Monitoring Rates -HCl and Mg

Essay by poisonous_acid August 2006

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Monitoring Rates.

You will design and carry out an experiment which allows you to monitor the rate of a reaction as it proceeds.

Background: You will use the reaction between magnesium ribbon and dilute HCl to make hydrogen gas. You will need to keep certain things constant and you will need to make sure that the quantities are suitable. Your group will need to make up its own method and then carry out the experiment. Data will need to be recorded in a suitable manner and analysed and presented in a suitable way.

Mg + 2HCl à H2 + MgCl2

Assessment: Planning (a) - partially (Hypothesis and Variables)

Planning (b)

Data Collection

Data Processing and Presentation

Conclusion and Evaluation

Aim: To observe the effect of time (s) on the rate of the reaction (measured by the

volume of H2 gas produced (mL)) between 4cm of Mg ribbon, and 5mL of 2 moldm-3 HCl.

Hypothesis: The rate of reaction is defined as either a decrease in the concentration of reactants per unit time, or the increase in the concentration of products per unit time. For the following reaction Mg + 2HCl à H2 + MgCl2, as the duration of the reaction increases, the rate of reaction will increase rapidly at first, decrease as time proceeds and eventually stop, as demonstrated by the volume of H2 gas produced. This is because there increasingly fewer H+, Mg2+ and Cl- particles available to collide (with the correct orientation and minimal activation energy) and react with, as they are being used up, with perhaps Mg as the limiting reagent.


INDEPENDENT - is changed.

For this particular investigation, we were only aiming to observe the effect of time on the rate of reaction between the strip of Mg and dilute HCl, hence we did not have an independent variable.

;DEPENDENT - is measured.

Volume of H2 gas produced when a 4cm Mg strip has completely dissolved in 5mL of 2moldm-3 HCl - this is a direct way of measuring the rate of reaction between Mg and dilute HCl. When the Mg strip had completely dissolved in 5mL of 2 moldm-3 HCl, approximately 75mL of hydrogen gas was eventually produced. To make sure no hydrogen gas escaped during the reaction, a rubber bung with a delivery tube was used to seal the testube holding the 4cm Mg strip and the added 5mL of 2moldm-3 HCl. The tube directed the hydrogen gas up into an inverted graduated 100mL measuring cylinder standing in a 2L ice-cream container, both of which were filled with water. Hydrogen gas is insoluble in water, so it pushed some water out of the measuring cylinder, and we were able to take regular readings from the water level in the measuring cylinder.

· FIXED/ CONSTANT - kept the same.

o Air pressure - if the air pressure varied during this investigation, our results would have been affected as different volumes of hydrogen gas would have been obtained. We assumed that the air pressure would remain constant/ or vary insignificantly during this investigation, however to take extra precautions and minimise the effect of air pressure on our results, we performed our actual experiment in the same day, in the same room within a short time of 15 minutes. We should have used a barometer to check the air pressure regularly before, during, and after the investigation.

o Temperature the experiment was performed at - reaction rates increase at higher temperatures because the reacting particles have more kinetic energy hence they move faster, collide more frequently, and with more energy hence a larger proportion of particles have sufficient energy (i.e. more than the minimal activation energy required for a reaction). We were only concerned with observing the rate of reaction as the duration of the reaction increases, so we endeavoured to maintain a constant temperature by performing the experiment in the same room in the shortest time possible, without the influence of fans or heaters. We also used a water bath filled with tap water at room temperature (21ºC), although our faulty method (see method and evaluation below) did not allow the HCl to equilibrate with the temperature of the water in the water bath. We also changed the water in the water bath after every trial as we discovered the reaction between Mg and HCl to be exothermic, so this may have increased the temperature of the surrounding water slightly, which in turn could potentially increase the reaction rate of subsequent trials.

o Type of reactants used - the type of acid (weak/ strong) and the type of metal (position on the reactivity series) will produce different results, therefore it was crucial that we used the same Mg ribbon (the same metal at least!) and 2 moldm-3 HCl throughout the investigation.

o Surface area of the 4cm strip of Mg ribbon - a larger surface area (i.e. cutting the Mg ribbon into smaller pieces) exposes more particles, so there are more available to collide and react with, hence the frequency of collisions and therefore the reaction rate increases. We only cut the sandpapered Mg ribbon into equal lengths of 4cm for each trial, and hence kept and used the surface area already provided.

o Condition of 4cm Mg strip used - this is closely related to above. This specifically refers to whether the 4cm Mg strips were sanded immediately before performing each trial. Foreign substances, oxide coatings and impurities may affect results, so we tried to reduce this (apart from impurities) by sanding the 4cm Mg strips immediately before use.

o Length of Mg ribbon - as mentioned before, the concentration of reactants will affect the results (e.g. unless the HCl was a limiting reagent, increasing the length of the Mg ribbon would most likely increase the volume of hydrogen gas produced), and because we only wanted to observe the reaction rate over time, we assumed that the width of the Mg ribbon provided was consistent throughout the entire coil (although we could have measured this), we cut 3 4cm lengths of Mg to use throughout the investigation.

o Concentration of HCl used - increasing the concentration of reactants means that there is a higher number of reacting particles to collide with, hence the frequency of collisions and reaction rate increases, and vice versa for decreasing the concentration of reactants. After three trials with different concentrations, we found that 5 moldm-3 HCl and 3 moldm-3 HCl was too strong (excessive number of H+ particles to collide and react with the Mg), and we could not take measurements fast enough to produce an informative graph, so we used 2 moldm-3 HCl (by adding 2mL of 5moldm-3 HCl to 3mL of distilled water) throughout the investigation.

o Volume of 2 moldm-3 HCl used in each trial (5mL) - using a small volume of HCl may cause the 4cm Mg strip to dissolve incompletely (limiting reagent) so we used 5mL of 2 moldm-3 HCl for each trial.

o Time allowed for the reaction to take place - while trialling different concentrations of HCl at the beginning of the investigation, we discovered that the volume of hydrogen gas produced began levelling out after about 80 seconds (by this time, the Mg had completely dissolved, although bubbles were still being produced), and was consistent at 100 seconds (no frothing), so we decided to let the reaction continue for 100 seconds for the actual experiment.

o Handling of the boiling tube containing reactants - physically stirring, shaking etc during the reaction could further speed up the reaction rate because it may cause the reacting particles to move around faster, and according to the collision theory, increase the frequency of collisions between Mg and HCl. We did left the boiling tube containing the Mg strip and HCl in the water bath, and did not handle it during the reaction.

Apparatus: (does not include amounts used in trialling)

· 3 clean boiling tubes

· 12cm Mg ribbon (4cm for each repeat (3 repeats altogether))

· 6mL 5 moldm-3 HCl (2mL for each repeat)

· 9mL distilled water (3mL for each repeat)

· Tap water

· 1 Rubber bung with two holes (one for delivery tube, and the other for a 20mL syringe)

· 1 10mL graduated measuring cylinder

· 1 100mL graduated measuring cylinder

· 1 20mL syringe

· 1 250mL beaker

· 1 100mL beaker

· 2L ice cream container

· Thermometer

· Stopwatch

· Sandpaper

· Ruler

· Scissors


1. Because we were provided with 5 moldm-3 HCl, we needed to dilute it to

2 moldm-3 because the Mg dissolved to quickly to obtain an informative graph. This was done by combining 6mL of 5moldm-3 HCl and 9mL of distilled water together in an 100mL beaker to obtain 15mL of 2 moldm-3 HCl, as 5mL of HCl was needed for each of the three repeats.

2. We are able to directly determine the rate of reaction by measuring the volume of hydrogen gas produced during the reaction, and the time taken for a 4cm piece of Mg ribbon to completely dissolve in 5mL of 2 moldm-3 HCl. The hydrogen gas produced can be collected using an inverted graduated measuring cylinder full of water. As the insoluble hydrogen gas collects, it pushes water out from the measuring cylinder. So, we collected the ice cream container (three-quarters filled with tap water) and 100mL measuring cylinder, and set up the equipment as shown in the diagram below. Note that the inverted 100mL measuring cylinder should be initially completely filled with water. This was done by filling up the measuring cylinder, covering the opening with a hand, inverting it and quickly lowering the measuring cylinder into the ice cream container filled with water, while ensuring that the water inside the inverted measuring cylinder remained, did not escape.

3. We then sanded and cut the 12cm Mg ribbon provided, into 3 strips, each 4cm long. This was rolled up and then placed inside one boiling tube. The boiling tube was sealed with the rubber bung, and the delivery tube was carefully fed up under the inverted graduated measuring cylinder. One member of the group must hold the measuring cylinder perfectly upright so accurate measurements can be taken. The boiling tube was then placed into a 250mL beaker three-quarters filled with water. This is the water bath.

4. We then filled the clean 20mL syringe with 5mL of 2 moldm-3 HCl.

5. The tip of the syringe was inserted into the remaining hole of the bung. One member injected the 5mL of 2moldm-3 HCl into the testube containing the 4cm Mg strip, while the other group member simultaneously started the stopwatch.

6. Every 5 seconds, the timekeeper alerted the other member, who immediately recorded the new volume of hydrogen gas in the inverted measuring cylinder.

7. The timekeeper alerted the other recording member when 100 seconds had passed and the new volume of hydrogen gas in the inverted graduated measuring cylinder was recorded.

8. Empty the water in the water bath, ice cream container and measuring cylinder.

9. Repeat steps 2 to 6 for the remaining two repeats.