CHM 1311 Study Guide - Ammonia, Sodium Sulfide, Silver Sulfide

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14 Jul 2014

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During this portion of the lab we immediately observed the distinct, odourless and light blue aqueous solution of cuso4 along with the transparent, strongly odoured aqueous solution of nh3. Upon initial reaction a very soluble, deep blue coloured complex is formed, [cu(nh3)4]2+. The vibrant blue is due to the replacement of the. H2o molecule with the nh3 that results in a greater absorption of yellow light than the water molecule. This is because the nh3 has a much stronger formation constant. K f than water making the concentration of [cu(nh3)4]2+ to increase and the concentration of [cu(h2o)4]2+ to decrease. Thus, this reaction is shifting towards the products. Also it is found that the [cu(nh3)4]2+ complex is stronger than the known blue [cu(h2o)4]2+ solution used as a reactant. With the addition of the hcl solution we witnessed a change in colour from the opaque blue back to a more transparent, lighter blue.

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-------Question------
Write balanced equations that indicate the reaction happening inthe information

6) the reduction of Cu2+ to Cu is fairly slow, so we need to testto ensure that it is complete before recovering the copper metal. Asensitive test for the presence of any remaining Cu2+ in solutionis the formation of deep blue Cu(NH3)4^2+(aq) upon the addition ofexcess NH3 solution. Cu(NH3)4^2+, the tetramminecopper(II) ion, isanother example of a transition metal complex, like are ammonia,rather than water molecules(and there are only four of them)

NH3 is a ga, highly soluble in water, which hydrolyzes to formweakly basic solution:
NH3(aq) + H2O(l) <=> NH4^+(aq) + OH^-(aq)

The solution is usually called NH3 (aq). The reaction equilibriumlies far to the left and only a small amount of NH^+ and OH^- form.When a metal ion, M^2+(aq) is added to a small amount of ammoniasolution, a reaction with OH- causes a hydroxide precipitate toform. This is the case with Cu^2+, Zn2+, Cd2+, and a fewothers.

M^2+(aq) + 2NH3(aq) + 2H2O(l) <=> M(OH)2(s) +2NH4^+(aq)

If a large excess of NH3(aq) is added to the M(OH)2(s), thisprecipitate will rediscover as the soluble teraammine complexformed.

Cu(OH)2(s) + 4NH3(aq) <=> Cu(NH3)4^2+(aq) + 2OH-(aq)

A Zn^2+/Zn half cell was placed inside a beaker with a Cu^2+/Cu half cell and was hooked up to a voltmeter to record the voltage. 3 mL of 6 M Cu^2+ solution was added until a deep blue Cu(NH3)4^2+ complex ion was obtained and a voltage reading was recorded. Based on this information how can I use the experimental standard cell potential, experimental nonstandard cell potential, and 0.10 M of zinc to determine the concentration of copper in the solution?

CHM 1311 Study Guide - Ammonia, Sodium Sulfide, Silver Sulfide

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