Mg + 2HCl {r} MgCl2 + H2
Magnesium + Hydrochloric acid {r} Magnesium chloride +Hydrogen
(solid) (aqueous) (aqueous) (gas)
Magnesium will react with Hydrochloric acid, because it is higher in the reactivity series than Hydrogen (see diagram 1.1). When the two chemicals react a displacement reaction will take place and the Magnesium will displace the hydrogen in the Hydrochloric acid forming Magnesium chloride and Hydrogen gas.
Diagram 1.1
Depending on certain factors the rate that this reaction will take place will either increase or decrease. The factors that may affect the rate of reaction (the variables) are as follows:
1) Temperature of the Hydrochloric acid. 2) Concentration of the Hydrochloric acid.
3) Boiling tube shaken or not shaken.
4) Surface area of Magnesium.
5) Presence of catalyst.
6) Type of acid used.
All these factors can change the rate of reaction because of the “Collision Theory”.
This is a Theory that is used to predict the rate of reaction. The collision theory is based on the idea that for a chemical reaction to take place, it is necessary for the reacting particles to collide with each other with enough energy to break or form new bonds between the other particles, which is called a “successful collision.” If when they collide and they do not have enough energy to break or form new bond they will simply bounce off each other, causing an
“unsuccessful collision.”
RECORDING MY EXPERIMENT
There are several ways I could record my experiment .the possible ways are as follows:
Amount of gas evolved
I could use a gas syringe to collect the gas that will be given off in the reaction. I could use these results to calculate the initial (first) rate of reaction.
(This method would be very time consuming) Or
The weight before and after the experiment
(This method would be very time consuming)
I could put the conical flask with the chosen volume of Hydrochloric acid onto a set of accurate electronic scales and record the weight of it. I could then drop a piece of Magnesium into the conical flask and measure the decrease in weight at chosen intervals. The weight of the experiment will decease because as the hydrogen in the Hydrochloric acid is being displaced it is being released an will float up and out the conical flask, the weight change will not be very big, but there will be one.
How long the Magnesium takes to dissolve.
(This is a simple and efficient way to record the result)
I could measure the length of time it takes for the Magnesium to dissolve. (By looking at the Boiling tube and observing the magnesium till it is completely gone in the hydrochloric acid, or, by listening to the fizz of the Magnesium and timing it till the fizzing stopped). The only problem with recording the experiment this way is that you could only calculate the average rate of reaction and not the initial (first) rate of reaction.
Ways to measure the rate of reaction:
- Average rate of reaction
- Initial rate of reaction
I will be using the Average rate to calculate the rate of reaction as it can calculate the average rate.
This is how I am going to recode my results: Table
Diagram 1.2
Results
Factors
I have already mentioned the factors that I will be using; I am now going to into a bit more detail on each specific one.
1) Temperature of the Hydrochloric acid.
Why rate changes with temperature?
If the temperature of the acid is different each time the reaction will not be fair.
At low temperature the particles do not have much energy. When Substances are given heat the particles take in energy. When energy is given to particles they move faster and collide more often. This will give a successful collision. The rate of reaction increases and is faster.
Low Temp High Temp
Acid
Magnesium
2) Type of acid used
If I change the type of acid then the rate of reaction would change. Hydrochloric, Sulphuric and Nitric acid would all produce a different rate of reaction, so if I do change the type of acid then all three kinds would produce a different set of results.
3) Concentration of the Hydrochloric acid
The more concentrated the reactants, the grater the rate of reaction will be, because the higher the concentration of acid then the more acid particles per 10cm³ so more collisions per second there for there will be more successful collisions per second.
More acid less acid
Acid
Water
4) Boiling tube shaken or not shaken.
Shaking the Boiling tube whilst the reaction is taking place can alter the rate of reaction because when the reaction is taking place there is already collisions taking place in the Boiling tube. When the particles gain kinetic energy by movement this helps to make the reaction faster.
Shaken Not Shaken
Acid
Magnesium
5) Presence of catalyst.
A catalyst is a substance, which can alter the rate of reaction, but remains chemically unchanged itself at the end of the reaction. Catalysts usually speed up the reaction. A catalyst, which slows down a reaction, is called a negative catalyst, or inhibitor. Catalysts speed up reactions by providing an alternative pathway for the reaction, i.e. one that has much lower activation energy. More collisions will, therefore, have enough energy for this new pathway.
(Extracts from: Letts study guide, GCSE CHEMISTRY)
Catalysts work best when one of the reactants has a big
surface area.
6) Surface area of the Magnesium.
When of the reactants is a solid, the reaction must take place on the surface area of the solid. By breaking up the solids into smaller pieces, the surface area is increased, giving a greater area of collisions to take place as well as causing an increase in the rate of reaction. Note that the smaller the pieces the larger the surface area of the given amount.
Less surface more surface
area area
Acid
Water
Magnesium
Safety
Safety is a very important part of this experiment because we are working with a corrosive chemical. The things that you will need to do to keep your experiment a safe one for yourself and other students around you are as follows:
-
OBEY ALL LABORATORY RULES
- Wear safety Goggles all the time you are using Hydrochloric Acid.
- Care to eyes and skin and all the other people is always vital and necessary.
- Take care in using glassware since it is sharp when broken and can cut skin.
- Take care when returning all glassware and equipment at the end of the experiment.
- When finished with all chemical make sure you disposal of it safely (down the sink).
- Wash all your equipment after your experiment.
- Don’t leave any thing lying around (e.g. Boiling tube Bench on the table, it might roll off).
- Make sure all the boiling tubes are in the rack.
- Listen to all instructions that are given.
- If you get acid on your hand or skin wash it off immediately.
- When measuring out the different concentration of acid, be careful not spill it onto on your hands or clothing.
Fair test
In order to keep your experiment a fair test you will have to make sure that you keep the following factors the same:
- Volume of differing concentration acid. (Cubic centimetre)
- Length of magnesium.
- Surface area of magnesium.
- Use the same acid (hydrochloric Acid).
- Do the same experiment three times to get an average result.
- Make sure there is no excessive water or acid in the boiling tube left from the previous test.
- Make sure you time the experiment accurately.
- Use one method of recording the result e.g. observing the magnesium till it dissolves away in the hydrochloric acid or by listening to the fizz of the Magnesium and timing it until the fizzing stops.
- Ensure that all equipment is working properly.
Apparatus
The apparatus used during the experiment are as follows:
- Stop watch
- Water
- Hydrochloric Acid
- Magnesium ribbon (2 cm)
- Test tube rack
- Boiling tube
- Ruler
- Scissors
- Measuring cylinder
10. Goggles
Method
Make sure you understand everything before starting the experiment read through the method a couple of times.
I am going to go through the experiment step by step.
Diagram 1.3
For my experiment the first thing I did was to get all the apparatus and set it up (like in Diagram 1.3). Before doing any thing I put on my goggles. It is very helpful if two people do the same experiment together. I decided to work with a partner.
- Collect all apparatus.
- Put on goggles.
- Place the three boiling tubes in the rack.
- Cut the Magnesium in to 2cm sized pieces using a ruler and a pair of scissors (do a few to save time).
- Make a table to record your result (use diagram 1.2)
- Make the concentrated of acid you require using the measuring cylinder (exact 10 cm³).
- Pour the right amount of acid solution in the three boiling tubes.
- Make sure that the stopwatch is on 0.00.
- Ether you or your partner take one of the boiling tube from the rack and hold the boiling tube in the hand
10. Then the same person has to place the 2cm piece of
magnesium over the boiling tube.
11. The other person gets ready to start the stopwatch.
12. Then together, drop the Magnesium and start the
watch when the magnesium hits the surface of the
hydrochloric acid start the stop watch.
13. The person holding the boiling tube immediately
start shaking the boiling tube when the magnesium
is reacting with the hydrochloric acid.
14. Both of you should then observe the magnesium to
notice when it has dissolved.
15. At the time one of you spot the magnesium has
disappeared then without delay stop the stop watch.
16. Record the time on the table.
17. Repeat the same experiment with the same
concentrated of acid (Repeat steps 6 to 16 changing
the concentration of the acid).
18. Record all results and then work out the average.
19. Convert the time in second for all your results.
20. Then work out rate 1/t, and plot the results on a
graph (see diagram 1.4). Make two graphs:
- Time against concentration of acid.
- Rate 1/t against concentration of acid.
GRAPHS
Diagram 1.4
200 Graph 1
100
90
80
70
60
50
40
30
20
10
0M 1M 2M
Concentration of acid
Graph 2
0M 1M 2M
Concentration of acid
Preliminary Work
Before doing the actual experiment I decided to do some preliminary work. This was to tell me the details that I would need to know for my investigation to be successful. I saw from these preliminary investigations that the magnesium ribbon started to react with the hydrochloric acid the moment that I dropped it in. I decided it would be a good idea to start timing the second that I dropped it in. When the ribbon had been eaten away by the acid, it stopped fizzing. I decided that I would stop timing the second that the magnesium had gone. I also discovered from my preliminary experiment that when I used a low concentration of hydrochloric acid, it took a long time for the magnesium ribbon to be eaten away. I decided that it would be impractical to spend time on the low concentrated of hydrochloric acid. I furthermore learnt from my preliminary work that it was sometimes quite difficult to stop timing on the exact moment that the magnesium had dissolved. I decided therefore that I would carry out each experiments three times and find the average time as this would result in more accurate figure.
When magnesium reacts with hydrochloric acid, magnesium chloride is formed. I wrote down the equation to show this:
Magnesium + Hydrochloric acid = Magnesium chloride +Hydrogen
Mg + 2HCl {r} MgCl2 + H2
The equipment I needed for the experiment were:
- Stop watch
- Water
- Hydrochloric Acid 2moler
- Magnesium ribbon (2 cm)
- Test tube rack
- Boiling tube
- Ruler
- Scissors
- Measuring cylinder
- Goggles
I decided to do the experiment, three times each, using all the information that I gained while I was doing my preliminary experiment.
Results
