One of the classic methods for determining the manganese...

Key Concepts

Spectrophotometry

Spectrophotometry is an analytical technique used to measure the amount of light absorbed by a solution at a specific wavelength. In this procedure, the deep color of the permanganate ion allows for the determination of its concentration, which in turn is directly related to the manganese content in the steel sample. This method is a fundamental tool in analytical chemistry for quantifying substances based on light absorption.

Oxidation States

Understanding oxidation states is critical in redox chemistry because it helps track the electron transfer that occurs during the reaction. In this method, manganese is initially present in the +2 oxidation state and is converted into the +7 oxidation state in the permanganate ion. This change is key to both the redox reaction mechanism and the resulting color change that is utilized for quantification in analytical procedures.

Redox Reactions

Redox reactions are chemical processes involving the transfer of electrons between species, where one substance is oxidized (loses electrons) and another is reduced (gains electrons). In the context of the problem, the transformation of manganese(II) to the permanganate ion is a redox process that is central to the analysis, as it changes the oxidation state of manganese and produces a species with a characteristic deep color for spectrophotometric measurement.

Balancing Chemical Equations

Balancing chemical equations involves adjusting the stoichiometric coefficients so that the number of atoms of each element and the overall charge are the same on both sides of the reaction. This process is essential for accurately representing the chemical transformations taking place in the method, ensuring the conservation of mass and charge, and serving as the basis for quantitative analysis.

Transcript

00:01 This question says that one of the classic methods for determining the manganese content in steel involves converting all the manganese to deeply colored per manganate ion and then measuring the absorption of light.

00:12 The steel is first dissolved in nitric acid, producing the manganese 2 ion and nitrogen dioxide gas.

00:20 The solution is then reacted with an acidic solution containing the periodate ion.

00:25 The products are per manganate and iodate ions.

00:28 It asks us to write the balanced chemical equations for both of these steps.

00:33 So here i've worked out most of the first step.

00:36 We have this is kind of the net ionic equation, but it's not balanced.

00:40 This is the nitrate ion from nitric acid, the solid manganese that's in the steel, the manganese 2 ion that we're going to remove from the steel, and the nitric acid, or sorry, the no2 gas that's produced as a product.

00:54 So to balance this, we need to figure out, or we need to use the half -reaction method to look at both the oxidation and the reduction reaction separately, and then combine them.

01:05 So up top here, we have manganese solid going to manganese 2 plus.

01:08 This is the oxidation reaction, of course.

01:10 It's losing electrons.

01:12 And to balance this, we don't need to change the number of manganese or add h -plus molecules or water molecules.

01:18 We just need to add two electrons to make sure that the charges are equal on both sides.

01:22 So this one's pretty straightforward.

01:25 For the nitrate ion, we're going from no3 minus to.

01:29 No2 with no charge.

01:31 The nitrogen is going from a plus five to a plus four oxidation state.

01:35 It's gaining electrons becoming reduced.

01:39 And so to balance this, we don't have to change the number of nitrogen.

01:41 They're the same.

01:42 We have to add one molecule, one molecule of water to the right side to get this extra oxygen.

01:47 So now there are three oxygens on both sides.

01:50 But we added two hydrogens.

01:52 So we have to add two protons over here.

01:54 And then we have a net plus one charge on this side and no charge on this side.

01:59 So we have to add an electron.

02:01 Now the charges and the number of each atom are balanced.

02:04 Now to combine these two, we have to find the lowest common multiple between one electron and two electrons.

02:10 Of course, that's two.

02:11 If we simply double this, we'll have two electrons on the left side, two electrons on the right side, and we can balance.

02:18 So i'm going to cross these out.

02:19 These will cancel.

02:20 And now we can just combine these two.

02:22 We have manganese solid plus two protons plus the nitrate ions.

02:42 Oh, but i forgot that we are doubling these.

02:45 I was trying to save space.

02:49 So we need to double.

02:51 There's two here, but we doubled this whole reaction, so there are four protons.

02:55 There's one nitrate, but we're doubling it, so there's two nitrates.

03:02 So this was combining the left side of the equations.

03:05 Now let's do the right.

03:06 We have just one, emin -2 ion.

03:18 We are doubling the bulk.

03:19 Bottom, so we have two no2 gas, and we're doubling waters, so we have two h2 liquids.

03:37 And we can go back and check all of this.

03:39 Most of it should balance manganese, one each, nitrogen's two each, hydrogens four each, oxygens, we have six each, and on the left we have plus two, and on the right we have plus two.

03:55 So this is the balanced chemical equation for that first step in this process.

04:00 And then here i've worked out most of the second step.

04:03 It tells us the manganese 2 plus ion is combining with the per iodate ion to form per manganate and iodate.

04:12 Again, manganese starts at 2 plus and then it goes to plus 7.

04:18 So it's losing electrons becoming oxidized...

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