If all the molecules of acid dissociate completely then the acid has a very low Ph number (1-2), the acid is described as strong.
HCl H+ +Cl-
H2SO4 2H+SO42-
If only a few of the molecules of the acid dissociate then the acid has a Ph of less than 7 but is not as low as 1 or 2 and the acid has as weak.
When a metal which is higher up the reactivity series that hydrogen is placed in an aqueous solution of an acid, the metal can displace the hydrogen from the acid as hydrogen gas and at the same time, the metal forms a cation which combines with the anion from the acid to form a salt.
Reactivity series
Metals can be placed in order of there reactivity. This is called the reactivity series. Magnesium position is quite high up this list. This is because it gives away electrons easily and so it form Ions readily.
Potassium
Sodium
Calcium
Magnesium
Aluminum
Zinc
Iron
Lead
Copper
When a metal like magnesium, which is higher up the reactivity series than hydrogen, is placed in an aqueous solution of sulphuric acid, the metal can displace the hydrogen from the acid as hydrogen Ions gas is given off.
At the same time magnesium forms magnesium cations, which combine with the sulphate anions from the acid to form a salt. Since the hydrogen is in solution, they are moving rapidly and randomly. Some of them collide with a piece of magnesium. If there is sufficient energy in the collision a chemical reaction takes place (collision theory).
The higher the concentration of sulphuric acid, the faster the rate of the reaction with magnesium.
Factors
The factor that I intend in investigate is the effect of concentration on the rate of reaction. I picked this factor because it is easy to measure.
Variables
The independent variable in this investigation is the concentration of sulphuric acid. We want to keep this constant because There will be more hydrogen Ions per unit volume if the concentration of sulphuric acid is increased. This means that there is more chance of a collision between the Magnesium Atoms and the Hydrogen Ions. The more successful collisions that take place with the magnesium atoms per unit of time, the faster the rate of reaction.
The dependent variable in this investigation is the time taken for the effervesce to be completed.
The controlled variables in this investigation the temperature of the sulphuric acid, the volume of acid, extent of swirling and size of magnesium.
Prediction
I predict that the higher the concentration of the sulphuric acid the faster the rate of reaction because the more hydrogen Ions the more collisions there will be and the faster the rate of reaction.
This is what I predict the results will look like;
Rate of reaction is proportional to concentration of acid.
Apparatus
2x retort stands and burette clamps
2x burettes
Sand paper
Ruler
Scissors
Stop clock
Conical flask
2xfunnels
White tiles
Safety goggles
Chemicals
Sulphuric acid (100g/cm3)
Magnesium ribbon (10cm)
Distilled water
Method
- Set up the apparatus as shown above.
- Clean the magnesium ribbon with sandpaper to remove the oxide layer.
- Then cut a 1cm length of magnesium using the scissors.
-
Measure 25cm3 of sulphuric acid using the burette in to the conical flask. N.E read the burette at the eye level from the bottom of the meniscus.
- Then add 1cm of magnesium and start the stop clock.
- Swirl conical flask once and watch for the reaction to be completed. I.e. the effervesce has stop.
- Record the time in seconds in the results table.
- Repeats steps 3-7 with same concentration of acid to ensure the result are reliable.
- Repeat steps 2-8 with the different concentration shown in the results table.
Safety
-
Make sure you are wearing goggles because sulphuric acid is corrosive and if any off it got into your eyes it would be dangerous.
-
A corrosive substance has the of visibly destroying or irreversibly damaging living tissue by chemical action.
- If any acid spills on your hands wash them immediately.
- Make sure you wear goggles at all times.
Strategy for dealing with results.
I will present my results in a graph like the one shown below;
Rate is directly proportional to concentration of acid rate and concentration.
When the concentration of acid is zero, the rate of reaction will also be zero. I expect this shape of graph because I have studied rates of reaction in my double award chemistry course.
I will use the following equations to complete my tables;
(1)concentration of sulphuric acid
(G/dm3)=volume of stock solution X concentration of stock
Total volume of solution solution
How I will insure that results are precise and reliable
- I use the burettes because there are very accurate.
- I read the meniscus at eye level. Using a white tile to help me because I will be able to make sure it is level and that I can see it clearly.
- I sanded the magnesium ribbon to remove the oxide layer.
- I used a stop clock because I wanted to have an accurate and reliable.
- I started clock as soon as the magnesium was added to the sulphuric acid because than when the reaction started.
- I stopped the clock immediately when the effervescence stopped because when the reaction has ended.
- I repeated the experiment to make sure the results were accurate.
- I checked my result to ensure they were consistent. It was necessary to repeat if my results are more than 10 seconds apart, I would repeat the experiment and use the 2 closest and take the average.
I will use the following equations to complete my table;
(1) Concentration of sulphuric acid (g/dm3)
Volume of stock solution X concentration of stock
Total volume of solution
(2) Average time= Time 1 + Time 2
2
(3) Rate= 1000
Average time
Obtaining evidence
(1) Concentration of sulphuric acid (g/dm3)
Volume of stock solution X concentration of stock
Total volume of solution
(2) Average time= Time 1 + Time 2
2
(3) Rate= 1000
Average time
I did not have to repeat any of my experiments for a third time because in each case time 1 and time 2 were within 10 seconds of each other.
Interpreting
From the table of result it is obvious that as the concentration of acid is increased the time taken for the effervesce to stop decrease e.g. at a concentration of 20 g/dm3 of H2SO4 the average time taken equal to 27 seconds at concentration of 10 g/dm3 of H2SO4 the average time taken is for the effervesce to stop is 143 seconds.
I can conclude from my investigation as the concentration of sulphuric acid increase the rate of reaction increases.
From my graph I can say that my prediction is correct because at a concentration of 40 the rate of reaction is 6.5 and at concentration of 48 the rate of reaction is 12 and at a concentration of 60 the rate of reaction is 18 and at a concentration of 80 the rate of reaction is 37 and at a concentration of 100 the rate of reaction is 50. This show that as the concentration increases so does the rate of reaction.
The Collision Theory
For particles to react they must collide with each other with sufficient energy to cause a successful reaction. In the case of Magnesium and sulphuric acid, the magnesium atom must collide with the hydrogen Ions from the sulphuric acid. The collision must have enough energy for the reaction to be successful.
Step 1
The Hydrogen Ions are continually moving around in the solution.
In doing this they will to collide with Magnesium.
Step 2
If the reaction is enough energy from the collision the reaction will happen. Hydrogen gas and Magnesium Ions are produced.
If there is a successful reaction the word equation will be;
Magnesium+ Sulphuric Acid → Magnesium+ Hydrogen
Mg (s) + H²SO4 → Mg SO4 +H2
In a given amount of time if there are a lot of successful collisions, a lot of hydrogen gas will be produced.
The rate of a reaction depends on how many successful collisions there are in a given amount of time.
If there are not many collisions, then the rate of reaction is slow.
The hydrogen Ion bounces away if
The Collisions do not have
Enough Energy. No reaction
Takes place.
The concentration of sulphuric acid.
There will be more hydrogen Ions per unit volume if the concentration of sulphuric acid is increased. This means that there is more chance of a collision between the Magnesium Atoms and the Hydrogen Ions. The more successful collisions that take place with the magnesium atoms per unit of time, the faster the rate of reaction.
See the diagram on the next page
Evaluating
Points which do not fall on the curve of best fit are anomalous results
Above the line.
to fast H2SO4 may have been more concentrated than . should have been of Mg . . to slow
Below the line to slow
Mg to big
H2SO4 to dilute
How ever there is a clear general trend in the graph showing that as concentration increase so does the rate of reaction.
I can conclude my results.
Burettes
Stop clock
Ruler
Scissors X scalpel more accurate
Conical flask prevents acid spray
Swirling X use a magnetic stirrer
Sanding Mg X may not have been evenly removed . affected the amount of Mg present.
Any changes or improvements that I would do would be; repeat each experiment for a third time, take a class set of results and process to ensure the result were more reliable and use a magnetic stirrer instead of swirling.
