BENTUK ORBITAL

Bentuk orbital ditentukan oleh subkulit dari elektron atau ditentukan bilangan kuantum azimutnya. Jadi, apabila suatu elektron memiliki bilangan kuantum azimut sama, maka bentuk orbitalnya juga sama, sehingga yang membedakan hanyalah tingkat energinya. Dengan memahami uraian berikut, akan diketahui bentuk orbital s, p, d, dan f.

a. Orbital s

Orbital yang paling sederhana adalah orbital s. Setiap subkulit s terdiri atas 1 buah orbital yang berisi 2 elektron. Orbital s berbentuk bola simetri yang menunjukkan bahwa electron memiliki kerapatan yang sama, jika jarak dari inti atom juga sama. Semakin jauh letak elektron dari inti atom, kerapatannya semakin rendah. Nilai bilangan kuantum utama suatu orbital memengaruhi ukuran orbital. Semakin besar nilai bilangan kuantum utama, ukuran orbitalnya juga semakin besar.

b. Orbital p

Bentuk orbital p seperti balon terpilin. Kepadatan elektron tidak tersebar merata, melainkan terkonsentrasi dalam dua daerah yang terbagi sama besar dan terletak pada dua sisi berhadapan dari inti yang terletak di tengah.

Subkulit p terdiri atas 3 orbital, tiap orbital mempunyai bentuk yang sama. Perbedaan ketiga orbital terletak pada arah, di mana terkonsentrasinya kepadatan elektron. Biasanya orbital p digambarkan menggunakan satu kumpulan sumbu x, y, dan z, sehingga diberi tanda px, py dan pz.

c. Orbital d dan f

Setiap subkulit d terdiri atas 5 orbital dengan bentuk kelima orbital yang tidak sama. Orientasi orbital d dilambangkan dengan dxy, dxz, dyz, dx2-y2 dan dz2.

Empat orbital mempunyai bentuk yang sama dan setiap orbital mempunyai 4“lobe” kepadatan elektron. Adapun perbedaannya terletak pada arah berkumpulnya kepadatan elektron. Sementara itu, satu orbital lagi mempunyai bentuk berbeda, tetapi memiliki energi yang sama dengan keempat orbital d lainnya.

Orbital f mempunyai bentuk orbital yang lebih rumit dan lebih kompleks daripada orbital d. Setiap subkulit f mempunyai 7 orbital dengan energi yang setara. Orbital ini hanya digunakan untuk unsur-unsur transisi yang letaknya lebih dalam.

Chemical equilibria

Reversible reactions
A reversible reaction is one where there is a forward and backward reaction occurring:
The double arrow signifies a reversible reaction.
If in the above reaction the concentrations of A, B, C, D do not change, although the reaction is still in progress, then the forward rate must equal the backward rate. A situation known as dynamic equilibrium has been reached.
Equilibrium constants
Any dynamic equilibrium can be described in terms of its equilibrium constant, Kc.
The equilibrium constant is the product of the molar concentrations of the products raised to the power of its coefficient in the stoichiometric equation, divided by the product of molar concentrations of the reactants, each raised to the power of its coefficient in the stoichiometric equation.
So for the reaction:
The equilibrium constant is given by:
Where [] represents the concentration of the species in moldm-3.
For gaseous systems, we use Kp instead of Kc. Here, the species are shownin the equilibrium equation in terms of their partial pressures. (In a mixture of gases, the proportion of the total pressure due to a particular gas is dependant on its mole fraction).

2. Le Chatelier Principle
Introduction
Le Chateliers principle states:
The position of the equilibrium of a system changes to minimise the effect of any imposed change in conditions.
This principle applies to any reaction that is in equilibrium.
The effect of concentration changes on equilibrium
Changing concentration of a reactant or product does not change the numerical value of the equilibrium constant, but it does change the position of the equilibrium.
In general, the position of the equilibrium is shifted towards the right if the concentration of a reactant is increased or to the left if the concentration of a product is increased.
At the start, when the change is made, the mixture is not at equilibrium, but equilibrium is eventually restored.
The effect of pressure changes on equilibrium
For a reaction involving gases, altering the pressure may cause a change in the position of the equilibrium.
For a reaction where there is an increase in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the left.
Where there is a decrease in the number of moles from reactants to products, increasing the pressure moves the equilibrium to the right. The equilibrium constant remains the same.
The effect of temperature changes on equilibrium
The change that takes place when temperature is changed depends upon whether the forward reaction is exothermic or endothermic.
If the forward reaction is exothermic then the backward one is endothermic.
If the temperature is increased, the equilibrium moves to the left, since an endothermic reaction will tend to reduce the temperature.
Conversely, if the temperature is decreased then the equilibrium, moves to the right.
The effects of catalysts on equilibrium
A catalyst has no effect on the position of the equilibrium. However, it does increase the rate of both the forward and backward reactions, decreasing the time taken to reach equilibrium.

RATE REACTION

1. The rate of a chemical reaction

What is rate?

Rate is a measure of how fast or slow something is. In chemistry, we speak of a rate of reaction, this tells us how fast or slow a reaction is.

Why do chemists want to know the rate of a reaction?

If you are making a product, it is important to know how long the reaction takes to complete, before the product is produced.

Rate is a measure of a change that happens over a single unit time. That unit time is most often a second, a minute, or an hour.

Reaction between zinc and dilute hydrochloric acid

What we observe over time is that gradually the zinc disappears and bubbles of gas appear. After a few minutes the bubbles of gas form less and less quickly until finally no bubbles appear because all the acid has been used up, some zinc remains.

To summarise, during this reaction zinc chloride and hydrogen gas are been formed at the same time as zinc and hydrochloric acid react.

How to measure rate

Using the reaction between zinc and hydrochloric acid as an example, the following are methods by which you could measure the rate of that reaction.

1. Measure that amount of zinc used up per minute

2. Measure the amount of hydrochloric acid used up per minute

3. Measure the amount of zinc chloride been formed per minute

4. Measure the amount of hydrogen been produced per minute

When choosing which method to measure rate always choose the most straightforward.

In the example above, by far the easiest would be to collect the bubbles of hydrogen and measure its volume.

Methods Used for Measuring Rate

Measuring volume of gas evolved:

To measure the hydrogen gas released in the above reaction we use the apparatus as shown. As the bubbles of gas are given off, the plunger in the syringe moves out as hydrogen gas fills it. After, say every 20 seconds we read the volume of gas in the syringe. The reaction is complete when the syringe no longer moves.

To find the actual rate we plot a graph of volume of hydrogen (cm³) against time (seconds).

Note:

1. The rate is not a constant throughout the reaction - it changes!

2. The reaction is fastest at the start, gradually becoming slower as the reaction proceeds.

3. From the graph, the fastest part of the reaction is shown by the steepest curve.

4. The curve on the graph goes flat when the reaction is complete. This is because, as time goes on the volume of the gas evolved does not change.

Measuring the Rate of Loss of a Gaseous Product:

In the reaction between calcium carbonate (marble chips) and hydrochloric acid we can use the apparatus below to find the rate of reaction.

Marble chips and acid are placed in the flask but separated by a piece of card - preventing the reaction from proceeding. This apparatus is placed on a balance and the mass of the flask and its contents is read.

To start the reaction, the flask is gently lent to one side, causing the card to fall and the marble chips and acid to mix.

A piece of cotton wool is placed in the neck of the flask to allow carbon dioxide gas to escape. As the gas escapes the mass of the flask reduces. Take readings of mass loss over a time interval, e.g. 30 seconds.

To find the actual rate we plot the loss in mass (grams) against time (seconds)

As with the previous experiment, the steepest part of the curve is at the start, hence the fastest part of the reaction is at the start.

Gradually the curve becomes less and less steep as the reaction slows down. Eventually a flat curve appears indicating the end of the reaction.

2. Changing the rate of a reaction

How to change the rate of a reaction

There are 4 methods by which you can increase the rate of a reaction:

1. Increase the concentration of a reactant.

2. Increase the temperature of the reactants.

3. Increase the surface area of a reactant.

4. Add a catalyst to the reaction.

Before, we discover the reasons for the above causing an increase in rate, we must first look at what is needed to cause a reaction to occur!

If we take the reaction between magnesium and hydrochloric acid, in order for them to react together:

1. They must collide with each other

2. The collision must be with sufficient energy.

The rate of a reaction depends on how many successful collisions there are in a given unit of time.

The Effect of Concentration

If the concentration of acid (a reactant) is increased, the reaction proceeds at a quicker rate.

In dilute acid there are less acid particles. This means there is less chance of an acid particle hitting a magnesium particle as compared with acid of a higher concentration.

In concentrated acid there are more acid particles, therefore there is a greater chance of an acid particle hitting a magnesium particle.

Remember: the more successful collisions there are the faster the reaction.

The graph below shows results from two experiments. Experiment A was with concentrated acid and experiment B used dilute acid.

As you can see, the greater the concentration of the acid used in a reaction the steeper the curve and the shorter the reaction time. Hence, these results show that an increase in concentration increases the rate of a reaction.

The Effect Of Temperature

At low temperatures the reacting particles have less energy. When particles are heated they gain energy. The gaining of energy enables the particles to move around quicker, this increases their chance of colliding but also, the increase in energy increases the possibility of a collision occurring with sufficient energy. Therefore rate of reaction increases with increasing temperature.

The Effect of Surface Area

The rate of reaction between magnesium and hydrochloric acid increases as you increase the surface area of the magnesium.

For example: powdered metal (greater surface area) reacts quicker with acid than strips of metal (lower surface area).

The greater the surface area of the metal the more of its particles are exposed to the acid. This increase in exposure increases the frequency of successful collisions.

The Effect of a Catalyst

Some reactions may be speeded up by using a catalyst. A catalyst reduces the energy required for the reactants to successfully collide. The result is more collisions become successful, hence the rate of a reaction increases.

Test Atomic Structure

1. What is the total number of electrons in the 2p sublevel of a chlorine atom in the ground state? (1) 6; (2) 2; (3) 3; (4) 5.
2. Which is the electron configuration of an atom in the excited state? (1) 1s1,2s ; (2) 1s2,2s2,2p1 ; (3) 1s2,2s2,2p; (4) 1s2,2s2,2p2,3s1
3. A Ca (+2) ion differs from a Ca atom in that the Ca (2+) ion has (1) more protons; (2) fewer protons; (3) more electrons; (4) fewer electrons.
4. At the end of 12 days, 1/4 of an original sample of a radioactive element remains. What is the half-life of the element? (1) 24 days; (2) 48 days; (3) 3 days; (4) 6 days.
5. The total number of orbitals in the 4f sublevel is (1) 1; (2) 5; (3) 3; (4) 7.
6. Which electron transition is accompanied by the emission of energy? (1) 1s to 2s; (2) 2s to 2p; (3) 3p to 3s; (4) 3p to 4p.
7. What is the total number of nucleons (protons and neutrons) in an atom of selenium (atomic no.=34, atomic mass= 79)? (1) 34; (2) 45; (3) 79; (4) 113.
8. What is the total number of principal energy levels that are completely filled in an atom of magnesium in the ground state? (1) 1; (2) 2; (3) 3; (4) 4.
9. What is the maximum number of electrons that can occupy the 4d sublevel? (1) 6; (2) 2; (3) 10; (4) 14.
10. Which sublevels are occupied in the outermost principal energy level of an argon atom in the ground state? (1) 3s and 3d; (2) 3s and 3p; (3) 2s and 3p; (4) 2p and 3d.
11. Which electron configuration represents an atom in the excited state? (1) 1s,2s2 ; (2) 1s2,3p1 ; (3) 1s2 ,2s2,2p5 ; (4) 1s2,2s2,2p6.
12. Which element has an atom in the ground state with the most loosely bound electron? (1) He; (2) As; (3) Xe; (4) Cs.
13. The half-life of C-14 is 5730 years. What fraction of a 1 gram sample of C-14 would remain after 17,190 years? (1) 1/2; (2) 1/4; (3) 1/8; (4) 1/16.
14. Isotopes of an element have a different (1) number of electrons; (2) number of protons; (3) atomic number; (4) mass number.
15. A neutral atom of an element has an electron configuration of 2-8-2. What is the total number of p electrons in this atom? (1) 6; (2) 2; (3) 10; (4) 12.
16. A neutral oxygen atom (O) differs from an oxide ion in that the atom has (1) more electrons; (2) fewer electrons; (3) more protons; (4) fewer protons.
17. Which is the electron configuration of a hydrogen atom with an atomic mass of 3 in1the ground state? (1) 1s; (2) 1s2; (3) 1s2,2s1; (4) 1s22,2s.
18. When an electron in an atom of hydrogen moves from the second to the first principal energy level then the result is the emission of (1) a beta particle; (2) an alpha particle; (3) quantized energy; (4) gamma rays.
19. How many occupied sublevels are in an atom of carbon in the ground state? (1) 5; (2) 6; (3) 3; (4) 4.
20. What is the total number of electrons in the 2nd principal energy level of a chlorine atom in the ground state? (1) 5; (2) 7; (3) 8; (4) 17.

Test Acids and Bases

1. The pH of a solution is 2 at 25 °C. What is the pOH of this solution?
   (1) 0; (2) 2:(3) 12; (4) 14.
2. In a solution with a pH of 3 the color of (1) litmus is red; (2) litmus is
   blue;(3) phenolphthalein is red; (4) phenolphthalein is blue.
3. What is the total number of moles of H1+ ions that will neutralize 2.0 moles
   of OH1-ions? (1) 1.0; (2) 2.0; (3) 17; (4) 34.
4. Which salt hydrolyzes in water to form a solution that is acidic? (1) KCl;
   ( 2) NH4Cl;(3) NaCl; (4) LiCl.
5. Which particle is amphiprotic? (1) HNO3 ; (2) NO31- ; (3) NH3 ; (4) NH41+ .
6. As 50. milliliters of 0.1 M HCl is added to 100 milliliters of 0.1 M NaOH the
   pH of theNaOH solution (1) decreases; (2) increases; (3) remains the same.
7. Which compound is an electrolyte? (1) C6H12O6 ; (2) C12H22O11; (3) C2H5OH
   (4) CH3COOH.
8. The hydroxide ion concentration is greater than the hydronium ion concentration in a 0.1 M solution of (1) NaOH; (2) CH3OH; (3) HNO3 ; (4) H2SO4.
9. What is the pH of a solution whose hydronium ion concentration is 0.0001 moles per
   liter? (1) 1; (2) 10; (3) 14; (4) 4.
10. The conjugate base of the bisulfite ion is the (1) sulfide ion; (2) sulfite ion;
    (3) bisulfate ion; (4) sulfate ion.
11. A weak acid (HX) has an equilibrium constant of 1.0 X 10-8. What is the pH of a 1.0
    M solution of this acid? (1) 1; (2) 7; (3) 8; (4) 4.
12. If 6 milliliters of 1M HCl is exactly neutralized by 3 milliliters of KOH then the
    molarity of the KOH is (1) 1M; (2) 2M; (3) 3M; (4) 9M.
13. The pH of 0.001M HCl is (1) 1; (2) 2; (3) 3; (4) 4.
14. NH3 + H2O = NH4 1+ + OH1- Given the above reaction the two Bronsted acids are
    (1) NH3 and H2O; (2) NH3 and OH1- ; (3) NH4 1+ and H2O ; (4) NH4 1+ and OH1- .
15. An organic compound whose water solution turns litmus red is (1) CH3OH; (2) C6H12;(3) CH3COOH; (4) C6H12O6 .
16. Which could act as either a Bronsted acid or a Bronsted base? (1) chloride ion;
    (2) sulfide ion; (3) bisulfide ion; (4) carbonate ion.
17. Which solution will be exactly neutralized by 1.0 liter of 1.0M NaOH? (1) 1.0 liter
    of 0.50M HCl; (2) 1.0 liter of 2.0M HCl; (3) 0.50 liter of 0.50M HCl; (4) 0.50 liter
    of 2.0M HCl.
    17. Which solution will be exactly neutralized by 1.0 liter of 1.0M NaOH? (1) 1.0 liter
    of 0.50M HCl; (2) 1.0 liter of 2.0M HCl; (3) 0.50 liter of 0.50M HCl; (4) 0.50 liter
    of 2.0M HCl.
18. Which pH indicates the highest concentration of hydronium ions? (1) 1; (2) 7;
    (3) 10; (4) 14.
19. Which 0.1M aqueous solution is the poorest conducter of electricity? (1) C2H5OH;
    (2) HCl; (3) H2SO4 ; (4) NH3 .
20. Which of the following 0.1 M solutions is the best conductor of electricity? (1) HNO3
    ; (2) HNO2 ; (3) NH3 ; (4) CH3COOH .
21. Which of the following is the weakest Bronsted acid? (1) HBr; (2) HI; (3) HCl;
    (4) HF.
22. When K2CO3 dissolves in water the resulting solution turn litmus paper (1) red
    and is acidic; (2) blue and is acidic; (3) red and is basic; (4) blue and is basic.
23. A 30 milliliter sample of HCl is completely neutralized by 10. milliliters of a 1.5 M
    NaOH solution. What is the molarity of the HCl solution? (1) 0.25; (2) 0.50;
    (3) 1.5; (4) 4.5.
24. Which could be the pH of a solution whose hydronium ion concentration is less than
    the hydroxide ion concentration? (1) 9; (2) 2; (3) 3; (4) 4.
25. A solution of which metallic ion would have an acid pH? (1) sodium; (2) aluminum;
    (3) potassium; (4) lithium.
26. Which compound is the weakest electrolyte? (1) HCl; (2) HNO3 ; (3) H2S;
    (4) H2SO4 .
27. Which hydrogen ion concentration indicates the most acidic solution? (1) 1 x
    10-11M; (2) 1 x 10-9 M; (3) 1 x 10-7 M; (4) 1 x 10-5 M.
28. Which compound is correctly classified as a salt? (1) KNO3 ; (2) HNO3 ; (3) CH3COOH
    ; (4) C2H5OH .
29. How many milliliters of 2.0 M NaOH are needed to exactly neutralize 50 mL of 2.0
    M HCl? (1) 25; (2) 50; (3) 100; (4) 200.
30. In the reaction H2O + H2O ------> H3O1+ + OH1- water is acting as (1) a Bronsted
    acid only; (2) a Bronsted base only; (3) neither a Bronsted acid nor base; (4) both
    a Bronsted acid and base.
31. Which of the following combinations when dissolved in water would produce an
    acidic buffer? (1) KCl and NaCl; (2) NaOH and HOH; (3) NaCl and HCl;
    (4) CH3COOH and NaCH3COO .
32. When titrating a strong acid and a weak base the stoichiometric point may have a
    pH of (1) 0; (2) 7; (3) 5; (4) 9.
33. Which 0.1 molar aqueous solution contains the highest concentration of OH1- ions?
    (1) CH3OH ; (2) NaOH ; (3) C2H5OH ; (4) NH3 .
34. Red litmus will turn blue when placed in an aqueous solution of (1) HCl; (2) CH3COOH;
    (3) KOH; (4) CH3OH.
35. Which ion is amphiprotic? (1) Cl1- ; (2) HSO41- ; (3) O2- ; (4) NH41+ .
36. A solution at 25 degrees Celsius with a pH of 7 contains (1) more hydronium ions
    than hydroxide ions; (2) fewer hydronium ions than hydroxide ions; (3) an equal
    number of hydronium ions and hydroxide ions; (4) no hydronium ions or hydroxide
    ions.
37. Which acid is almost completely ionized in a dilute solution at 25 °C? (1) CH3COOH;
    (2) H2S; (3) H3PO4 ; (4) HNO3 .
38. A solution of potassium carbonate would have a pH closest to (1) 1; (2) 5; (3) 3;(4) 8.
39. How many liters of 2.5 M HCl are required to exactly neutralize 1.5 liters of 5.0 M
    NaOH? (1) 1.0; (2) 2.0; (3) 3.0; (4) 4.0.
40. What are the Bronsted-Lowry bases in the following reaction:
    H2S + H2O ------> H3O1+ + HS1- ? (1) H2S and H2O; (2) H 2S and H3O1+ ; (3) HS1-and H2O; (4) HS1- and H3O1+ .
41. When additional solid NaCl dissolves in a solution of NaCl in water the pH of the
    solution (1) decreases; (2) increases; (3) remains the same.
42. When hydrochloric acid is neutralized by sodium hydroxide then the salt formed is
    sodium (1) hydrochlorate; (2) chlorate; (3) chloride; (4) perchloride.
43. The ionization constant of 1.8 x 10-5 is for a weak acid. A reasonable pH for a 0.1 M
    solution of this acid would be (1) 1; (2) 9; (3) 3; (4) 14.
44. A 1 molal solution of magnesium chloride has a higher boiling point than a 1 molal
    solution of solution of (1) FeCl3 ; (2) CaCl2 ; (3) BaCl2 ; (4) NaCl.
45. A water solution of which gas contains more hydroxide ions than hydronium ions?
    (1) HCl; (2) NH3 ; (3) CO2 ; (4) SO2 .
46. The following are hydrogen ion concentrations. Which hydrogen ion concentration
    indicates the strongest acid? (1) 1.0 x 10-4; (2) 2.0 x 10-5; (3) 3.0 x 10-6; (4) 4.0 x
    10-7.
47. How many milliliters of 0.200 molar NaOH are needed to neutralize 100 milliliters
    of 0.100 molar HCl? (1) 40.0; (2) 50.0; (3) 100.; (4) 200.
48. Given the following reaction: H2SO4 + H2O ======> HSO41- + H3O1+ . The two
    Bronsted-Lowry acids are (1) H2SO4 and H2O; (2) H2SO4 and H3O1+ ; (3) H2O and
    H3O1+ ; (4) HSO4 1- and H3O1+ .
49. Neutralization of 15 milliliters of a 1.0-molar solution of KOH requires 5 milliliters
    of hydrochloric acid. What is the molarity of the HCl solution? (1) 1.0; (2) 2.0;
    (3) 3.0; (4) 6.0.
50. Which solution is the best conductor of electricity? (1) 1.0 M boric acid; (2) 1.0 M
    carbonic acid; (3) 1.0 M acetic acid; (4) 1.0 M hydrochloric acid.