A direct-current power supply of low voltage (less than 10 volts) has lost the markings that indicate which
output terminal is positive and which is negative. A chemist suggests that the power supply terminals be connected
to a pair of platinum electrodes that dip into 0.1-molar KI solution. Which of the following correctly identifies the
polarities of the power supply terminals?
The answer is: A gas will be evolved only at the negative electrode.
I thought this:
I2 (s) is reduced to 2I- at the cathode, so no gas there. But how do I know there'd be gas at the anode, the neg. electrode?
The K^+ ions are attracted to the negative electrodes but H2O is easier to reduce than K^+ so you get
2H2O + 2e ==>H2 + OH^-.
I think I get it, but just to clarify...
Because we have a power supply, we know we have an electrolytic cell, which needs energy from an external source. For electrolytic cells, cathodes are considered the negative electrodes. If K+ and H20 go to the cathode, they get reduced - rather, H20 gets reduced because K likes giving away electrons normally. Thus, the water, when reduced, forms hydrogen gas.
Have I filled in all the holes correctly? Also, what role does the platinum play in the problem? Is it just there, sitting?
I think
Well, it seems like you've stumbled upon one of life's greatest mysteries - the magical world of gassy electrodes!
Let me shed some light on this perplexing situation. When the power supply terminals are connected to a pair of platinum electrodes that dip into a 0.1-molar KI solution, a chemical reaction takes place at each electrode. At the cathode (negative electrode), Iodine ions (I-) are reduced to form Iodine gas (I2). However, the real party happens at the anode (positive electrode).
At the anode, water molecules (H2O) are oxidized to form Oxygen gas (O2) and Hydrogen ions (H+). These Hydrogen ions, being a mischievous bunch, really love their KI buddies. So, they team up with the Iodide ions (I-) from the KI solution and become Hydrogen iodide gas (HI).
So, in a nutshell, at the negative electrode (cathode), no gas is evolved, while at the positive electrode (anode), Hydrogen iodide gas (HI) is formed.
Remember, this is just a scientific explanation, so you should take it with a pinch of comedic salt. Gassy electrodes certainly make for a fascinating and amusing topic!
To determine which electrode is positive and which is negative, we can use the concept of electrolysis. In this case, we are using a 0.1-molar KI (potassium iodide) solution.
During the electrolysis of an aqueous solution, positive ions migrate towards the negative electrode (cathode), while negative ions migrate towards the positive electrode (anode).
Since the chemist suggests that the power supply terminals be connected to a pair of platinum electrodes that dip into the KI solution, we can infer that the platinum electrode connected to the terminal that corresponds to the negative power supply terminal is the cathode, while the platinum electrode connected to the terminal that corresponds to the positive power supply terminal is the anode.
Now, let's consider the reduction and oxidation reactions that occur during electrolysis.
At the cathode (negative electrode), the reduction reaction occurs. Here, Iodide ions (I-) will be reduced to iodine (I2) according to the equation:
2I- (aq) + 2e- -> I2 (s)
This reaction does not involve the evolution of gas. Therefore, we can conclude that no gas will be evolved at the cathode.
At the anode (positive electrode), the oxidation reaction occurs. Here, water molecules can undergo oxidation or oxidation of iodide ions could also occur. However, it is important to note that in this particular case, the solution is 0.1-molar KI.
In a KI solution, the oxidation of water molecules is favored over the oxidation of iodide ions. The oxidation of water can produce oxygen gas (O2) according to the reaction:
2H2O (l) -> O2 (g) + 4H+ (aq) + 4e-
Therefore, we can conclude that gas will be evolved at the anode (negative power supply terminal) during the electrolysis process.
In summary, when the power supply terminals are connected to a pair of platinum electrodes dipping into the 0.1-molar KI solution:
- The terminal connected to the platinum electrode where gas is evolved is the negative power supply terminal (anode).
- The terminal connected to the platinum electrode where no gas is evolved is the positive power supply terminal (cathode).
So, the correct answer would be that gas will be evolved only at the negative electrode (anode).