Post your chemistry questions here. (1 Viewer)

[umz93]

oxidation/reduction states refers to the charge an atom has or the number of extra or reduced electrons an atom has on the outer valence shell.

Thanks but could you please be a bit more specific in terms of what Oxidation is and what reduction is. Also how do we solve questions asking about what has oxidised/reduced the most?

 

[_deloso]

Thanks but could you please be a bit more specific in terms of what Oxidation is and what reduction is. Also how do we solve questions asking about what has oxidised/reduced the most?

Oxidation is when a metal (mostly) or other substances loses an electron/s and will usually turn into an ion- this is known as a reductant (don't get this confused... other people will think that since it is a reductant, then it will reduce).
In contrast, reduction is the gain of electron/s. this known as the oxidant)
now the name is opposite to what it is doing because the name represents what it will do to the other substance. therefore, if something is a reductant (gets oxidised), it will reduce the other substance. If something is an oxidant (gets reduced), it will oxidise the other substance.

just remember OILRIG
OIL- oxidation is loss of electron and RIG- reduction is gain of electron

we can solve questions asking about what has oxidised/reduced the most by looking at the standard reduction table (given in the exam)
-the higher it is in the table, the more active is the metal. It is more likely to lose electrons and thus oxidise
-the lower it is in the table, the less active is the metal. it is more likely to gain electron and thus reduce.

 

[funnytomato]

or compare the change of oxidation states

 

[Vidhya]

How does the use of a catalyst affect the Haber process (name an example of a catalyst)? Refer to Le Chatelier's principle etc etc

 

[b3kh1t]

First to the question, mass of 6969 molecules of glucose so find the number of moles; 6969/6.022x10^23 = 1.157x10^-20 moles
therefore the mass of 6969 molecules is; 1.57x10^-20 X (6X12.01+12X1.008+6X16) = 2.095x10^-18 g/molecule

and if the question asks for the mass of a cellulose molecule with 6969 monomers then you must subtract only 6968 molecules of water as the monomers on the end of the molecule do not decompose there hydrogen and hydroxyl group to form water.
therefore 6969 x (6X12.01+12X1.008+6X16) - 6968 X ( 2X1.008+16) = 1129971.676 g/mol or 1.13x10^6 g/mol
then if you would like to find the mass of one molecule of this cellulose polymer then 1.13x10^6/6.022x10^23 = 1.876x10^-18 g/molecule (that is where ohexploitable got her answer from I assume)

 

[b3kh1t]

PS the polymerisation for cellulose in Conquering Chem page 23 has the info and page 24 has an example question (question no. 24)

 

[b3kh1t]

How does the use of a catalyst affect the Haber process (name an example of a catalyst)? Refer to Le Chatelier's principle etc etc

the catalyst used in the Haber process is the magnetite . this catalyst is needed because if the temperature of reaction becomes to high the yield of ammonia becomes to low for desirable yields. Although a higher temperature will provide a higher successful collision rate per second, Le Chatelier's principle states a system at equilibrium will try to oppose any change made to it, therefore the reverse endothermic reaction will occur decomposing ammonia. Hence the catalyst is used to reduce the activation energy required and allow the reaction to take place at a lower temperature which will increase the yield of ammonia.

 

[Vidhya]

Another Haber process ques: Describe the changes that would occur when the system is moved to a container of lower volume.

 

[b3kh1t]

Another Haber process ques: Describe the changes that would occur when the system is moved to a container of lower volume.

the equation for the Haber process is .

When the system is moved to container of lower volume the pressure will increase, according to Le Chatelier's principle the equilibrium will try to oppose the change in pressure, therefore the equilibrium will move to the side with less gas molecules, which can be seen from the equation is the right hand side with only 1 gas molecule compared to 4 gas molecules on the left hand side. Thus the equilibrium will shift right providing a higher concentration of ammonia and decreasing the concentration of nitrogen and hydrogen.

 

[Lemiix]

Hey Guys,
I got a few question from my Chemistry Prelim multiple choice that I got wrong and don't understand. Hope you guys could help.
Q1. Which Particles both have an electron configuration of an inert gas.
A) Chlorine atom, chloride ion
B) Sodium atom, sodium ion
C) Helium atom, lithium ion
D) Hydrogen atom, helium atom

Q2.Nitric Oxide (NO) reacts with oxygen to form nitrogen dioxide according to the following equation:
2NO(g) + O2(g) <--> 2NO2(g)
What mass of oxygen is required to produce 23 grams of NO2?
A) 8
B) 16
C) 32
D) 46

Q3. Which molecule would produce similar intermolecular forces to water?
A) SO2
B) NH3
C) H2S
D) CO2

 

[b3kh1t]

Hey Guys,
I got a few question from my Chemistry Prelim multiple choice that I got wrong and don't understand. Hope you guys could help.
Q1. Which Particles both have an electron configuration of an inert gas.
A) Chlorine atom, chloride ion
B) Sodium atom, sodium ion
C) Helium atom, lithium ion
D) Hydrogen atom, helium atom

well the atoms which are inert, can be found in the last coloumn on the periodic table, and the only atoms which are found in this coloumn is the helium atom, therfore it will be C or D. The atom is the form in which it is found naturally in the environment, this is what is shown in the periodic table. Therefore the hydrogen atom will not be inert obviously as it is not in the last column. Thus making the the correct answer C. Also, the ions refer to an atom which are charged, and since lithium is a metal it will become negatively charged as it will lose an electron. The lose of the electron will result in the outer shell now being full, making the ion inert.

Q2.Nitric Oxide (NO) reacts with oxygen to form nitrogen dioxide according to the following equation:
2NO(g) + O2(g) <--> 2NO2(g)
What mass of oxygen is required to produce 23 grams of NO2?
A) 8
B) 16
C) 32
D) 46

23 grams of NO2 = 23/(14+2x16)
= 0.5 moles of NO2
the number of moles of NO2 produced is equal twice the number of moles of oxygen.
therefore no. moles of O2 = 0.5/2
= 0.25 moles of oxygen used
therefore the mass of oxygen used = 0.25x(2x16)
= 8 grams, therefore the correct answer is A.

Q3. Which molecule would produce similar intermolecular forces to water?
A) SO2
B) NH3
C) H2S
D) CO2

A water molecule is composed of hydrogen and oxygen atoms, thus hydrogen bonds, dipole-dipole and dispersion forces are present but the strongest bond of these is the hydrogen bond (note that the only molecules that can form hydrogen bonds are flourine, oxygen and nitrogen; an easy way to remember them is FON). Therefore we need to look for another molecule which can form hydrogen bonds; A is made of sulfur and oxygen which can not form a hydrogen bond, B is made of nitrogen and hydrogen (remembering FON, where N is nitrogen) therefore this molecule may be able to form hydrogen bonds, C is formed of hydrogen and sulfur which can not form a hydrogen bond, D also can not form hydrogen bonds as it is composed of carbon and oxygen.

 

[RANK 1]

ok so now i've got a question for the other guys who are good at chem.

i came across my notes whilst looking at ion testing and i found an interesting one for phosphate where to test for phosphate ions, you react the phosphate ions with molybdate ions and ascorbic acid which will give you a blue molybdenum-phosphate complex

now can someone explain to me, this reaction cause the shit i find on the internet just confuse me.

 

[Carl5]

Ok, so in one book I've used the refluxing apparatus for esterification is directed downwards towards the floor, however, in a multichoice question in my success one book, it says it must be completely vertical. Does it really matter? If so, why?

 

[RANK 1]

Ok, so in one book I've used the refluxing apparatus for esterification is directed downwards towards the floor, however, in a multichoice question in my success one book, it says it must be completely vertical. Does it really matter? If so, why?

the volatile liquids when condensed have to fall back down to the bottom and the reflux apparatus in esterification is around the round bottum flask so it'll have to be vertical unless u clamp the whole thing diagonally.

the book is probably referring to general reflux apparatus which doesnt have to be vertically set up, but for esterification it should be vertical

ie

 

[b3kh1t]

ok so now i've got a question for the other guys who are good at chem.

i came across my notes whilst looking at ion testing and i found an interesting one for phosphate where to test for phosphate ions, you react the phosphate ions with molybdate ions and ascorbic acid which will give you a blue molybdenum-phosphate complex

now can someone explain to me, this reaction cause the shit i find on the internet just confuse me.

There are many tests for phospate ions, one is making the solution alkali and adding barium ions, adding magnesium ions and ammonium ions, and the last is adding ammonium molybdate, , and ascorbic acid (the only purpose for this acid is to provide hydrogen ions), and the complex ion those form however from my knowledge it is a yellow colour and it is called polynuclear phosphomolybdate complex, .

the equation is basically as follows


Simply it must be acidified first but adding the acid and then the ammonium molybdate solution is added, reacting with theh phosphate ion and producing the yellow precipitate.

Um well I don't think you need to know that specific way of testing for phosphate just know the barium ion method or the magnesium and ammonium method.

 

[RANK 1]

There are many tests for phospate ions, one is making the solution alkali and adding barium ions, adding magnesium ions and ammonium ions, and the last is adding ammonium molybdate, , and ascorbic acid (the only purpose for this acid is to provide hydrogen ions), and the complex ion those form however from my knowledge it is a yellow colour and it is called polynuclear phosphomolybdate complex, .

the equation is basically as follows


Simply it must be acidified first but adding the acid and then the ammonium molybdate solution is added, reacting with theh phosphate ion and producing the yellow precipitate.

Um well I don't think you need to know that specific way of testing for phosphate just know the barium ion method or the magnesium and ammonium method.

ty

 

[mathemalia]

First to the question, mass of 6969 molecules of glucose so find the number of moles; 6969/6.022x10^23 = 1.157x10^-20 moles
therefore the mass of 6969 molecules is; 1.57x10^-20 X (6X12.01+12X1.008+6X16) = 2.095x10^-18 g/molecule

and if the question asks for the mass of a cellulose molecule with 6969 monomers then you must subtract only 6968 molecules of water as the monomers on the end of the molecule do not decompose there hydrogen and hydroxyl group to form water.
therefore 6969 x (6X12.01+12X1.008+6X16) - 6968 X ( 2X1.008+16) = 1129971.676 g/mol or 1.13x10^6 g/mol
then if you would like to find the mass of one molecule of this cellulose polymer then 1.13x10^6/6.022x10^23 = 1.876x10^-18 g/molecule (that is where ohexploitable got her answer from I assume)

You don't have to work with moles, do you?
I mean for every 2 units, there is one water condensed out. The first and last units are,however, not connected by both sides. So if they ask for mass for n units of glucose:

m(n cellulose)= M(glucose)*n - M(water)*(n-1)

?

 

[b3kh1t]

You don't have to work with moles, do you?
I mean for every 2 units, there is one water condensed out. The first and last units are,however, not connected by both sides. So if they ask for mass for n units of glucose:

m(n cellulose)= M(glucose)*n - M(water)*(n-1)

?

It's always best to work with moles because equations are done with molar ratios, and with polymerisation of glucose the hydroxyl of the end of one and the hydrogen ion of the other glucose which it will join are removed this produces the water, the end glucose will keep the hydroxl and the other end glucose will keep the hydrogen ion, this is why only one water is removed.