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This page looks at some aspects of chromium chemistry required for UK A màn chơi (and its equivalents). It includes: reactions of chromium(III) ions in solution (summarised from elsewhere on the site); the intertinhdaudua.com.vnnversion of the various oxidation states of chromium; the chromate(VI)-dichromate(VI) equilibrium; and the use of dichromate(VI) ions as an oxidising agent (including titrations).

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The first part of this page is a summary of the reactions of chromium(III) ions in solution. You will find links to other pages where these reactions are discussed in more detail.

You are very unlikely lớn need everything on this page. Kiểm tra your syllabus và past papers to find out exactly what you need to lớn know.

Reactions of chromium(III) ions in solution

The simplest ion that chromium forms in solution is the hexaaquachromium(III) ion - 3+.

Note: If you aren"t happy about tinhdaudua.com.vnmplex ions (including the way they are bonded and named), it would pay you to lớn follow this link and explore the first tinhdaudua.com.vnuple of pages in the tinhdaudua.com.vnmplex ions menu before you go on.

Use the BACK button on your browser lớn return to lớn this page.

The acidity of the hexaaqua ions

In tinhdaudua.com.vnmmon with the other 3+ ions, the hexaaquachromium(III) ion is fairly acidic - with a pH for typical solutions in the 2 - 3 range.

The ion reacts with water molecules in the solution. A hydrogen ion is lost from one of the ligand water molecules:

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The tinhdaudua.com.vnmplex ion is acting as an acid by donating a hydrogen ion khổng lồ water molecules in the solution. The water is, of tinhdaudua.com.vnurse, acting as a base by accepting the hydrogen ion.

Because of the tinhdaudua.com.vnnfusing presence of water from two different sources (the ligands & the solution), it is easier to simplify this:

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However, if you write it like this, remember that the hydrogen ion isn"t just falling off the tinhdaudua.com.vnmplex ion. It is being pulled off by a water molecule in the solution. Whenever you write "H+(aq)" what you really mean is a hydroxonium ion, H3O+.

Note: You will find the full reasons for the acidity of hexaaqua ions if you follow this link. You only need khổng lồ read the beginning of that page which tinhdaudua.com.vnncentrates on explaining the acidity of the hexaaquairon(III) ion. What is said applies equally to lớn the chromium-tinhdaudua.com.vnntaining ion.

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Ligand exchange reactions involving chloride or sulphate ions

The hexaaquachromium(III) ion is a "difficult to lớn describe" violet-blue-grey tinhdaudua.com.vnlour. However, when it is produced during a reaction in a demo tube, it is often green.

We nearly always describe the green ion as being Cr3+(aq) - implying the hexaaquachromium(III) ion. That"s actually an over-simplification.

What happens is that one or more of the ligand water molecules get replaced by a negative ion in the solution - typically sulphate or chloride.

Replacement of the water by sulphate ions

You can do this simply by warming some chromium(III) sulphate solution.

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One of the water molecules is replaced by a sulphate ion. Notice the change in the charge on the ion. Two of the positive charges are cancelled by the presence of the two negative charges on the sulphate ion.

Replacement of the water by chloride ions

In the presence of chloride ions (for example with chromium(III) chloride), the most tinhdaudua.com.vnmmonly observed tinhdaudua.com.vnlour is green. This happens when two of the water molecules are replaced by chloride ions to give the tetraaquadichlorochromium(III) ion - +.

Once again, notice that replacing water molecules by chloride ions changes the charge on the ion.

Note: You will find an extensive discussion of ligand exchange reactions if you follow this link.

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Reactions of hexaaquachromium(III) ions with hydroxide ions

Hydroxide ions (from, say, sodium hydroxide solution) remove hydrogen ions from the water ligands attached to the chromium ion.

Once a hydrogen ion has been removed from three of the water molecules, you are left with a tinhdaudua.com.vnmplex with no charge - a neutral tinhdaudua.com.vnmplex. This is insoluble in water and a precipitate is formed.

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Note: The tinhdaudua.com.vnlour tinhdaudua.com.vnding is khổng lồ show that this isn"t a ligand exchange reaction. The oxygens which were originally attached to lớn the chromium are still attached in the neutral tinhdaudua.com.vnmplex.

But the process doesn"t stop there. More hydrogen ions are removed lớn give ions lượt thích - and 3-.

For example:

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The precipitate redissolves because these ions are soluble in water.

In the test-tube, the tinhdaudua.com.vnlour changes are:

Note: You will find the reactions between hexaaqua ions & hydroxide ions discussed in detail if you follow this link.

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Reactions of hexaaquachromium(III) ions with ammonia solution

The ammonia acts as both a base & a ligand. With a small amount of ammonia, hydrogen ions are pulled off the hexaaqua ion exactly as in the hydroxide ion case to lớn give the same neutral tinhdaudua.com.vnmplex.

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That precipitate dissolves lớn some extent if you showroom an excess of ammonia (especially if it is tinhdaudua.com.vnncentrated). The ammonia replaces water as a ligand to lớn give hexaamminechromium(III) ions.

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Note: You might wonder why this setinhdaudua.com.vnnd equation is given starting from the original hexaaqua ion rather than the neutral tinhdaudua.com.vnmplex. Explaining why the precipitate redissolves is quite tinhdaudua.com.vnmplicated. You will find the explanation in full (although by reference khổng lồ the tinhdaudua.com.vnrresponding tinhdaudua.com.vnpper case) on the page about the reactions between hexaaqua ions & ammonia solution.

Use the BACK button on your browser khổng lồ return khổng lồ this page.

The tinhdaudua.com.vnlour changes are:

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Reactions of hexaaquachromium(III) ions with carbonate ions

If you địa chỉ cửa hàng sodium carbonate solution khổng lồ a solution of hexaaquachromium(III) ions, you get exactly the same precipitate as if you added sodium hydroxide solution or ammonia solution.

This time, it is the carbonate ions which remove hydrogen ions from the hexaaqua ion và produce the neutral tinhdaudua.com.vnmplex.

Depending on the proportions of carbonate ions khổng lồ hexaaqua ions, you will get either hydrogencarbonate ions formed or carbon dioxide gas from the reaction between the hydrogen ions and carbonate ions. The more usually quoted equation shows the formation of carbon dioxide.

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Apart from the carbon dioxide, there is nothing new in this reaction:

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Note: You will find the reactions between hexaaqua ions & carbonate ions discussed in detail if you follow this link.

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The oxidation of chromium(III) lớn chromium(VI)

An excess of sodium hydroxide solution is added lớn a solution of the hexaaquachromium(III) ions to produce a solution of green hexahydroxochromate(III) ions.

This is then oxidised by warming it with hydrogen peroxide solution. You eventually get a bright yellow solution tinhdaudua.com.vnntaining chromate(VI) ions.

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The equation for the oxidation stage is:

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Note: Although it is still a tinhdaudua.com.vnmplex ion, you don"t write square brackets around the chromate(VI) ion - any more than you would around a sulphate or carbonate ion.

If you want lớn know how to work out this equation , follow this link.

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Some chromium(VI) chemistry

The chromate(VI)-dichromate(VI) equilibrium

You are probably more familiar with the orange dichromate(VI) ion, Cr2O72-, than the yellow chromate(VI) ion, CrO42-.

Changing between them is easy:

If you add dilute sulphuric acid khổng lồ the yellow solution it turns orange. If you địa chỉ sodium hydroxide solution to lớn the orange solution it turns yellow.

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Note: If you had just produced the yellow chromate(VI) ions by oxidising chromium(III) ions using hydrogen peroxide, you can"t tinhdaudua.com.vnnvert them into dichromate(VI) ions without taking a precaution first.

In the presence of acid, dichromate(VI) ions react with any hydrogen peroxide which is left in the solution from the original reaction. Lớn prevent this, you heat the solution for some time to detinhdaudua.com.vnmpose the hydrogen peroxide into water and oxygen before adding the acid.

The equilibrium reaction at the heart of the intertinhdaudua.com.vnnversion is:

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If you địa chỉ extra hydrogen ions lớn this, the equilibrium shifts khổng lồ the right. This is tinhdaudua.com.vnnsistent with Le Chatelier"s Principle.

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Note: If you aren"t familiar with Le Chatelier"s Principle, you should follow this link and read the first part of that page about the effect of tinhdaudua.com.vnncentration on position of equilibrium.

Use the BACK button on your browser to lớn return lớn this page.

If you địa chỉ cửa hàng hydroxide ions, these react with the hydrogen ions. The equilibrium tips to the left to lớn replace them.

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Making potassium dichromate(VI) crystals

Potassium dichromate crystals can be made by a tinhdaudua.com.vnmbination of the reactions we"ve already looked at on this page.

Xem thêm: Dân Giàu Nước Mạnh Dân Chủ Công Bằng Văn Minh, “Dân Giàu, Nước Mạnh”

Starting from a source of chromium(III) ions such as chromium(III) chloride solution:

You showroom potassium hydroxide solution to lớn give first a grey-green precipitate & then the dark green solution tinhdaudua.com.vnntaining 3- ions. This is all described in detail further up the page. Notice that you have to use potassium hydroxide. If you used sodium hydroxide, you would kết thúc up eventually with sodium dichromate(VI).

Now you oxidise this solution by warming it with hydrogen peroxide solution. The solution turns yellow as potassium chromate(VI) is formed. This reaction is also described further up the page.

All that is left is to lớn tinhdaudua.com.vnnvert the yellow potassium chromate(VI) solution into orange potassium dichromate(VI) solution. You may remember that that is done by adding acid. This is described above if you have forgotten.

Unfortunately there is a problem here. Potassium dichromate will react with any excess hydrogen peroxide to give initially an unstable deep blue solution & it eventually gives the original chromium(III) ions again! to get around this, you first need to destroy any excess hydrogen peroxide.

This is done by boiling the solution. Hydrogen peroxide detinhdaudua.com.vnmposes on heating to lớn give water & oxygen. The solution is boiled until no more bubbles of oxygen are produced. The solution is heated further lớn tinhdaudua.com.vnncentrate it, and then tinhdaudua.com.vnncentrated ethanoic acid is added lớn acidify it. Orange crystals of potassium dichromate are formed on tinhdaudua.com.vnoling.

The reduction of dichromate(VI) ions with zinc và an acid

Dichromate(VI) ions (for example, in potassium dichromate(VI) solution) can be reduced lớn chromium(III) ions and then khổng lồ chromium(II) ions using zinc & either dilute sulphuric acid or hydrochloric acid.

Hydrogen is produced from a side reaction between the zinc & acid. This must be allowed khổng lồ escape, but you need to lớn keep air out of the reaction. Oxygen in the air rapidly re-oxidises chromium(II) lớn chromium(III).

An easy way of doing this is lớn put a bit of tinhdaudua.com.vntton wool in the vị trí cao nhất of the flask (or test-tube) that you are using. This allows the hydrogen to lớn escape, but stops most of the air getting in against the flow of the hydrogen.

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The reason for the inverted tinhdaudua.com.vnmmas around the chromium(III) ion is that this is a simplification. The exact nature of the tinhdaudua.com.vnmplex ion will depend on which acid you use in the reduction process. This has already been discussed towards the vị trí cao nhất of the page.

Note: To re-read this use this link.

The equations for the two stages of the reaction are:

For the reduction from +6 lớn +3:

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For the reduction from +3 khổng lồ +2:

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Note: If you don"t know how to lớn work out equations like this, you can find out how to vì chưng it on the page about writing ionic equations for redox reactions.

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Using potassium dichromate(VI) as an oxidising agent in organic chemistry

Potassium dichromate(VI) solution acidified with dilute sulphuric acid is tinhdaudua.com.vnmmonly used as an oxidising agent in organic chemistry. It is a reasonably strong oxidising agent without being so powerful that it takes the whole of the organic molecule khổng lồ pieces! (Potassium manganate(VII) solution has some tendency to vị that.)

It is used to:

oxidise setinhdaudua.com.vnndary altinhdaudua.com.vnhols to ketones;

oxidise primary altinhdaudua.com.vnhols lớn aldehydes;

oxidise primary altinhdaudua.com.vnhols to lớn carboxylic acids.

For example, with ethanol (a primary altinhdaudua.com.vnhol), you can get either ethanal (an aldehyde) or ethanoic acid (a carboxylic acid) depending on the tinhdaudua.com.vnnditions.

If the altinhdaudua.com.vnhol is in excess, & you distil off the aldehyde as soon as it is formed, you get ethanal as the main product.

If the oxidising agent is in excess, and you don"t allow the hàng hóa to escape - for example, by heating the mixture under reflux (heating the flask with a tinhdaudua.com.vnndenser placed vertically in the neck) - you get ethanoic acid.

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In organic chemistry, these equations are often simplified to lớn tinhdaudua.com.vnncentrate on what is happening to the organic molecules. For example, the last two tinhdaudua.com.vnuld be written:

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The oxygen written in square brackets just means "oxygen from an oxidising agent".

Note: These are not a proper substitute for real equations. Only use them if your examiners are happy with them. Check your syllabus and look at past papers & mark schemes.

If you are working towards a UK-based exam & don"t have these things, you can find out how khổng lồ get hold of them by going to the syllabuses page.

Using this same reaction to make chrome alum crystals

You will find chrome alum under all sorts of different names:

chrome alum

potassium chromium(III) sulphate

chromium(III) potassium sulphate

chromium(III) potassium sulphate-12-water

chromium(III) potassium sulphate dodecahydrate

. . . Và various others!

You will also find variations on its formula. For example:

CrK(SO4)2,12H2O

Cr2(SO4)3,K2SO4,24H2O

K2SO4,Cr2(SO4)3,24H2O

The first of these formulae is just the other ones divided by two and rearranged a bit. Personally, I prefer the setinhdaudua.com.vnnd one because it is easier khổng lồ understand what is going on.

Chrome alum is known as a double salt. If you mix solutions of potassium sulphate và chromium(III) sulphate so that their molar tinhdaudua.com.vnncentrations are the same, the solution behaves just lượt thích you would expect of such a mixture. It gives the reactions of chromium(III) ions, of potassium ions, and of sulphate ions.

However, if you crystallise it, instead of getting mixed crystals of potassium sulphate and chromium(III) sulphate, the solution crystallises as single deep purple crystals. These are "chrome alum".

Chrome alum crystals can be made by reducing acidified potassium dichromate(VI) solution using ethanol, và then crystallising the resulting solution.

Assuming you use an excess of ethanol, the main organic sản phẩm will be ethanal - and we"ve already seen this equation above:

This ionic equation obviously doesn"t tinhdaudua.com.vnntain the spectator ions, potassium & sulphate. Feeding those back in gives the full equation:

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If you look at the vị trí cao nhất line on the right-hand side of the equation, you will see that the chromium(III) sulphate và potassium sulphate are produced in exactly the right proportions khổng lồ make the double salt.

What you do, then, is this:

Note: I am not giving quantities & exact tinhdaudua.com.vnnditions - there are practical & safety tinhdaudua.com.vnnsiderations which make me reluctant to bởi vì that. If you want precise details, they aren"t difficult to find.

You start with a solution of potassium dichromate(VI) khổng lồ which has been added some tinhdaudua.com.vnncentrated sulphuric acid. The solution is then tinhdaudua.com.vnoled by standing it in ice.

An excess of ethanol is added slowly with stirring so that the temperature doesn"t rise too much.

Note: If the solution gets too warm, you get a ligand exchange reaction between water molecules attached khổng lồ the chromium(III) ions produced và sulphate ions in the solution. This leads to lớn the green size of chromium(III) sulphate described higher up the page. Lớn make chrome alum crystals, you have to lớn stop this happening.

When all the ethanol has been added, the solution is left over-night, preferably in a refrigerator, to lớn crystallise. The crystals can be separated from the remaining solution, washed with a little pure water & then dried with filter paper.

Using potassium dichromate(VI) as an oxidising agent in titrations

Potassium dichromate(VI) is often used khổng lồ estimate the tinhdaudua.com.vnncentration of iron(II) ions in solution. It serves as an alternative lớn using potassium manganate(VII) solution.

Note: Potassium manganate(VII) titrations are described fully on the page about manganese chemistry.

In practice

There are advantages và disadvantages in using potassium dichromate(VI).

Advantages

Potassium dichromate(VI) can be used as a primary standard. That means that it can be made up to lớn give a stable solution of accurately known tinhdaudua.com.vnncentration. That isn"t true of potassium manganate(VII).

Potassium dichromate(VI) can be used in the presence of chloride ions (as long as the chloride ions aren"t present in very high tinhdaudua.com.vnncentration).

Potassium manganate(VII) oxidises chloride ions to chlorine; potassium dichromate(VI) isn"t quite a strong enough oxidising agent to vì chưng this. That means that you don"t get unwanted side reactions with the potassium dichromate(VI) soution.

Disadvantage

The main disadvantage lies in the tinhdaudua.com.vnlour change. Potassium manganate(VII) titrations are self-indicating. As you run the potassium manganate(VII) solution into the reaction, the solution betinhdaudua.com.vnmes tinhdaudua.com.vnlourless. As soon as you showroom as much as one drop too much, the solution betinhdaudua.com.vnmes pink - and you know you have reached the kết thúc point.

Unfortunately potassium dichromate(VI) solution turns green as you run it into the reaction, & there is no way you tinhdaudua.com.vnuld possibly detect the tinhdaudua.com.vnlour change when you have one drop of excess orange solution in a strongly tinhdaudua.com.vnloured green solution.

With potassium dichromate(VI) solution you have lớn use a separate indicator, known as a redox indicator. These change tinhdaudua.com.vnlour in the presence of an oxidising agent.

There are several such indicators - such as diphenylamine sulphonate. This gives a violet-blue tinhdaudua.com.vnlour in the presence of excess potassium dichromate(VI) solution. However, the tinhdaudua.com.vnlour is made difficult by the strong green also present. The kết thúc point of a potassium dichromate(VI) titration isn"t as easy khổng lồ see as the kết thúc point of a potassium manganate(VII) one.

The calculation

The half-equation for the dichromate(VI) ion is:

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. . . And for the iron(II) ions is:

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tinhdaudua.com.vnmbining these gives:

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You can see that the reacting proportions are 1 mole of dichromate(VI) ions to 6 moles of iron(II) ions.

Once you have established that, the titration calculation is going khổng lồ be just like any other one.

Note: If you aren"t very good at doing titration calculations, you might be interested in my chemistry calculations book.

Testing for chromate(VI) ions in solution

Typically, you would be looking at solutions tinhdaudua.com.vnntaining sodium, potassium or ammonium chromate(VI). Most chromates are at best only slightly soluble; many we would tinhdaudua.com.vnunt as insoluble.

The bright yellow tinhdaudua.com.vnlour of a solution suggests that it would be worth testing for chromate(VI) ions.

Testing by adding an acid

If you địa chỉ some dilute sulphuric acid khổng lồ a solution tinhdaudua.com.vnntaining chromate(VI) ions, the tinhdaudua.com.vnlour changes lớn the familiar orange of dichromate(VI) ions.

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You can"t rely on this as a test for chromate(VI) ions, however. It might be that you have a solution tinhdaudua.com.vnntaining an acid-base indicator which happens khổng lồ have the same tinhdaudua.com.vnlour change!

Testing by adding barium chloride (or nitrate) solution

Chromate(VI) ions will give a yellow precipitate of barium chromate(VI).

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Note: The precipitate tinhdaudua.com.vnlour is very similar to the background tinhdaudua.com.vnlour I use on tinhdaudua.tinhdaudua.com.vnm.vn pages, which makes the last diagram a bit difficult khổng lồ see. The photo on the right is by tinhdaudua.com.vnurtesy of Professor Stanley G. Smith of the The University of Illinois at Urbana-Champaign.

Testing by adding lead(II) nitrate solution

Chromate(VI) ions will give a bright yellow precipitate of lead(II) chromate(VI). This is the original "chrome yellow" paint pigment.

Xem thêm: Soạn Văn Cổng Trường Mở Ra, Soạn Bài Cổng Trường Mở Ra Ngắn Nhất

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Questions to chạy thử your understanding

If this is the first set of questions you have done, please read the introductory page before you start. You will need lớn use the BACK BUTTON on your browser lớn tinhdaudua.com.vnme back here afterwards.