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[化學 Chem] HKDSE Chemistry Question Review (2019-06-21)



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1. Given a tube of K2Cr2O7 (aq).

(a) What will happen if I add NaOH to it?
(b) What is this type of reaction?

引用:
Answer:

(a) The solution changes color from orange to yellow.

K2Cr2O7 + 2 NaOH --> Na2CrO4 + K2CrO4 + H2O

This is NOT a neutralization reaction.

(b) Hydrolysis. This is NOT a redox reaction.
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2. Given the third law of thermodynamic: A reaction is spontaneous if ΔG < 0

Denote ΔG = (-nF/RT)ln(K)
where n is no. of mole of electrons in a electrochemical cell,
T is temperature in Kelvin scale,
F, R are constant.

What is the significance of ln(K) in this equation?

引用:
Answer:

K is the equilibrium constant
If K>1, ln(K) >0 and thus ΔG < 0
i.e. the forward reaction is spontaneous.

[ 本帖最後由 Aeroblast 於 2019-6-20 04:22 AM 編輯 ]
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3. K2C2O7 can oxidize ethanol into acetic acid.

(a) Balance the ionic equation for this reaction.
(b) Given the third law of thermodynamics:

ΔG = ΔH - TΔS

where ΔG <0 denotes spontaneous reaction.
Also by the definition of entropy:
S = kln(W)
where k is constant and W is directly related to volume of a fluid.
We know that the reaction does not require catalyst or heat supply.
state the sign of ΔH and ΔS,
where ΔH = enthalpy change and  ΔS = entropy change

引用:
Answer:

(a) 8[H]+ + [Cr2O7]2- + 3 CH3CH2OH --> 3 CH3COOH + 2 [Cr]3+ + 4 H2O
(b) No. of moles of products < No. of moles of reactants
So ΔS <0
Hence ΔG < ΔH - TΔS <0
ΔH < TΔS  
|ΔH| > T|ΔS| as T>0
The reaction must be exothermic.




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4. Does H2(g) diffuse faster than O2(g)? Explain your answer mathematically.
引用:
Answer:

Holding room temperature. This implies that the average kinetic energy of both gas molecules is constant. Then we have:



Untitled.png

Since mass of O2 > mass of H2,
the answer is obviously in the affirmative.


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