Saturday, May 29, 2010
Tuesday, May 25, 2010
Answers to Katy
Hi Katy,
In the problem of amalgam fillings, Saliva is working as the conducting medium, so it's equivalent to the solutions and salt bridge in a galvanic cell
As for the question about the galvanic cell, this is true. The galvanic cell stops when the anode strip depletes or when ions of the cathode deplete.
In the problem of amalgam fillings, Saliva is working as the conducting medium, so it's equivalent to the solutions and salt bridge in a galvanic cell
As for the question about the galvanic cell, this is true. The galvanic cell stops when the anode strip depletes or when ions of the cathode deplete.
Monday, May 24, 2010
Answers to Katy
Hi Katy, concerning the reaction between nitrogen and hydrogen gas to produce ammonia
N2 + H2 → NH3
The oxidation number of N decreases from zero to -3, this is reduction.
(N + 3e → N-3) × 2 so 2N + 6e→ 2N-3
(H → H+ + 1e) × 6 so 6H → 6H+ + 6e
2N + 6H → 2N-3 + 6H+
so the balanced equation:
N2 + 3H2 → 2NH3
The other question: The cell in which electroplating occurs is called an electrolytic cell. Eectroplating is a form of electrolysis.Whenever you are passing an electric current to start a chemical reaction, electrolysis is involved.
Third question: Electrolysis of water in the presence of KNO3
Cathode 1/2 reaction:
2H2O + 2e → H2 + 2OH-
in this 1/2 reaction, the O.N. of H decreases from +1 to 0. this is reduction. But only 2 hydrogens of the 4 hydrogens in the reactants were reduced. The other 2 hydrogens were neither oxidized nor reduced. In the compound OH-, the oxidation number of H is +1, which indicates that the O.N. of the 2 hydrogens in 2OH- did not change from that of reactant.
Reduction 1/2 reaction:
2H2O → O2 + 4H+ + 4e
in this 1/2 reaction, the O.N. of oxygen increased from -2 to 0, so this is oxidation. On the other hand, the oxidation number of hydrogen in the reactants and products is the same: +1
As for the question about the galvanic cell,
In the anode half cell, the amount of cations in the solution are increasing (remember that the atoms of the metal are being oxidized to cations) , so the amount of positive charges increases, therefore, anions flow from the salt bridge to maintain electric neutrality at this half cell.
conversely, cations flow from the salt bridge to the cathode half cell solution, because the solution is losing cations (cations are reduced to atoms), and the amount of anions in solution (sulfate ions for exampe if the solution is zinc sulfate) is becoming much more than the cation.
If you're question is: why do ions flow from salt bridge to maintain electric neutrality? What makes them flow in the first place?
Well, this would be an advanced question...flow of ions is controlled by a special diffusion called Fickian diffusion...If you like to read about it, you can..but it's not required from you this year
N2 + H2 → NH3
The oxidation number of N decreases from zero to -3, this is reduction.
(N + 3e → N-3) × 2 so 2N + 6e→ 2N-3
(H → H+ + 1e) × 6 so 6H → 6H+ + 6e
2N + 6H → 2N-3 + 6H+
so the balanced equation:
N2 + 3H2 → 2NH3
The other question: The cell in which electroplating occurs is called an electrolytic cell. Eectroplating is a form of electrolysis.Whenever you are passing an electric current to start a chemical reaction, electrolysis is involved.
Third question: Electrolysis of water in the presence of KNO3
Cathode 1/2 reaction:
2H2O + 2e → H2 + 2OH-
in this 1/2 reaction, the O.N. of H decreases from +1 to 0. this is reduction. But only 2 hydrogens of the 4 hydrogens in the reactants were reduced. The other 2 hydrogens were neither oxidized nor reduced. In the compound OH-, the oxidation number of H is +1, which indicates that the O.N. of the 2 hydrogens in 2OH- did not change from that of reactant.
Reduction 1/2 reaction:
2H2O → O2 + 4H+ + 4e
in this 1/2 reaction, the O.N. of oxygen increased from -2 to 0, so this is oxidation. On the other hand, the oxidation number of hydrogen in the reactants and products is the same: +1
As for the question about the galvanic cell,
In the anode half cell, the amount of cations in the solution are increasing (remember that the atoms of the metal are being oxidized to cations) , so the amount of positive charges increases, therefore, anions flow from the salt bridge to maintain electric neutrality at this half cell.
conversely, cations flow from the salt bridge to the cathode half cell solution, because the solution is losing cations (cations are reduced to atoms), and the amount of anions in solution (sulfate ions for exampe if the solution is zinc sulfate) is becoming much more than the cation.
If you're question is: why do ions flow from salt bridge to maintain electric neutrality? What makes them flow in the first place?
Well, this would be an advanced question...flow of ions is controlled by a special diffusion called Fickian diffusion...If you like to read about it, you can..but it's not required from you this year
Sunday, May 23, 2010
Answer to Katy
Katy, an electrolytic cell is a cell that converts electric energy to chemical energy.
This means, that if an electric current gives rise to a chemical reaction, then we have an electrolytic cell, so the word electrolysis is used.
In the case of electrolysis of water, we gave the cell a voltage that decomposed water into hydrogen gas and oxygen gas.
In the case of electroplating, we gave the cell a voltage so that the chemical reaction of electroplating took place.
In both cases, we have an electric current that is used to allow the occurance of an oxidation-reduction reaction. So both cells are called electrolytic cells
This means, that if an electric current gives rise to a chemical reaction, then we have an electrolytic cell, so the word electrolysis is used.
In the case of electrolysis of water, we gave the cell a voltage that decomposed water into hydrogen gas and oxygen gas.
In the case of electroplating, we gave the cell a voltage so that the chemical reaction of electroplating took place.
In both cases, we have an electric current that is used to allow the occurance of an oxidation-reduction reaction. So both cells are called electrolytic cells
Chemistry Problem (4)
Galvanic effect of the amalgam filling
Amalgam fillings are made of a combination of metals including silver, tin, copper and mercury. Amalgam is extremely durable and able to withstand the grinding and chewing of your back teeth over long periods of time. However, researches have shown that the presence of mercury in amalgam fillings is dangerous, especially when mercury is found in an area close to the human brain. Mercury is extremely toxic to the nervous system, kidneys, and livers. Mercury may result in defects in fetuses for pregnant women. People with an amalgam filling in their mouths have experienced mild electric shocks when the amalgam came in contact with an aluminum foil, aluminum fork or spoon. Since saliva contains electrolytes, it works as a conductor of the formed electric current. The reaction between mercury and aluminum is summarized by the following equation:
Al +Hg+→ Al 3+ + Hg
a) Prove that this reaction is redox.
b) Write the oxidation and reduction half-reactions, then the overall redox reaction.
c) Identify the metal that has more tendency to lose electrons in this reaction, and the metal that has less tendency to lose electrons.
d) Explain the feeling of electric shock in people with amalgam fillings.
e) It is said that Aluminum and mercury form a "battery" effect in your mouth. Explain this statement.
f) Refer to the passage to answer these questions:
i) What is the medium that conducts the formed electric current in your mouth? Why?
ii) Why are amalgam fillings used in dentistry?
iii) What are the toxic effects of mercury to humans?
g) Based on this information, would you approve using amalgam fillings in your teeth? Why?
*picture was taken from naturaldentistry.us
Amalgam fillings are made of a combination of metals including silver, tin, copper and mercury. Amalgam is extremely durable and able to withstand the grinding and chewing of your back teeth over long periods of time. However, researches have shown that the presence of mercury in amalgam fillings is dangerous, especially when mercury is found in an area close to the human brain. Mercury is extremely toxic to the nervous system, kidneys, and livers. Mercury may result in defects in fetuses for pregnant women. People with an amalgam filling in their mouths have experienced mild electric shocks when the amalgam came in contact with an aluminum foil, aluminum fork or spoon. Since saliva contains electrolytes, it works as a conductor of the formed electric current. The reaction between mercury and aluminum is summarized by the following equation:
Al +Hg+→ Al 3+ + Hg
a) Prove that this reaction is redox.
b) Write the oxidation and reduction half-reactions, then the overall redox reaction.
c) Identify the metal that has more tendency to lose electrons in this reaction, and the metal that has less tendency to lose electrons.
d) Explain the feeling of electric shock in people with amalgam fillings.
e) It is said that Aluminum and mercury form a "battery" effect in your mouth. Explain this statement.
f) Refer to the passage to answer these questions:
i) What is the medium that conducts the formed electric current in your mouth? Why?
ii) Why are amalgam fillings used in dentistry?
iii) What are the toxic effects of mercury to humans?
g) Based on this information, would you approve using amalgam fillings in your teeth? Why?
*picture was taken from naturaldentistry.us
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