Lisa chem exam question

[jaja]

hey
just did a lisachem 2008 exam

Q14 (done thx chem nerd)
q16 (done thx chem nerd)

Q5 e)... Why is electrolyzing a dilute aqueous solution of NACL a more effiecient way to produce hydrogen than electrolyzing water??
 
could this mean that nacl is more concentrated thus electrolyzing would be more effiecient???

Q4b)  " the acid -base indicator bromphenol blue is an equilibrium mixture of a weak acid and its conjugate base. The equilibrium may be represented by the equation...........HIn + H20 _arrow____ H3o + IN...... at What PH will the ratio [HIn]/[In] in bromphenol blue be 100/1?"

the answer replaces the HIn to 100 and In to 1 in the Ka formulae.. why do they do this?

Q6e).... " Why is that copper can be produced by the electrolysis of an aqueous solution of Cu2+ ions, but lithium can not be produced by the electrolysis of an aqueous solution of Li + ions?

isn't H20 a stronger oxidant than cu2+??? however the answer says that h20 is below cu2+ thus Cu is stronger

[chem-nerd]

Q16
it says in the sentence underneath the cell that the solution is aqueous. Double check all the information before making any assumptions.

Q14 
during discharge, lead to lead sulfate is an oxidation reaction at the anode (negatively charged as it is a spontaneous production of electrons)

during recharge, lead sulfate to lead is a reduction reaction at the cathode (negatively charged as it is a non-spontaneous reaction with the electrons being forced in by the power supply)

[jaja]

ohh i get that now.. thanks

[Mao]

Q5 e)... Why is electrolyzing a dilute aqueous solution of NACL a more effiecient way to produce hydrogen than electrolyzing water??
 
could this mean that nacl is more concentrated thus electrolyzing would be more effiecient???

Having electrolytes in the solution allows a faster movement of charge in the solution, in a way having a 'supercharged' salt bridge. Remember that the rate of electron transfer (the current) is limited by the slowest chain in the system. Having pure water, with a total ion concentration of at 25 degrees, it is a very poor conductor and the motion of electrons will be restricted by this.

Q4b)  " the acid -base indicator bromphenol blue is an equilibrium mixture of a weak acid and its conjugate base. The equilibrium may be represented by the equation...........HIn + H20 _arrow____ H3o + IN...... at What PH will the ratio [HIn]/[In] in bromphenol blue be 100/1?"

the answer replaces the HIn to 100 and In to 1 in the Ka formulae.. why do they do this?

Since it is given as a ratio, the units and relative magnitude cancels out. Substituting [HIn] = 1 and [In] = 0.01 will yield the same result, as the ratio is the same. Substituting [HIn] = 0.1 and [In] = 1*10^-3 will also yield the same result. Etc.
The simplest thing is to substitute the ratio itself in.

Q6e).... " Why is that copper can be produced by the electrolysis of an aqueous solution of Cu2+ ions, but lithium can not be produced by the electrolysis of an aqueous solution of Li + ions?

isn't H20 a stronger oxidant than cu2+??? however the answer says that h20 is below cu2+ thus Cu is stronger

Oxidant strength is on the left, where the stronger oxidant is found towards the top. The equation for oxidation of water (water is on the left hand side by itself with electron) is below Cu2+, and above Li+