  • 10mL graduated cylinder
  • 50mL graduated cylinder
  • 200mL beaker
  • Glass stirring rod

Experimental Design

The first step of this experiment will be to create 3 different samples of hydrochloric acid with different molarities (2 mol, 1 mol, 0.5 mol). To do this, the hydrochloric acid stock solution (with a known molarity) will be mixed with distilled water until the desired molarity has been achieved. 6 50mL samples of hydrochloric acid will be created (2 trials for each molarity). Next, 6 samples of 0.25g of magnesium metal will be mixed with each of the 3 hydrochloric acid solutions (2 trials each). The initial mass (including both the hydrochloric acid and magnesium metal) of each sample must be recorded. As the magnesium metal is added to the hydrochloric acid solution, the timer will be started. The mass of the solution will be recorded at 30-second intervals. Once the reaction is complete (no more bubbles are being released and there is no magnesium metal remaining), the timer is stopped and the final time and mass are recorded. Observations should be recorded in a table similar to the one below:

Molarity of HClTrial #Initial massTimeMassFinal mass
2 mol/L130 sec
60 sec
90 sec
230 sec
60 sec
90 sec
1 mol/L130 sec
60 sec
90 sec
230 sec
60 sec
90 sec
0.5 mol/L130 sec
60 sec
90 sec
230 sec
60 sec
90 sec

Method:

  • To create the 2mol hydrochloric acid solution, 22.4mL of the 8.9mol hydrochloric acid stock solution was mixed with 77.6mL of distilled water in an Erlenmeyer flask and stirred. The solution was then divided into two 50mL samples.
  • To create the 1mol hydrochloric acid solution, 11.2mL of the 8.9mol hydrochloric acid stock solution was mixed with 88.8mL of distilled water in an Erlenmeyer flask and stirred. The solution was then divided into two 50mL samples.
  • To create the 0.5mol hydrochloric acid solution, 5.6mL of the 8.9mol hydrochloric acid stock solution was mixed with 94.4mL of distilled water in an Erlenmeyer flask and stirred. The solution was then divided into two 50mL samples.
  • Six 0.25g samples of magnesium metal were set aside in weighing boats.
  • 50mL of the 2mol hydrochloric acid solution in a beaker and one weigh boat containing 0.25g of magnesium metal were placed on an electronic balance and the initial mass was recorded.
  • The sample of magnesium metal was then added to the hydrochloric acid sample (still remaining on the scale). At the same time, the timer was started.
    **NOTE: Be sure to keep the weigh boat on the scale after adding the magnesium metal. This is so that the initial mass and final mass reflect only the gas being released.
  • Every 30 seconds the mass was recorded in the data table.
  • Steps 5-7 were repeated with the second samples of hydrochloric acid and magnesium metal (using the same molarity of hydrochloric acid).
  • Steps 5-8 were repeated using 1mol hydrochloric acid and then 0.5mol hydrochloric acid.

Observations

Molarity of HClTrial #Initial massTimeMassFinal mass
2 mol/L1100.91g30 sec100.67g100.45g
60 sec100.56g
90 sec100.45g
2102.6130 sec102.40g102.14g
80 sec102.26g
90 sec102.14g
1 mol/L199.82g30 sec99.76g99.48g
60 sec99.72g
90 sec99.69g
120 sec99.65g
150 sec99.62g
180 sec99.59g
210 sec99.56g
240 sec99.54g
270 sec99.52g
300 sec99.50g
330 sec99.49g
345 sec99.48g
2100.24g30 sec100.20g99.92g
60 sec100.15g
90 sec100.12g
120 sec100.08g
150 sec100.05g
180 sec100.02g
210 sec99.99g
240 sec99.97g
270 sec99.94g
300 sec99.93g
330 sec99.92g
0.5 mol/L196.45g30 sec96.43g96.17g
60 sec96.42g
90 sec96.40g
120 sec96.39g
150 sec96.38g
180 sec96.37g
210 sec96.36g
240 sec96.35g
270 sec96.34g
300 sec96.33g
330 sec96.32g
360 sec96.31g
390 sec96.31g
420 sec96.29g
450 sec96.26g
480 sec96.25g
510 sec96.24g
540 sec96.23g
570 sec96.23g
600 sec96.22g
630 sec96.21g
660 sec96.20g
690 sec96.20g
720 sec96.19g
750 sec96.17g
780 sec96.17g
2101.42g30 sec101.41g101.15g
60 sec101.40g
90 sec101.39g
120 sec101.38g
150 sec101.37g
180 sec101.36g
210 sec101.35g
240 sec101.32g
270 sec101.32g
300 sec101.30g
330 sec101.30g
360 sec101.29g
390 sec101.28g
420 sec101.27g
450 sec101.26g
480 sec101.25g
510 sec101.24g
540 sec101.23g
570 sec101.22g
600 sec101.21g
630 sec101.20g
660 sec101.19g
690 sec101.18g
720 sec101.16g
750 sec101.16g
780 sec101.15g

Challenges

One issue that our group encountered when performing this experiment was that when using the 0.5mol hydrochloric acid, the reaction proceeded very slowly and we had not allowed for enough time for the reaction to complete. This could be resolved by using a molarity between 1mol/L and 2mol/L instead of decreasing the molarity to 0.5mol/L.

Sources of Error

In chemistry you can almost never have the perfect environment and perfect accuracy to receive the perfect result. Each experiment, though done multiple times, will almost always have different results. In our experiment there were many such sources of error. The Magnesium metal, when poured into the HCl acid, left its residue of the measuring boat so not every single particle of Mg was used which, in turn, might have caused a minute difference in the final result. Another factor is that we might not have had exactly 50 ml of HCl. Its cohesive properties left residue on the sides of the beaker which might have caused a small difference. The graduated cylinder also is only exact to the 0.01 ml which leaves a 1% margin of error. Also, the hydrogen gas release condensed around the sides of the beaker which might have also had a change in the mass of the final product.

Concentration and How It Affects The Rate of Reaction

According to our findings and the collision theory, concentration clearly affects the rate of reaction. This is because by increasing the concentration of the reactant you increase the number of molecules of the reactant which, in turn, increases the frequency of the collisions. The more particles there are the easier it is to find a particle to collide with, which increases the time it takes for the reaction to occur.

Concentration of HCl (Mol/L)Initial Mass of Reactants

(Grams)

Final Mass of Reactants

(Grams)

Difference in Mass

(Grams)

Time Taken for Reaction to Occur (Seconds)Rate of Reaction

∆M(Mm of H2)X2/Time

2 Molar (Trial 1)100.67g100.45g0.22g900.0098755 Mol/L/Sec
2 Molar (Trial 2)102.40g102.14g0.26g900.0116711

Mol/L/Sec

1 Molar (Trial 1)99.76g99.48g0.28g3450.0032788

Mol/L/Sec

1 Molar (Trial 2)100.20g99.92g0.28g3300.0034279

Mol/L/Sec

0.5 Molar (Trial 1)96.43g96.17g0.26g7800.0013466

Mol/L/Sec

0.5 Molar (Trial 2)101.41g101.15g0.26g7800.0013466

Mol/L/Sec

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