ALL CIE CHEMISTRY DOUBTS HERE !!

[Twinkle Charms]

(i) The material X can be pumice or ceramic used such as brick or broken crockery. Write the chemical name of the substance that might be present in one of these materials.

(ii) Suggest why X needs to be heated strongly?

(iii)The material X becomes black in color. Suggest what substance is responsible for this black color and how it arises in the reaction?

[Hissa]

I have some few questions

1) Increasing the temperature of the propanoic acid solution causes the pH to decrease.
What does this tell you about the enthalpy of dissociation?

2) Why is an indicator usually added in iodine/thiosulphate titrations but not in titrations involving potassium managanate (VII)?

[Alpha]

Just telling you not to wait for Deadly_King; he'll be absent for the week-end.

[Arthur Bon Zavi]

(i) The material X can be pumice or ceramic used such as brick or broken crockery. Write the chemical name of the substance that might be present in one of these materials.

(ii) Suggest why X needs to be heated strongly?

(iii)The material X becomes black in color. Suggest what substance is responsible for this black color and how it arises in the reaction?

I am trying :

(i) Silicon_(IV) carbide (SiC) / Silicon_(IV) nitride (Si₃N₄).

(ii) To overcome it's melting point (their melting points are considerably high)/ To have it in the liquid state.

(iii) Oxygen (or oxygen compound) from atmosphere, by heating.

[Ivo]

How would one attempt Qs. 4, 5 and 6 on the worksheet?  Please click here for the worksheet.

Many thanks in advance!

[Deadly_king]

I am trying :

(i) Silicon_(IV) carbide (SiC) / Silicon_(IV) nitride (Si₃N₄).

(ii) To overcome it's melting point (their melting points are considerably high)/ To have it in the liquid state.

(iii) Oxygen (or oxygen compound) from atmosphere, by heating.

Nice try but I don't think your answer to the second question is correct. It's not necessary that it turns into liquid for combustion to take place.

(ii) For it to reach activation energy.

The rest is good 

[Deadly_king]

I have some few questions

1) Increasing the temperature of the propanoic acid solution causes the pH to decrease.
What does this tell you about the enthalpy of dissociation?

2) Why is an indicator usually added in iodine/thiosulphate titrations but not in titrations involving potassium managanate (VII)?

1) According to the information given an increase in temperature causes the pH to decrease. Decrease in pH implies the solution becomes more acidic, i.e there is more H⁺ present in the solution.

Hence we can deduce that an increase in temperature has caused propanoic acid to dissociate more. In other words the enthalpy change is endothermic. this is because according to Hess's law, an increase in temperature will cause the equilibrium to shift so as to oppose the change which is in this case decrease temperature.

Since the forward reaction for dissociation was favoured, dissociation is an endothermic process.

2) Iodine and thiosulfate are colourless compounds which will react together to form colourless solutions. Hence we will never be able to identify the end-point unless an indicator is used.

Potassium Manganate however is purple in colour and will turn colourless as it is being titrated. Therefore the point at which the solution changes from purple to colourless will be the end-point. This can be observed with our own bare eyes.

Hope it helps

[Twinkle Charms]

thank you guys =]

[~ Miss Relina ~]

guys can anyone explain what is the ground state of an atom (related to the elctronic configuration)
alsowhat is the electron density maps
 
need help asap

[Deadly_king]

guys can anyone explain what is the ground state of an atom (related to the elctronic configuration)
alsowhat is the electron density maps
 
need help asap

Atoms can exist in 2 states:  Ground state and Excited state

The ground state of an atom is when its electronic configurations are at their lowest energy level. In other words all the electrons orbiting around the nucleus will be in the lowest sub-shells.

Excited state is just the opposite as the electrons may be found in higher energy sub-shells. An atom is more likely to undergo reactions in the excited state rather than the ground state since it has more energy.

Take Sodium for example, its electron configuration is: 1s² 2s² 2p⁶ 3s¹

Electrons that are excited have moved to another shell. So if a Sodium atom was excited it would still have the same amount of electrons but a different configuration. Example: 1s² 2s² 2p⁶ 3p¹

Electron density maps are used by scientists during an experiment called the X-ray crystallography so as to estimate the location of electrons around the nucleus of a molecule.

If you need more information about it, click here.

[ruby92]

m/j 2010 paper 42
Q2 b and c

[Master_Key]

Q2 B (i)  For Mg(OH)₂ = 2993 - (1890 + (2*550)) = 3 kJ mol^-1

            For Sr(OH)₂ = 2467 - (1414 + (2*550)) = -47 kJ mol^-1

(ii)  Sr(OH)₂ is more soluble in water as it releases energy when it dissolves in water (Exothermic).

(iii) Sr(OH)₂ is less soluble in hot water as it releases energy when it dissolves in water (Exothermic). The same principle as in equilibrium.

[elemis]

http://www.xtremepapers.net/CIE/International%20A%20And%20AS%20Level/9701%20-%20Chemistry/9701_s02_qp_1.pdf

Question 6, 9 and 16, please.

[Deadly_king]

http://www.xtremepapers.net/CIE/International%20A%20And%20AS%20Level/9701%20-%20Chemistry/9701_s02_qp_1.pdf

Question 6, 9 and 16, please.

Jun 02 p1

6. Make use of the formula PV = nRT

It is said in the question that temperature is constant and so is the number of moles. In other words P is inversely proportional to V.

Therefore P₁V₁ = P₂V₂

P₁V₁ is given by the sum of the initial P_xV_x + P_yV_y while the final volume V₂ will be V_x + V_y.

So (2 x 1) + ( 1 x 2) = P₂(1 + 2) ----> P₂ = 4/3

Answer is A.

9. Enthalpy change of atomisation is the energy required to convert one mole of hydrazine to its respective atoms.

In other words we just have to break all the bonds involved within its structure.

Energy required to break 1 N-N bond + 4 N-H bond. Use values from the data booklet to find the required answer.

Delta H = 160 + 4(390) = 1720 KJ

Answer is B.

16. You need to know this equation by heart.

NaCl + 2H₂O -----> Cl₂ + H₂ + 2NaOH

Answer is B.

[elemis]

http://www.xtremepapers.net/CIE/index.php?dir=International%20A%20And%20AS%20Level/9701%20-%20Chemistry/&file=9701_w02_qp_1.pdf

I dont understand how the answer is D for Question 11.

Please help me.