Monday 9 November 2009

HETEROGENEOUS CATALYSIS

Explain what is meant by the term " heterogeneous catalysis ".
Give an example and explain how the catalyst works.

In heterogeneous catalysis, the physical state of the catalyst is different from that of the reactants.

Example of heterogeneous catalysis is the reaction between gaseous iodine and hydrogen to form hydrogen iodide catalysed by solid platinum.
Pt(s)
H2(g) + I2(g) ------------> 2HI(g)

Solid platinum provides a suitable surface for the adsorption of the hydrogen and iodine molecules. This is possible because platinum has many empty or half-filled d-orbitals that can be used to form temporary bonds with the reacting molecules.

The adsorption process not only increases the concentration of the reactants on the platinum surface, it also weakens the covalent bonds in the molecules which leads to a decrease in the activation energy. Furthermore, the reacting molecules are now in the correct orientation for new bonds to be formed.



----H----H----I----I----H----H----I----I----H----H----
temporary bonds
____________________________________________
Platinum surface

* the bonds between H-H and I-I are being broken.
* the bonds between H-I are being formed


After that, the HI molecules are released from platinum surface so that other reactant molecules can be adsorbed.

HOMOGENEOUS CATALYSIS

Explain what is meant by the term "homogeneous catalysis".
Give an example and explain how the catalyst works.

Homogeneous catalysis is a reaction where the physical states of the catalyst is the same as that of the reactants.

Example of homogeneous catalysis is the reaction between aqueous iodide ion and aqueous peroxodisulphate ion catalysed by aqueous iron(III) ion.
Fe3+
S2O82-(aq) + 2I-(aq) ------------> I2(aq) + 2SO42-(aq)

In the presence of aqueous iron(III) ion, the reaction occurs in two steps.

Step 1 : 2Fe3+(aq) + 2I-(aq) ----------> 2Fe2+(aq) + I2(aq)
Step 2 : 2Fe2+(aq) + S2O82-(aq) ----------> 2Fe3+(aq) + 2SO42-(aq)

The catalysed reaction has lower activation energy because it involves the collision of oppositely charged particles while the uncatalysed reaction involves the collision of like charged particles.

Homogeneous catalyst works by forming an intermediate (Fe2+) and then being regenerated in the following step. Fe3+ ion manages to do so because it can vary its oxidation states between +3 and +2. By doing so, it helps to transfer the electron from I- to S2O82-.

Saturday 7 November 2009

SHAPE OF MOLECULES

For these compounds - H2S, CIF3, XeF4

a) predict the shape of the molecular orbitals around the central atom
b) explore the shape of the actual molecule and give the change in the angle size and the reason
for it

by joeyeehu...

Contributing In:
Chemistry
Best Answer - Chosen by Voters

S(6e) ; 2H(2e) ; Total = 8 electrons = 4 electron pairs
The basic shape is tetrahedral.
But there are 2 bond pairs and 2 lone pairs
So, it is V-shaped with bond angle around 104.5 degree
( the strength of electron pairs repulsion decreases from lone pair-lone pair > lone pair-bond pair > bond pair-bond pair )

Cl(7e) ; 3F(3e) ; Total = 10 electrons = 5 electron pairs
The basic shape is trigonal bipyramidal
But there are 2 lone pairs and 3 bond pairs.
The 2 lone pairs are situated on the trigonal plane because they are 120 degree apart.
So, it is T-shaped.

Xe(8e) ; 4F(4e) ; Total = 12 electrons = 6 electron pairs
The basic shape is octahedral.
But there are 2 lone pairs and 4 bond pairs.
The 2 lone pairs are opposite to each other.
So, its shape is square planar.

POLAR MOLECULE

Why is CBr2Cl2 considered to be polar?

by joeyeehu...

Contributing In:
Chemistry
Best Answer - Chosen by Voters

In order for a molecule to be polar, it must have polar bond and the molecule must be asymmetrical (not symmetry).

CBr2Cl2 is tetrahedral in shape. It has polar bonds in C-Br and C-Cl. Its shape is not symmetrical. Hence, it is a polar molecule.

Although its shape looks symmetrical, in fact it is not in 3D. The dipole moments of the two C-Br bonds cannot cancel out each other. The same is true of the two C-Cl bonds.

Tuesday 3 November 2009

RATE CONSTANT

Resolved Question

Chemistry - rate of constant (k) problem?
For a reaction aA → products, the initial concentration of A, [A]0 = 6.6 M, and the first half-life is 17 s and the second half-life is 34 s.
What is the numerical value of the rate constant k?

by joeyeehu...

Contributing In:
Chemistry
Best Answer - Chosen by Voters

You will notice that the second half life is two times the duration of the first half life. This is the characteristic of second order reaction.

The second order equation is 1/[A] = kt + 1/[A]o

At first half life t', [A] = [A]o/2
substitute into the equation

2/[A]o = kt' + 1/[A]o

kt' = 1/[A]o

k (17) = 1/6.6

k = 8.91 x 10^-3

CONDUCTIVITY IN METAL AND ELECTROLYTE

Resolved Question

Why is it that the conductivity of metals decrease with increase in temperature while that of electrolytes 'increase' with increase in temperature?

by joeyeehu...

Contributing In:
Chemistry
Best Answer - Chosen by Asker

In metal, electron is responsible as the charge carrier. A systematic flow of electrons in one direction will cause the metal to conduct electricity efficiently. An increase in temperature will cause the electrons to get excited and this causes them to move in a not so orderly manner. Hence, they become less efficient as the charge carrier and conductivity decreases.

In electrolyte, it is the ions (both positive and negative) that is responsible as the charge carrier. An increase in temperature will increase the energy of the ions and they will move faster. Hence its efficiency as charge carrier increases and this will cause the conductivity to increase.


Asker's Comment:
thankyou. dats exactly wat i needed to know

EQUILIBRIUM

Resolved Question
Equilibrium Question

CaCO3, CaO and CO2 are at equilibrium. The Following actions are taken:

1. More CO2(g) is added to the system
2. More CaCO3(s) is added to the system
3. Decreased the Volume of the system,
4. Increased the temperature of the system

One or more of there actions lead to a change in CO2(g) concentration after equilibrium is re-established.

Which statements are responsible?

by joeyeehu...


Contributing In:
Chemistry
Best Answer - Chosen by Voters

CaCO3(s) <=========> CaO(s) + CO2(g)

You should use the Le Chatelier's Principle to predict the outcome.

It states that when a system in equilibrium is disturbed, the system will shift its equilibrium position in the direction so as to partially couteract the changes made until a new equilibrium is reestablished.

1. when more CO2 is added, the equilibrium position will shift to the left

2. when more CaCO3(s) is added, it will not change the equilibrium position. The equilibrium
position does not depend on the amount of solid reactants or solid products as long as there is
some solid left in the system.

3. decrease the volume will cause the pressure to increase. Increase in pressure will cause the
equilibrium position to shift to the left as there is less number of moles of gas on the left hand
side of the equation (zero as oppose to one one the right hand side)

4. increase the temperature willl cause the equilibrium position to shift to the right because the
forward reaction is endothermic.So, the answer is 1, 3 and 4

CATHODIC PROTECTION

Resolved Question

Based of the standard reduction potentials provided below, which of the following metals could provide cathodic protection to keep iron metal from corroding?

(Select all that apply.)
Half-Reaction E° (V)

Ag+(aq) + e‾ → Ag(s) +0.80
Co2+(aq) + 2 e‾ → Co(s) -0.28
Cr3+(aq) + 3 e‾ → Cr(s) -0.74
Cu2+(aq) + 2 e‾ → Cu(s) +0.15
Fe2+(aq) + 2 e‾ → Fe(s) -0.44
Ni2+(aq) + 2 e‾ → Ni(s) -0.25
Mg2+(aq) + 2 e‾ → Mg(s) -2.37
Zn2+(aq) + 2 e‾ → Zn(s) -0.76

by joeyeehu...


Contributing In:
Chemistry
Best Answer - Chosen by Asker

For cathodic protection of iron from corroding, all you need is to find a metal which is more electropositive than iron. So, you have to choose the metals that have the standard reduction potentials which are more negative than that of iron.

Since E(Fe2+/Fe) = - 0.44 v, Cr(-0.74), Mg(-2.37) and Zn(-0.76) can be used.
Of course, Mg is the best and most efficient as it has the most negative SRP (most electropositive)

Asker's Comment:
Thank you

ANODISATION

What is the chemical process taking place in sulphuric acid anodizing?

by joeyeehu...


Contributing In:
Chemistry
Best Answer - Chosen by Voters

Anodising is a process to form a uniform layer of aluminium oxide on aluminium. This is done by the electrolysis process using aluminium as the anode, platinum or other inert substance as cathode and sulphuric acid as the electrolyte.

At the cathode, hydrogen gas is produced. 2H+ + 2e ----> H2
At the anode, oxygen gas is produced. 4OH- ---> 2H2O + O2 + 4e

The oxygen gas produced at the anode (aluminium) will react with Al to form aluminium oxide.
1 week ago
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