Assess how changing the electric current in the electrolysis of acidified water affects the rate at which hydrogen gas is produced.
WILLLIAM WEBSTER 5SK CHEMISTRY SC1
Electrolysis - Planning
In this investigation, I will assess how changing the electric current in the electrolysis of acidified water affects the rate at which hydrogen gas is produced. The solution to be electrolysed is made up using acid and water. It is of little consequence what acid is used however in this case I will use Sulphuric acid (H2SO4).
When H2SO4 is put in water it is dissociated and forms ions:
H2SO4 › 2H (2+) + SO4 (2-)
Ions are also present from the water in the solution:
H2O › H (+) + OH (-)
During the electrolysis process, the positive hydrogen ions move towards the cathode and the negative hydroxide and sulphate ions move towards the anode.
At the cathode the hydrogen ions gain an electron. They are discharged and are converted into hydrogen gas:
2H (+) + 2e (-) › H2
At the anode, the hydroxide, not the sulphate ions are discharged. Water and oxygen gas are formed:
4OH (-) › 2 H2O + O2 + 4e (-)
The hydrogen gas can be collected and measured. The greater the volume of hydrogen gas formed over a set period of time, the faster electrolysis is occurring.
In the experiment there are several possible ways of changing the electric current such as changing the voltage or the position of the electrodes within the electrolysis cell. However, it was found from preliminary work that the most effective way to change the current was to change the concentration of the acid solution. The preliminary work showed that the greater the concentration of the acid, the greater the current. Ohm's law states that R(resistance) = V(voltage)
I (current)
Therefore, I = V
R
Therefore, if V is constant, in order to increase I, R must decrease. Inversely, to decrease I, R must increase. The value of R depends on how easy it is for the electric charge to pass through the conductor. Therefore to decrease R, the passage must be made easier and to increase R, the passage must be made more difficult. This can be achieved by making the solution a better or poorer conductor. The preliminary work showed that the greater the acid concentration, the greater the current, therefore an increase in acid concentration will make the solution a better conductor and result in a decrease in resistance.
Method
. Set up the apparatus as shown in the diagram. Make sure that the distance between the electrodes is equal to the diameter of the reaction vessel.
2. Turn on the power supply (D.C. 7V) for 120 seconds (measure the time using the stopwatch) and record the volume of gas produced after 120 seconds and the current (ammeter reading) in the results table.
3. Taking care to ensure a fair test by taking all the possible variables (detailed below) into account repeat the experiment using the different acid concentrations specified in the results table.
4. If a result appears anomalous repeat that particular experiment.
Acid Volume (ml)
Water Volume (ml)
Current (A)
Gas Volume (cm³)
5
245
0
240
5
235
20
230
25
225
30
220
35
215
40
210
45
205
50
200
It will be possible to calculate the rate of hydrogen production per minute by dividing the final volume of gas produced by the total number of minutes that the experiment runs for (2).
Current (A)
Gas volume/2 (rate of H2 production per min)
In order to ensure a fair and accurate test, it is necessary to make sure that the current is the only variable. All other possible variables must be controlled. These include:
* Position of the electrodes within the solution - moving the electrodes closer together is a way of varying the current. It was found in the preliminary work that moving the electrodes closer together caused an increase in the current. As changing the acid concentration, not changing the position of the electrodes is my ...
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Current (A)
Gas volume/2 (rate of H2 production per min)
In order to ensure a fair and accurate test, it is necessary to make sure that the current is the only variable. All other possible variables must be controlled. These include:
* Position of the electrodes within the solution - moving the electrodes closer together is a way of varying the current. It was found in the preliminary work that moving the electrodes closer together caused an increase in the current. As changing the acid concentration, not changing the position of the electrodes is my chosen method for changing the current; the distance between the 2 electrodes must be uniform throughout the whole procedure. In my experiment, it will be constant at the diameter of the reaction vessel. The distance between the electrodes affects the current as the closer they are together the smaller the distance that the electrons, which carry the electric, charge have to travel. Thus, if the electrons have to travel a shorter distance through the solution, they will pass through it at a greater rate (current).
* Voltage - is a way of varying the current. It was found in the preliminary work that an increase in voltage resulted in an increase in current. This is in accordance with Ohm's law (R=V/I) as in any one experiment, R is constant therefore an increase in V must result in an increase in I. As changing the acid concentration is my chosen method for changing the current, the voltage will not be changed and will be kept constant at 7 volts. The voltage affects the current as volts measure how much electrical energy is given to each electron. Greater energy will cause the electrons to move at a greater rate (current).
* Time - the time the hydrogen gas is collected for must be constant in order to be able to calculate the rate of hydrogen production. It will be set at 120 seconds.
* Length and thickness of electrodes - this is a factor which affects the resistance of the circuit. Greater length results in greater resistance as the electric charge metal atoms blocking its passage. Thickness of the wire affects resistance as more electric charge can pass through as thick wire than a thin wire in the same time (i.e. thicker wires have smaller resistances). As, according to Ohm's law, resistance affects current, failure to keep this variable constant may affect my results.
* Quantity of solution - this is another possible variable which may affect the results if not kept constant. My constant value will be 250ml. For example if one experiment was performed with 250ml and one with 300ml of solutions of the same concentration, the rate of hydrogen production from the 300ml experiment would probably be greater as there would be more hydrogen ions present which could be discharged.
* Material of the electrodes - a change in the material the electrodes are made out of may change the current as different electrodes may have different resistances. For example copper electrodes have less resistance than nichrome electrodes. As, according to Ohm's law, resistance affects current, The material of the electrodes must be the same throughout the procedure. I will use the same nichrome electrodes throughout the procedure.
Safety
Acid solution is an irritant. Wear safety specs and wipe up any spillages.
Prediction
I predict that as the current increases, the amount of hydrogen gas produced will increase and thus the rate of hydrogen production will increase. This is because greater current results in a more energetic reaction and causes the ions to move to the electrodes at greater speed. All this results in an increase in the rate of the reaction and thus an increase in the rate of hydrogen production.
I predict that the increase in the rate of hydrogen production, for my range of values, will be directly proportional to the increase in current and that a graph of the two will have the following shape:
The shape of the graph of current vs. total amount of hydrogen produced would be identical to the above.
Electrolysis - Obtaining
The experiment to see how changing the electric current in the electrolysis of acidified water affects the rate at which hydrogen gas is produced was carried out. The results were as follows:
Acid Volume (ml)
Water Volume (ml)
Current (A)
Gas Volume (cm³)
5
245
0.05
0.7
0
240
0.1
.4
5
235
0.14
2.1
20
230
0.16
2.3
25
225
0.18
2.6
30
220
0.2
2.9
35
215
0.21
3.2
Electrolysis - Analysing
The results of the experiment clearly show that as the current increases the volume of hydrogen gas produced increases.
Acid Volume (ml)
Water Volume (ml)
Current (A)
Gas Volume (cm³)
5
245
0.05
0.7
0
240
0.1
.4
5
235
0.14
2.1
20
230
0.16
2.3
25
225
0.18
2.6
30
220
0.2
2.9
35
215
0.21
3.2
As more hydrogen gas is produced as the current increases, electrolysis must be occurring at an increasingly greater rate: i.e. the greater the current, the greater the rate of hydrogen production and the greater the rate of electrolysis.
A graph (graph 1) can be plotted of current Vs volume of hydrogen gas produced. It is clear that the trend can be best represented by a line of best fit. The line of best fit must pass through zero as when the current = 0, electrolysis cannot occur and thus no H2 gas will be produced. As a straight line is produced, the general case of
Y = MX + C can be used to find a relationship between the current and the volume of H2 gas produced:
Y = volume of H2 gas produced,
X = current,
M = gradient of the straight line on the graph of current Vs volume of H2 gas produced = 14.4,
C = the value at which the straight line crosses the vertical (y) axis = 0
Thus:
Y = 14.4X
This means that, in general, the volume of H2 gas produced during electrolysis lasting for 2 minutes is equal to the current multiplied by 14.4. For example, if the current were equal to 0.11 amps, the volume of H2 gas produced in 2 minutes would be equal to (14.4 x 0.11) which is equal to 1.584cm³. This can be checked by drawing a line from the point on the x axis where current = 0.11 vertically to the line of best fit and then drawing another to a point on the y axis from where the first line touches the line of best fit.
GRAPH 1:
As the only variable for each experiment was the current, all other possible variables were kept constant. This means that the time each experiment ran for was the same (2 mins). Thus a value for the rate at which electrolysis occurs when a certain current is passed through the solution can be calculated. By dividing the total amount of H2 gas produced in 2 mins by 2, a value for the amount of hydrogen produced per minute can be calculated. This is shown in the following table.
Current (A)
Gas Volume (cm³)
Gas vol produced per min (cm³)
0.05
0.7
0.35
0.1
.4
0.7
0.14
2.1
.05
0.16
2.3
.15
0.18
2.6
.3
0.2
2.9
.45
0.21
3.2
.6
As the volume of gas produced per minute is half the total volume of gas produced, and:
Total gas vol = 14.4 x current,
Vol gas produced per min (rate of gas production) = 7.2 x current
A graph of rate of gas production Vs current (graph 2) would have exactly the same shape as the total gas volume Vs current graph.
The attached graph would not continue with the same gradient indefinitely. There would come a point where the current would become large enough so as all the hydrogen ions in the 250ml solution would have been discharged within the 2-minute time limit. From this point onwards the gradient of the graph would become 0.
It has been found that an increase in the rate of electrolysis is directly proportional to an increase in current.
During the electrolysis of acidified water, the positive hydrogen ions move towards the cathode and the negative hydroxide and sulphate ions move towards the anode.
At the cathode the hydrogen ions gain an electron. They are discharged and are converted into hydrogen gas:
2H (+) + 2e (-) › H2
At the anode, the hydroxide, not the sulphate ions are discharged. Water and oxygen gas are formed:
4OH (-) › 2 H2O + O2 + 4e (-)
The factors which affect the rate at which the electrolysis process occurs are the abundance of ions in the solution and current. The abundance of ions affects how plentiful the supply of "raw materials" for the process is. The energy for the process is governed by the current. The current is a measure of the speed with which the electric charge passes through the conductor. The greater the speed, the greater the amount of electrical energy which is delivered to the conductor per second. Therefore, greater current results in a more energetic reaction. How energetic the reaction is determines how quickly the ions move towards the electrodes. If more energy is given to the ions, they will move faster.
Therefore, if the ions are moving faster, more will arrive at the electrodes per second. This will result in more ions being discharged and more atoms being formed.
For example, in the production of H2 gas from the electrolysis of acidified water. If:
When the current = Z, X ions arrive at the cathode every second and X hydrogen atoms are produced.
If the current is doubled:
When the current = 2Z, the energy level of the process will double and as /my results show the rate of hydrogen production will double. This means that 2X ions will arrive at the cathode every second and 2X hydrogen atoms will be produced per second.
In my prediction, I stated that I thought that the increase in the rate of hydrogen production would be directly proportional to the increase in current. My result and graphs clearly show that this is indeed the case and that the two are directly proportional. The shape of my predicted graph was uniform with the shape of the graphs plotted from my results. Therefore, it could be said that my conclusions show that my prediction was accurate.
Electrolysis - Evaluation
The experiment performed to assess how current affects the rate of hydrogen production during the electrolysis of acidified water was adequate to allow accurate conclusions to be drawn. The results can be considered as fairly accurate. None of my results were hugely anomalous. The results which were anomalous (i.e. line of best fit did not pass through them) were close to the best-fit line and were basically within the trend. They did not affect the accuracy of the conclusions, as they were not anomalous enough to make the best-fit line a curve as opposed to the straight line that is was found to be. Thus they did not affect my conclusion that an increase in the rate of hydrogen production is directly proportional to the increase in current.
The anomalies are circled in green on graph 2. There are several ways in which they can be explained. One, which can account for the anomalies in which the amount of hydrogen produced is greater than the general trend is the reaction time of the human body. There will be a period of time between the stopwatch showing that the 2-minute time limit has been reached and the power being turned off during which electrolysis still occurs and hydrogen is still being produced. This could be avoided by using some form of automated circuit which only allowed the current to flow for a set period of time. Another factor which can account for anomalies it the fact that the quantities which have to be measured (the current and the volume of H2 gas produced) are very small. The ammeter could only be read accurately to the nearest 0.01 amps and the measuring cylinder used to collect the H2 gas could only be read accurately to the nearest 0.1 cm³. There are several ways in which this could be avoided if the experiment was repeated. One such way would be to use an ammeter and measuring cylinder with a smaller scale. Another would be to start off using a much greater current which would produce more hydrogen gas which could be more easily measured. Another way would be to allow the experiment to run for longer
(i.e. time > 2mins) so that a larger and more easily measurable volume of hydrogen gas would be produced.
Further experiments could be carried out to assess what other factors affect the rate of hydrogen production during the electrolysis of acidified water. One such factor, which could be investigated, could be the effect of temperature of the rate of hydrogen production. It has been shown in this experiment that the rate of hydrogen production is affected by how energetic the reaction is. Increasing the temperature that the electrolysis process occurs at would, in theory, make the reaction more energetic and increase the rate of hydrogen production. For this experiment, current would become a constant and temperature the variable.