This increase in rate of reaction can be explained by the collision theory. An increase in concentration means there are more HCl particles in a given volume. This increases the frequency of collisions between HCl and Mg. This increases the number of successful collisions and results in a faster rate of reaction. The diagram below demonstrates this idea of increasing the concentration:
As well was the rate at which hydrogen gas is evolved is increased we can expect to see that when the concentration of HCl increases Magnesium dissolving will be much quicker.
Preliminary Work:
Before carrying out this investigation, we carried out similar experiments. We found that on average 32ml of hydrogen gas would be produced to completion of the reaction. This volume is what we would expect to see in our real experiments. We also observed the time taken for the magnesium ribbon to disappear in the acid solution. From this it was found that measuring the volume of gas produced would be a much more accurate indication to see if the reaction has gone to completion. This is because although the magnesium had disappeared, hydrogen gas was still being produced, showing the reaction had not been fully completed.
Apparatus
- 2 Measuring Cylinders (ml)
- Hydrochloric Acid (2M)
- Magnesium Ribbon (each 1.5cm, using two pieces at a time)
- Goggles
- Conical Flask
- Water
- Gas Syringe
- Ruler and Scissors
- Stop Clock
Diagram:
Method
- Using one of the measuring cylinders, measure 50cm³ of hydrochloric acid. Pour the acid into the conical flask.
- Then measure out 2 pieces of magnesium ribbon of 1.5cm, using a ruler, pencil and scissors.
- As soon as you have dropped the two magnesium strips into the acid close the flask using the gas syringe immediately and start the stop clock.
- Observe volume of gas every 5 seconds until 32ml of gas is produced. Record the time taken for this amount of gas to be produced.
- Repeat the above steps but change the water and hydrochloric acid volume ratio. Each time the experiment is repeated increase the volume of water by 10cm3 and decrease the volume of hydrochloric acid by cm³ maintaining a constant overall volume of 50cm³.
Fair Test
In order to keep my experiment a fair test I will have to make sure that I keep the following factors the same: -
The surface area of the magnesium must be maintained. This will be done by using strips of magnesium of equal length from the same length of magnesium. I will also have to make sure that the gas syringe is correctly connected and that it is placed quickly and tightly enough so that no hydrogen gas escapes. To make sure that all my results are as accurate as possible I will do each experiment three times and then take an average to prevent any anomalous results affecting the end conclusion in a big way.
Safety
- Wear goggles when handling acid and during when magnesium is reacted with acid.
- Clear all spillages.
- Avoid any contact with the acid.
Results:
Other Observations
As we decreased the concentration of acid, the time taken for the magnesium strip decreased. Also heat was evolved during the reactions suggesting that it was an exothermic reaction. In an exothermic reaction more energy is produced from the formation of new bonds than energy required to break existing bonds.
Analysis
The graph shows a linear relationship between the time taken for hydrogen gas to be evolved and the volume of water in the overall solution of hydrochloric acid. The graph shows that as the volume of water decreases and the volume of acid increases the time taken to produce 32ml of gas decreases. At 0cm³ of water the time taken to produce 32ml of hydrogen gas is 37.5s and at 30cm³ time taken is 75s. This means that hydrogen gas is being evolved much faster when the concentration of HCl increases and therefore the rate of reaction increases. This proves that our hypothesis is correct. An increase in volume of acid in the solution meant there was increase in concentration of HCl. Therefore there were more particles in a given volume. Therefore collisions between HCl and the magnesium occurred more often and so more collisions above the activation energy occurred increasing the rate of reaction.
Our graph can also enable us to find out if the decrease in concentration of HCl is proportional to the increase in time taken to produce 32ml of gas.
The table above shows that as the volume of acid decreases the time taken for hydrogen gas to be evolved increases. The table also shows that when the volume of acid decreases up to 20cm³ the difference between times decreases. This gives an indication of the rate of time taken for gas to be evolved. As the difference is decreasing the rate at which the volume of gas is being evolved is decreasing.
Evaluation:
The result of this investigation shows overall it was a successful investigation. However there was an anomaly, which suggests that there were errors in this investigation, which could be refined. This anomaly is circled on the graph. There could be a number of explanations for this anomaly. The time taken for the gas to evolve 32ml of hydrogen gas took longer than expected. When attaching the gas syringe to the conical flask, gas may have escaped into the atmosphere decreasing the amount released into the syringe. Also errors may have occurred when measuring the volume of acid and water to make up the solution. Another factor that may have caused this anomaly is the fact that they may have not been enough hydrogen in the solution to release a sufficient amount of gas. Also although the surface area of the magnesium appeared to be the same, the mass maybe significantly different, of lengths of magnesium strips. This could have the effect of producing more hydrogen gas, as there is now more reactant if the mass were greater.
Other than our anomaly, the rest of the results proved to be reliable as they all fit on the best line of fit and so a suitable procedure was used. It enabled us to draw a firm conclusion. Also they were accurate. Using a gas syringe enabled us to see the amount of gas evolved. This was a better method than just recording the time taken for the magnesium to disappear, as this would increase human error.
The investigation could be improved in a number of ways. o ensure that no hydrogen gas had escaped into the atmosphere whilst attaching the syringe we could keep the reactants separate whilst setting up the apparatus. We could also improve reliability by repeating the experiments a third time and then finding an average. This would help emphasise consistency of the results. To increase reliability we could have increased the range of concentrations. This would give a better line of best of it and accentuate the trends already shown. We could also investigate other factors like temperature and surface area and see their effects on the rate of reaction. For temperature we could carry out a series of experiments by doing the reactions in water baths of different temperatures. For surface area, the magnesium strips could be broken down into smaller pieces.
