1. Na2O, Al2O3 and SO3 have melting points of 1275oC, 2072oC and 17oC respectively.
a) Relate the melting points with the structure and bonding present in these oxides. [9]
b) Give an account of the solubility and reactions of these oxides with water.
Give the pH for the solutions formed and write relevant equations. [6]
Answer:
a)
Na2O and Al2O3 have giant ionic structures which are held together by ionic bonds. Strong electrostatic forces of attraction exist between the ions. Therefore Na2O and Al2O3 have high melting points.
Of the two oxides, Al2O3 has a higher melting point than Na2O. This is because the ionic bond between Al3+ ions and O2- ions is stronger than that between Na+ and O2-. The charge on the Al3+ ion is larger than that of the Na+ ion. The lattice energy of Al2O3 > Na2O.
The attractive force between sulphur trioxide molecules, SO3 are weak van der Waals forces of attraction. Therefore, it has a very low melting point.
b)
Na2O dissolves easily in water.
Na2O(s) + H2O(l) ----> 2NaOH(aq)
The solution formed is strongly alkaline. The pH for a strong alkali is 14. The reaction between Na2O and water is exothermic.
Al2O3 is insoluble and does not react with water. However, SO3 dissolves easily in water.
SO3 (g) + H2O(l) -----> H2SO4(aq)
The solution formed is a strong acid. The pH of a strong acid is 1. The reaction between SO3 and water is exothermic.
Wednesday 24 November 2010
stpm 1 (Free Radical Reaction)
2.
Each chlorine atom that is produced from chlorofluoroalkane in the stratosphere can destroy almost a hundred thousand ozone molecules. Explain. [5]
Answer:
Chorofluoroclkanes, such as Freon can initiate a free radical chain reaction.
Initiation step: CF2Cl2 + light (UV) ---> CF2Cl● + Cl●
Propagation step: Cl● + O3 --> ClO● + O2
ClO● + O3 --> Cl● + 2O2
In the propagation step, ozone is destroyed and the chlorine radical Cl● is regenerated. Therefore, one chlorine atom ( in the form of a radical) can destroy thousands of ozone molecules. The chlorine radical can only be annihilated by colliding with another chlorine radical.
Cl● + Cl● -> Cl2
Each chlorine atom that is produced from chlorofluoroalkane in the stratosphere can destroy almost a hundred thousand ozone molecules. Explain. [5]
Answer:
Chorofluoroclkanes, such as Freon can initiate a free radical chain reaction.
Initiation step: CF2Cl2 + light (UV) ---> CF2Cl● + Cl●
Propagation step: Cl● + O3 --> ClO● + O2
ClO● + O3 --> Cl● + 2O2
In the propagation step, ozone is destroyed and the chlorine radical Cl● is regenerated. Therefore, one chlorine atom ( in the form of a radical) can destroy thousands of ozone molecules. The chlorine radical can only be annihilated by colliding with another chlorine radical.
Cl● + Cl● -> Cl2
Friday 8 October 2010
Formation of Stalactites and Stalacmites
Stalactites are formed by the deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions.
Limestone is calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution.
The chemical formula for this reaction is:
CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)
This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down. When the solution comes into contact with air the chemical reaction that created it is reversed and particles of calcium carbonate are deposited.
The reversed reaction is:
Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda straw" stalactite.
Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws." Given enough time, these formations can meet and fuse to create columns.
Concrete stalactites
Stalactites can also form on concrete, and on plumbing where there is a slow leak and limestone (or other minerals) is in the water supply, although they form much more rapidly there than in the natural cave environment
The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide in concrete. This calcium oxide reacts with any rainwater that penetrates the concrete and forms a solution of calcium hydroxide.
The chemical formula for this is:
CaO(s) + H2O(l) → Ca(OH)2(aq)
Over time this calcium hydroxide solution reaches the edge of the concrete and, if the concrete is suspended in the air, for example, in a ceiling or a beam, then this will drip down from the edge.
When this happens the solution comes into contact with air and another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
When this solution drops down it leaves behind particles of calcium carbonate and over time these form into a stalactite. They are normally a few centimeters long and with a diameter of approximately half a centimeter.
Limestone is calcium carbonate rock which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution.
The chemical formula for this reaction is:
CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)
This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down. When the solution comes into contact with air the chemical reaction that created it is reversed and particles of calcium carbonate are deposited.
The reversed reaction is:
Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(aq)
Every stalactite begins with a single mineral-laden drop of water. When the drop falls, it leaves behind the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring. Eventually, these rings form a very narrow (0.5 mm), hollow tube commonly known as a "soda straw" stalactite.
Soda straws can grow quite long, but are very fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite.
The same water drops that fall from the tip of a stalactite deposit more calcite on the floor below, eventually resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws." Given enough time, these formations can meet and fuse to create columns.
Concrete stalactites
Stalactites can also form on concrete, and on plumbing where there is a slow leak and limestone (or other minerals) is in the water supply, although they form much more rapidly there than in the natural cave environment
The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide in concrete. This calcium oxide reacts with any rainwater that penetrates the concrete and forms a solution of calcium hydroxide.
The chemical formula for this is:
CaO(s) + H2O(l) → Ca(OH)2(aq)
Over time this calcium hydroxide solution reaches the edge of the concrete and, if the concrete is suspended in the air, for example, in a ceiling or a beam, then this will drip down from the edge.
When this happens the solution comes into contact with air and another chemical reaction takes place. The solution reacts with carbon dioxide in the air and precipitates calcium carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
When this solution drops down it leaves behind particles of calcium carbonate and over time these form into a stalactite. They are normally a few centimeters long and with a diameter of approximately half a centimeter.
Saturday 2 October 2010
SiO2 and TiO2
How i can selectively dissolve only silica from a mixture of titanium dioxide and silica?
Best Answer - Chosen by Voters
Silica is SiO2. It is an acidic oxide.
TiO2 is basic oxide.
SiO2 being an acidic oxide will dissolve in conc. sodium hydroxide solution whereas TiO2 will not.
Best Answer - Chosen by Voters
Silica is SiO2. It is an acidic oxide.
TiO2 is basic oxide.
SiO2 being an acidic oxide will dissolve in conc. sodium hydroxide solution whereas TiO2 will not.
Rate Law
What is the rate law for the uncatalyzed reaction?
2Ce4++(aq) + Tl+ (aq) --->2Ce3+(aq)+Tl3+(aq)?
Best Answer - Chosen by Voters
You can't simply create a rate law from a chemical equation. The rate law can only be determined by experiments.
There are various ways to determine the rate law by experiment, One way is by altering the concentration of the reactants and determine the effect on the rate of the reaction. By doing so, the order of reaction with respect to the reactant can be asertained.
Repeating the same process with other reactant and finally the overall order of reaction can be determined.
After the overall order of reaction is determined, then only the rate law can be established.
2Ce4++(aq) + Tl+ (aq) --->2Ce3+(aq)+Tl3+(aq)?
Best Answer - Chosen by Voters
You can't simply create a rate law from a chemical equation. The rate law can only be determined by experiments.
There are various ways to determine the rate law by experiment, One way is by altering the concentration of the reactants and determine the effect on the rate of the reaction. By doing so, the order of reaction with respect to the reactant can be asertained.
Repeating the same process with other reactant and finally the overall order of reaction can be determined.
After the overall order of reaction is determined, then only the rate law can be established.
Name of Complex
How to write systematic name of each compound?
eg: k3[Fe(CN)6].?
Best Answer - Chosen by Voters
You have to know the rules in naming the complex ions.
1) the cation is named first followed by the anion.
2) for identical ligands, use di, tri, tetra ect
3) the names of negative ligands end with 'o'
4) the ligands are named first followed by the metal or metal ion.
5) if the complex is cation, the metal is named as usual name
6) if the complex is anion, the name of metal should end with -ate.
7) the oxidation state of metal in the complex should be stated in roman in a bracket
So, K3 is the cation. Just called it potassium.
No need to mention the number of potassium.
[Fe(CN)6]3- is anion.
Six CN- is called hexacyano
Fe3+ is called ferrate because it forms a negative complex.
So, the name is potassium hexacyanoferrate(III)
eg: k3[Fe(CN)6].?
Best Answer - Chosen by Voters
You have to know the rules in naming the complex ions.
1) the cation is named first followed by the anion.
2) for identical ligands, use di, tri, tetra ect
3) the names of negative ligands end with 'o'
4) the ligands are named first followed by the metal or metal ion.
5) if the complex is cation, the metal is named as usual name
6) if the complex is anion, the name of metal should end with -ate.
7) the oxidation state of metal in the complex should be stated in roman in a bracket
So, K3 is the cation. Just called it potassium.
No need to mention the number of potassium.
[Fe(CN)6]3- is anion.
Six CN- is called hexacyano
Fe3+ is called ferrate because it forms a negative complex.
So, the name is potassium hexacyanoferrate(III)
Tuesday 14 September 2010
Which of the following contains an sp2-hybridized carbon?
CH4
CH3-
CH3CH3
CH3+
HC(triple bond)CH
Best Answer - Chosen by Voters
It is CH3+
There are 3 pairs of electrons around the carbon atom.
This suits the sp2 hybridised orbitals of carbon and it will take the shape of trigonal planar.
CH3- is not sp2 hybridised.
It is sp3 hybridised.
There are 4 pairs of electrons around the carbon atom.
Although it is sp3 hybridised, the shape is trigonal pyramidal because there is a lone pair of electrons.
CH4
CH3-
CH3CH3
CH3+
HC(triple bond)CH
Best Answer - Chosen by Voters
It is CH3+
There are 3 pairs of electrons around the carbon atom.
This suits the sp2 hybridised orbitals of carbon and it will take the shape of trigonal planar.
CH3- is not sp2 hybridised.
It is sp3 hybridised.
There are 4 pairs of electrons around the carbon atom.
Although it is sp3 hybridised, the shape is trigonal pyramidal because there is a lone pair of electrons.
DOT AND CROSS DIAGRAM
How do you draw the dot and cross diagrams for:?
Aluminium oxide
Carbon Tetrachloride
andSulfur dioxide?
Best Answer - Chosen by Asker
Aluminium oxide is an ionic compound. So, there is a complete transfer of electrons from aluminium to oxygen atom.
In aluminium oxide ( Al2O3), each atom of Al will transfer three electrons to the oxygen atoms. So, the two Al atoms will transfer a total of six electrons to the three oxygen atoms (each oxygen atom will receive two electrons).
[: Al :]^3+ [ O ]^2-
{All togather there are eight dots around Al and O is surrounded by six cross and two dots. Dots are electrons from Al and crosses are electrons of O}
For carbon tetrachloride, CCl4, Write the C at the centre and is surrounded by four Cl. Between each of the C and Cl is placed a dot and a cross.
Then, six dots around each of the Cl atom.For sulphur dioxide,
SO2, Write the S at the centre and the two O on each side ( in 'V' shaped).
Put two dots and two crosses between S and O.
Then another two dots for S.
Lastly, put four crosses around each O. [ You will fine that there are ten electrons around S.
This is possible because of the expansion of octet]
Asker's Comment:
I managed to get Aluminium oxide and Carbon Tetrachloride before you replied, but I forgot about the expansion of octet and it was troubling me when I went to sleep :)Thanks for your description, I'll do the SO2 as you say it (in my original answer I left the last O atom with only 6 electrons).
Aluminium oxide
Carbon Tetrachloride
andSulfur dioxide?
Best Answer - Chosen by Asker
Aluminium oxide is an ionic compound. So, there is a complete transfer of electrons from aluminium to oxygen atom.
In aluminium oxide ( Al2O3), each atom of Al will transfer three electrons to the oxygen atoms. So, the two Al atoms will transfer a total of six electrons to the three oxygen atoms (each oxygen atom will receive two electrons).
[: Al :]^3+ [ O ]^2-
{All togather there are eight dots around Al and O is surrounded by six cross and two dots. Dots are electrons from Al and crosses are electrons of O}
For carbon tetrachloride, CCl4, Write the C at the centre and is surrounded by four Cl. Between each of the C and Cl is placed a dot and a cross.
Then, six dots around each of the Cl atom.For sulphur dioxide,
SO2, Write the S at the centre and the two O on each side ( in 'V' shaped).
Put two dots and two crosses between S and O.
Then another two dots for S.
Lastly, put four crosses around each O. [ You will fine that there are ten electrons around S.
This is possible because of the expansion of octet]
Asker's Comment:
I managed to get Aluminium oxide and Carbon Tetrachloride before you replied, but I forgot about the expansion of octet and it was troubling me when I went to sleep :)Thanks for your description, I'll do the SO2 as you say it (in my original answer I left the last O atom with only 6 electrons).
VSEPR THEORY
For the rules of VSEPR - predicting shapes of molecular compounds.. please help with question below?
a) predict shape of molecular orbitals around central atom and give the angle/s between orbitals of GeH4, PCl5, SF6 (all numbers are the little numbers on the bottom (not top))
b) predict the shape of the molecular orbitals around the central atom and explore the shape of the actual molecule - give the change in the angle size and reason for itif anyone could please help assist answering this question or any part of it that would be AWESOME
by joeyeehu...
Best Answer - Chosen by Voters
In VSEPR theory, the electron pairs will be placed as far apart as possible to reduce repulsion.
GeH4:
Ge = 4e
4H = 4e
Total = 8e
= 4 electron pairs
The shape is tetrahedral with a bond angle of 109.5 degree
P = 5e
5Cl = 5e
Total = 10e = 5 electron pairs
The shape is trigonal bipyramidal. It has two types of bond angles: 90 and 120 degree
S = 6e
6F = 6e
Total = 12 e = 6 electron pairs
The shape is octahedral with bond angles 90 degree.
a) predict shape of molecular orbitals around central atom and give the angle/s between orbitals of GeH4, PCl5, SF6 (all numbers are the little numbers on the bottom (not top))
b) predict the shape of the molecular orbitals around the central atom and explore the shape of the actual molecule - give the change in the angle size and reason for itif anyone could please help assist answering this question or any part of it that would be AWESOME
by joeyeehu...
Best Answer - Chosen by Voters
In VSEPR theory, the electron pairs will be placed as far apart as possible to reduce repulsion.
GeH4:
Ge = 4e
4H = 4e
Total = 8e
= 4 electron pairs
The shape is tetrahedral with a bond angle of 109.5 degree
P = 5e
5Cl = 5e
Total = 10e = 5 electron pairs
The shape is trigonal bipyramidal. It has two types of bond angles: 90 and 120 degree
S = 6e
6F = 6e
Total = 12 e = 6 electron pairs
The shape is octahedral with bond angles 90 degree.
LEWIS STRUCTURE AND SHAPE
What is the best lewis structure of CLF2+?
Best Answer - Chosen by Voters
First, calculate the total number of electrons around the central atom,
that is Cl.
Cl - 7e
2F - 2e ( F is the terminal atom, so each atom will contribute one electron )
The total is 8e as you have to minus one electron because of the one positive charge.
So, there are 8 electrons around the central atom Cl. ( 4 electron pairs)
The 4 electron pairs will be placed as far apart as possible around the Cl atom.
Tetrahedral arrangement is the most likely geometry.
Out of the 4 electron pairs, 2 of them will be the bond pairs ( forming bonds with F in Cl-F) and the other 2 will be the lone pairs.
Hence, the Cl atom will be surrounded by 2 lone pairs and 2 bond pairs of electrons and it will take the shape of 'V' or bent.
The Lewis structure: [:F- Cl - F:]+
* There will be two dots each at the bottom and above the F-Cl-F
* For your information, F can only form one bond.
Best Answer - Chosen by Voters
First, calculate the total number of electrons around the central atom,
that is Cl.
Cl - 7e
2F - 2e ( F is the terminal atom, so each atom will contribute one electron )
The total is 8e as you have to minus one electron because of the one positive charge.
So, there are 8 electrons around the central atom Cl. ( 4 electron pairs)
The 4 electron pairs will be placed as far apart as possible around the Cl atom.
Tetrahedral arrangement is the most likely geometry.
Out of the 4 electron pairs, 2 of them will be the bond pairs ( forming bonds with F in Cl-F) and the other 2 will be the lone pairs.
Hence, the Cl atom will be surrounded by 2 lone pairs and 2 bond pairs of electrons and it will take the shape of 'V' or bent.
The Lewis structure: [:F- Cl - F:]+
* There will be two dots each at the bottom and above the F-Cl-F
* For your information, F can only form one bond.
STOICHIOMETRIC PROBLEM
Magnesium reaction can you solve it?
12g of Magnesium reacts with an excess off dilute sulfuric acid. Calculate the volume of gas produced at 25 degrees celcius and 100kPa.
by joeyeehu...
Best Answer - Chosen by Voters
Mg + H2SO4 -----------> MgSO4 + H2
24g of Mg will produce 1 mol of H2
12g of Mg will produce 0.5 mol of H2
Applying ideal gas law,
PV = nRTV = (0.5 x 8.31 x 298 ) / 100 x 1000... = 1.24 x 10^-2 m3
12g of Magnesium reacts with an excess off dilute sulfuric acid. Calculate the volume of gas produced at 25 degrees celcius and 100kPa.
by joeyeehu...
Best Answer - Chosen by Voters
Mg + H2SO4 -----------> MgSO4 + H2
24g of Mg will produce 1 mol of H2
12g of Mg will produce 0.5 mol of H2
Applying ideal gas law,
PV = nRTV = (0.5 x 8.31 x 298 ) / 100 x 1000... = 1.24 x 10^-2 m3
ISOTOPIC AND ATOMIC MASS
Resolved QuestionShow me another »
Difference between relative ISOTOPIC mass and relative ATOMIC mass?
Can anyone tell me is there any difference between relative ISOTOPIC mass and relative ATOMIC mass???
In my class they are introduced separately, but it seems like they are same when I read some other textbooks/reference.
Besides difference, how are they similar to each other??
Please don't copy the paragraphs in Wikipedia "Atomic Mass".........Thanks.
so, do they refer to the same thing or just different??and... what is RIM?
by joeyeehu...
Member since:
15 July 2006
Total points:
Best Answer - Chosen by Voters
Most natural occurring elements can exist in a few isotopic form. Some has two, some has three and some has six ....ect.For example, chlorine can exist as two isotopic forms, namely chlorine-35 and chlorine-37.
We can say that the relative isotopic mass of chlorine-35 is 35 and the relative isotopic mass of chlorine-37 is 37.
To determine the relative atomic mass of chlorine, then you must know the relative abundance of isotope Cl-35 and that of isotope Cl-37. It is found that the relative abundances of Cl-35 and Cl-37 are 75% and 25%.
The relative atomic mass of Cl is the weighted average of the relative isotopic mass of Cl-35 (75%) and the relative isotopic mass of Cl-37 (25%).
So, relative atomic mass of chlorine ={ (75 x 35) + (25 x 37) } / 100..............................… 35.5
So, RIM of element X refers to the relative mass of a particular isotope only.
RAM of element X refers to the average relative mass of all isotopes of X.
1 Rating: Good Answer
0 Rating: Bad Answer
Difference between relative ISOTOPIC mass and relative ATOMIC mass?
Can anyone tell me is there any difference between relative ISOTOPIC mass and relative ATOMIC mass???
In my class they are introduced separately, but it seems like they are same when I read some other textbooks/reference.
Besides difference, how are they similar to each other??
Please don't copy the paragraphs in Wikipedia "Atomic Mass".........Thanks.
so, do they refer to the same thing or just different??and... what is RIM?
by joeyeehu...
Member since:
15 July 2006
Total points:
Best Answer - Chosen by Voters
Most natural occurring elements can exist in a few isotopic form. Some has two, some has three and some has six ....ect.For example, chlorine can exist as two isotopic forms, namely chlorine-35 and chlorine-37.
We can say that the relative isotopic mass of chlorine-35 is 35 and the relative isotopic mass of chlorine-37 is 37.
To determine the relative atomic mass of chlorine, then you must know the relative abundance of isotope Cl-35 and that of isotope Cl-37. It is found that the relative abundances of Cl-35 and Cl-37 are 75% and 25%.
The relative atomic mass of Cl is the weighted average of the relative isotopic mass of Cl-35 (75%) and the relative isotopic mass of Cl-37 (25%).
So, relative atomic mass of chlorine ={ (75 x 35) + (25 x 37) } / 100..............................… 35.5
So, RIM of element X refers to the relative mass of a particular isotope only.
RAM of element X refers to the average relative mass of all isotopes of X.
1 Rating: Good Answer
0 Rating: Bad Answer
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