Concentration & Rate Factors Lab Answers

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) -> MgCl_2(s) + H_2(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 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 HCl	Trial #	Initial mass	Time	Mass	Final mass
2 mol/L	1		30 sec
60 sec
90 sec
2		30 sec
60 sec
90 sec
1 mol/L	1		30 sec
60 sec
90 sec
2		30 sec
60 sec
90 sec
0.5 mol/L	1		30 sec
60 sec
90 sec
2		30 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 was 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 HCl	Trial #	Initial mass	Time	Mass	Final mass
2 mol/L	1	100.91g	30 sec	100.67g	100.45g
60 sec	100.56g
90 sec	100.45g
2	102.61	30 sec	102.40g	102.14g
80 sec	102.26g
90 sec	102.14g
1 mol/L	1	99.82g	30 sec	99.76g	99.48g
60 sec	99.72g
90 sec	99.69g
120 sec	99.65g
150 sec	99.62g
180 sec	99.59g
210 sec	99.56g
240 sec	99.54g
270 sec	99.52g
300 sec	99.50g
330 sec	99.49g
345 sec	99.48g
2	100.24g	30 sec	100.20g	99.92g
60 sec	100.15g
90 sec	100.12g
120 sec	100.08g
150 sec	100.05g
180 sec	100.02g
210 sec	99.99g
240 sec	99.97g
270 sec	99.94g
300 sec	99.93g
330 sec	99.92g
0.5 mol/L	1	96.45g	30 sec	96.43g	96.17g
60 sec	96.42g
90 sec	96.40g
120 sec	96.39g
150 sec	96.38g
180 sec	96.37g
210 sec	96.36g
240 sec	96.35g
270 sec	96.34g
300 sec	96.33g
330 sec	96.32g
360 sec	96.31g
390 sec	96.31g
420 sec	96.29g
450 sec	96.26g
480 sec	96.25g
510 sec	96.24g
540 sec	96.23g
570 sec	96.23g
600 sec	96.22g
630 sec	96.21g
660 sec	96.20g
690 sec	96.20g
720 sec	96.19g
750 sec	96.17g
780 sec	96.17g
2	101.42g	30 sec	101.41g	101.15g
60 sec	101.40g
90 sec	101.39g
120 sec	101.38g
150 sec	101.37g
180 sec	101.36g
210 sec	101.35g
240 sec	101.32g
270 sec	101.32g
300 sec	101.30g
330 sec	101.30g
360 sec	101.29g
390 sec	101.28g
420 sec	101.27g
450 sec	101.26g
480 sec	101.25g
510 sec	101.24g
540 sec	101.23g
570 sec	101.22g
600 sec	101.21g
630 sec	101.20g
660 sec	101.19g
690 sec	101.18g
720 sec	101.16g
750 sec	101.16g
780 sec	101.15g

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 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 on 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)	100.67g	100.45g	0.22g	90	0.0098755 Mol/L/Sec
2 Molar (Trial 2)	102.40g	102.14g	0.26g	90	0.0116711   Mol/L/Sec
1 Molar (Trial 1)	99.76g	99.48g	0.28g	345	0.0032788   Mol/L/Sec
1 Molar (Trial 2)	100.20g	99.92g	0.28g	330	0.0034279   Mol/L/Sec
0.5 Molar (Trial 1)	96.43g	96.17g	0.26g	780	0.0013466   Mol/L/Sec
0.5 Molar (Trial 2)	101.41g	101.15g	0.26g	780	0.0013466   Mol/L/Sec