Two dark solids

Two dark solids, one giant ionic, the other covalent molecular.

OLYMPUS DIGITAL CAMERA

Iodine

Iodine is a molecular solid. It consists of I2 molecules. The molecules are non-polar and are held together by weak intermolecular attractions in the iodine crystals.

OLYMPUS DIGITAL CAMERA

Potassium manganate (VII)

Potassium manganate (VII) is an ionic solid, made up of K+ ions and MnO4 ions in a giant crystal lattice structure.

The differences in bonding and structure between the two materials means they have quite different properties.

Solubility

Water dissolves many polar molecules and ionic solids.

Here water is being added to iodine and potassium manganate (VII).

2016_11_30_water_mno4_i2

Which tube contains the iodine and which the potassium manganate (VII) ?

Hexane is a non-polar organic liquid which dissolves non-polar molecules, but not ionic solids.

Here hexane is being added to iodine and potassium manganate (VII).

2016_11_30_hexane_kmno4_i2

Which tube contains iodine and which potassium manganate (VII) ?

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with tap water and subjected to 10 volts?

2016_11_29_kmno4_elec_h2o

In potassium manganate (VII) the K+ ions are colourless, whilst the MnO4 ions are deep purple in colour.

Which way do the purple ions move and why?

Iodine dissolves in potassium iodide solution

The I2 molecules in iodine do not dissolve very well in pure water, but they do dissolve in a solution of potassium iodide.  I3 ions are formed when iodine dissolves in potassium iodide.

Behaviour in an electric field

What would happen if solid iodine and solid potassium manganate (VII) were placed on some filter paper moistened with potassium iodide solution and subjected to 10 volts?

2016_11_29_kmno4_elec_ki

Why does the brown colour move to the left at the top of the slide?

This one maybe harder to explain fully. So let’s close with a little rhyme:

“Two dark solids, sometimes purple, sometimes brown. One ionic, one molecular, with behaviour that can make you frown.”

 

Spooky

img_4962

Sometimes things don’t go the way the way they’re planned. When this happens in chemistry the first thing is to see if you can repeat the observation. Some examples follow, maybe you can explain them.

Here’s an experiment we did recently using the juice extracted from red cabbage as an acid/base indicator. We were surprised by the colour produced in strong alkali on the far left of the picture.

img_4699

Yellow at pH 12 to 14?

We checked with a second extract and got the same result

img_4704

Red cabbage juice in 1M NaOH and 2M HCl

In another experiment we tried to grow one big crystal of sodium chloride by suspending a ‘seed’ crystal in a saturated solution of brine. The seed crystal fell off and then something magical happened around the string.

OLYMPUS DIGITAL CAMERA

Boxes, little boxes..

Finally, when we tried to demonstrate diffusion by dissolving a small lump of potassium manganate (VII) in a beaker of water, things didn’t go as smoothly as planned:

2016_09_19_KMnO4_480_360_10_2.gif

Jittery time lapse animation (overnight)

Science, the fun part is in figuring out what happened.

 

Silane

Silane is toxic and all reactions involving magnesium silicide to produce silane should be carried out in a fume cupboard.

In the August 2016 blog we looked at the reaction of magnesium with oxygen and chlorine.

Magnesium is so reactive it will take the oxygen away from silicon in sand, (silicon dioxide, SiO2) .

Magnesium reacts with sand to produce magnesium oxide and silicon.

2Mg + SiO2  -> 2MgO + Si

If an excess of magnesium is used, magnesium silicide (Mg2Si) is also formed.

2Mg + Si  ->  Mg2Si

Silane is produced when magnesium silicide is reacted with dilute acids, such as hydrochloric and sulfuric acids, to produce silane.

For example,  4HCl + Mg2Si  ->  SiH4 + 2MgCl2

Silane bursts into flames as soon as it is produced.

For example, SiH4 + 2O2  ->  SiO2  + 2H2O

Experiment 1 – heating magnesium and silicon dioxide using a Fresnel lens

OLYMPUS DIGITAL CAMERA

The Fresnel lens here was taken from an old overhead projector

2016_09_30_mgsio2_reaction1_427_240_10

Quite a violent reaction, over in a few seconds

OLYMPUS DIGITAL CAMERA

Boiling tube cracked by the ferocity of the reaction

Experiment 2 – heating magnesium and silicon dioxide using a Fresnel lens

mvi_4011-mov_000027068_427

Powdered magnesium and silicon dioxide

2016_09_30_mgsio2_expt2_427_240_10

Heat using the focussed rays of the sun

Experiment 3 – heating magnesium and silicon dioxide in a crucible using a Fresnel lens

OLYMPUS DIGITAL CAMERA

Adjust the lens to focus the suns rays onto the mixture

2016_09_30_mgsio2_crucible_427_240_10

An exothermic reaction, over in a flash

OLYMPUS DIGITAL CAMERA

The product mixture contains magnesium silicide which must be left to cool for several minutes

Experiment 4 – adding dilute sulfuric acid to the product in a boiling tube

2016_06_17_SILANE_IN_BOILING_TUBE.gif

Experiment 5 – adding dilute hydrochloric acid to the product mixture

OLYMPUS DIGITAL CAMERA

The product mixture containing magnesium silicide

2016_09_30_mgsilicide_h2so4dark_427_240_20

Small, sputtering explosions

OLYMPUS DIGITAL CAMERA

Caught in the act

Experiment 6 – slow motion animation of adding dilute hydrochloric acid to the product mixture

2016_09_30_mgsilicide_hcl_427_270_20

Slow motion

Links to movies on You Tube:

Experiment 1
Magnesium and silicon (IV) oxide reaction 1 – boiling tube / Fresnel lens
16 seconds

Experiment 2
Magnesium and silicon (IV) oxide reaction 2 – boiling tube / Fresnel lens
39 seconds

Experiment 3
Magnesium and silicon (IV) oxide reaction 3 -crucible / Fresnel lens
24 seconds

Experiment 4
Magnesium silicide and sulfuric acid – boiling tube
14 seconds

Experiment 5
Magnesium silicide and sulfuric acid 3 – watch glass in the dark
40 seconds

Experiment 6
Magnesium silicide and hydrochloric acid at 120fps, no sound (watch glass)
56 seconds

Don’t look into the flame!

At the end of the last post which was about the reactions of the alkali metals with chlorine, I asked what you would see if burning magnesium were lowered into a gas jar containing chlorine gas. The answer is you would see a bright white flame.

The flame produced by magnesium in such reactions is so bright you must not look at it directly, because of the risk of damaging your eyes. Always wear eye protection and follow the recommended safety advice in your laboratory.

Sometimes its easier, although less exciting to look at a movie of such reactions. Movie here, (51 seconds).

MVI_4390.MOV_000021312

Magnesium burns in oxygen with a bright, white flame

What is the product of the reaction between magnesium and oxygen? Can you write a balanced symbol equation for the reaction?

Magnesium also reacts violently with chlorine, producing the same bright, white flame. In the movie here (54 seconds), burning magnesium ribbon is lowered into a gas jar containing a mixture of chlorine gas and air. Listen for the crackles at about 18 seconds into the movie, indicative of the very energetic nature of the reaction.

MVI_4381.MOV_000020020

Magnesium reacting with chlorine and oxygen from the air

What is the product of the reaction between magnesium and chlorine? Can you write a balanced symbol equation for the reaction?

Another movie of the reaction between magnesium and chlorine from the RSC Science Skool can be seen here,(1 minute 28 seconds).

Magnesium is so reactive, it will also take the oxygen away from silicon in silicon (IV) oxide. Carry out some research and find out the products of this reaction and an equation for it.

Answers to all of the questions in this post next time.

 

Making lithium chloride, sodium chloride and potassium chloride by direct combination

The alkali metals lithium, sodium and potassium all react with chlorine gas to produce their respective chlorides.

2Li + Cl2 –> 2LiCl

2Na + Cl2 –> 2NaCl

2K + Cl2 –> 2KCl

MVI_4099.MOV_000000083_labelled

Experimental set-up

In the animation below samples of lithium, sodium and potassium metals are shown being heated in a blue Bunsen burner flame until they catch fire. The flaming metals are then dropped into gas jars containing chlorine gas. Each metal reacts showing its characteristic flame colour and a product seen as a white smoke (solid particles of the chloride).

2016_07_01_LiNaK_chlorine_495_270_02

Flaming metals and chlorine

Here are three pictures taken from a movie of the reactions which can be viewed on You Tube here (3 minutes).

MVI_4099.MOV_000061686

Lithium reacting with chlorine to make lithium chloride

MVI_4099.MOV_000109109

Sodium reacting with chlorine to make sodium chloride

MVI_4099.MOV_000138888

Potassium reacting with chlorine to make potassium chloride

We carried out the experiments in broken porcelain crucibles because when we used combustion spoons the iron metal, in the steel from which the combustion spoons were made, also reacted with the chlorine gas (movie here). Dense red smoke, presumably of iron (III) chloride, was produced in addition to the alkali metal chlorides.

We had to use broken crucibles because whole crucibles would not fit inside our gas jars.

MVI_4084.MOV_000058933

Combustion spoon: lithium and chlorine

MVI_4084.MOV_000121287

Combustion spoon: sodium and chlorine

MVI_4084.MOV_000164080

Combustion spoon: potassium and chlorine

MVI_4099.MOV_000153361

Products from experiments carried out in porcelain crucibles were all white crystalline solids

As for comparing the reactivity of the three metals, potassium reacted the fastest, with the reaction over in a couple of seconds. Both lithium and sodium reacted for several seconds, with longer lasting flames.

However, the reaction between potassium and chlorine shown in the animation above did not go to completion because there was unreacted chlorine gas at the end of the experiment, (visible in the picture). There was also unreacted potassium and this reacted violently with water when the gas jar was was washed out, after allowing it to cool.

An animation of the reaction between potassium and chlorine in another experiment is shown below (movie here). This one did not react so violently, but was also over in a couple of seconds.

2016_06_23_01_960_540_10

Close-up: potassium reacting with chlorine

Thus, burning potassium reacts quicker than lithium and sodium with chlorine gas, but its almost as if the potassium chloride produced smothers the reacting metal.

What colour flame would you expect to see if burning magnesium ribbon was dropped into a gas jar full of chlorine gas?

 

Li sizzles, Na balls & K-pot explodes

Comparing the reactions of lithium, sodium and potassium with water.

MVI_4079.MOV_000014681_768_432

A lump of potassium explodes

The alkali metals all react speedily with water and with oxygen.

For this reason they are stored in oil to limit the oxygen and water in air getting to them in the chemical store.

IMG_4078_rot_labelled

Lumps of lithium, sodium and potassium metals in hexane prior to dropping them into water

Students at GCSE are often asked to compare the trend in reactivity of the metals on going down the group, by comparing the observations made as each one reacts with water.

To this end experiments are sometimes carried out, demonstrating the reactions of lithium, sodium and potassium with water.

All three metals float on water and react quickly with it producing hydrogen gas.

MVI_4079.MOV_000014431_768

Potassium always catches fire producing a lilac coloured flame and explodes if too big a chunk is used.

2016_06_30_800_450_10

“Li sizzles, Na balls and K-pot explodes lilac”

Equations for the reactions are:

2 Li   +   2H2O  –>  2LiOH   +   H2
2 Na   +   2H2O  –>  2NaOH   +   H2
2 K   +   2H2O  –>   2KOH   +   H2

Students are also often asked to predict how rubidium and caesium metals would react with water.

The answer is that they would both react producing similar products according to the equations above, but even more vigorously than potassium, with caesium being the most reactive.

Links to movies on You Tube from which the above gif animation was made are here:

1. Short version 39 seconds

2. Longer version 2 minutes and 3 seconds

 

Why has no one has ever dropped a lump of Francium into water and made a movie of it?

 

 

Fresnel Thermite

We carried out some experiments recently lighting a length of magnesium ribbon with a Fresnel lens. The Fresnel lens was used to focus light rays from the sun on the magnesium. The magnesium ribbon acted as a fuse to ignite a thermite reaction mixture in which it was embedded. The experiments went rather well providing a relatively safe and unhurried way of starting this energetic reaction.

P5100938_cut.mov_000017684

Using a Fresnel lens to start a thermite reaction

P5100938_cut.mov_000023823

The power of the sun

2016_05_22_Thermite_Fresnel_480_270_10.gif

We used 20g of thermite mixture made from aluminium powder (5g) and iron (III) oxide (15g) which reacts according to the following equation:

Fe2O3 + 2 Al → 2 Fe + Al2O3

2016_05_11_Fresnel_thermite_480_270_15

Stand well back

P5110941cut.mov_000075775

and focus

OLYMPUS DIGITAL CAMERA

a sample of iron can be obtained once the mixture has cooled down

The experiments were carried out in the tropics and it would be interesting to see if the same method works in a more temperate clime.

Links to movies of the experiments on You Tube can be found here:

  1. Fresnel / Thermite 1
  2. Fresnel / Thermite 2

 

 

Blue ruby

This post contains a collection of animated gifs and other pictures from experiments we have been doing recently.

First a heart made by putting copper foil into silver nitrate solution (copper is more reactive than silver and displaces silver ions from solution).

OLYMPUS DIGITAL CAMERA

Silver heart

Next, heating a beaker of ice to water and steam.

2016_04_20_heating_ice_water_2_30

Ice water steam

Those are anti-bumping granules you can see at the bottom of the beaker in the boiling water.

Now for a rather pretty B/Z reaction we carried out recently.

2016_04_22_BZ_blue_15

A Belousov-Zhabotinsky reaction

Captured using time-lapse mode

IMG_3930_320

99 pictures at 15 seconds interval

Finally, the curious case of the colour of chromium (III) ions in water. And hence the title of this post ‘Blue ruby’.

When asked “How do you make a blue heart red?”
“With a torch and chromium (III) ions”, I said

Jim Clark, writing on Chemguide.co.uk notes that “The simplest ion that chromium forms in solution is the hexaaquachromium(III) ion – [Cr(H2O)6]3+.” He also goes onto say that “The hexaaquachromium(III) ion is a “difficult to describe” violet-blue-grey colour.”

Our AQA A’ level Chemistry describes [Cr(H2O)6]3+ as being ruby in colour, but our solution of chromium (III) chloride definitely looks blue to me.

IMG_3956_320

chromium (III) chloride solution

That is, until one looks at it under torchlight (tungsten filament bulb), then
indeed it does appear a ruby colour.

2016_04_30_blue_ruby

blue or ruby?

IMG_3961_320

Use a torch

IMG_3962_320

shine a light

IMG_3963_320

then you’ll see ruby delight

2016_04_22_MVI_3950_33percent2.gif

off on off

2016_04_30_on_off_300

has that boiling tube got a red eye?

 

Sweet TLC hydrolysis

Aspartame sweetener is a modified dipeptide. When aspartame is reacted with 6M hydrochloric acid the peptide bond should be hydrolysed (broken), producing aspartic acid and phenylalanine methyl ester. The methyl ester may also be hydrolysed, depending on the severity of the conditions.

2016_03_19_aspartame_whiteboard

Whiteboard of the aspartame hydrolysis by my friend and colleague Tony Pluck

We carried out a hydrolysis reaction by boiling aspartame sweetener (one individual sachet) with 10ml of 6M HCl in a boiling water bath for 30 minutes. The results were analysed using thin layer silica chromatography, (silica TLC).

IMG_3885

Running a chromatogram (silica TLC plate)

Samples of aspartame sweetener were spotted before and after the hydrolysis, together with phenylalanine and aspartic acid standards.

The plate was run in a solvent system of butan-1-ol (12ml), glacial acetic acid (3ml) and distilled water (6ml) taken from the old Nuffield A’level Chemistry textbook .

After allowing the solvent to run up to within a few cm of the top of the plate it was removed from the beaker and dried using a hot hair dryer. The TLC plate was then sprayed with ninhydrin and heated in an oven at 110 C for approximately 10 minutes.

IMG_3889

Samples left to right, phenylalanine, hydrolysis mixture, aspartic acid and aspartame sweetener.

2016_03_20_aspartame_expt_02

Same as the above with labels on the plate

Another experiment was carried out, identical to the above except with a much shorter hydrolysis time (of about 3 minutes). This was to see if the whole experiment could be done in a 55 minute period of chemistry at school.

2016_03_20_aspartame_expt_01

The same again with a much shorter hydrolysis time of a few minutes

We want to run further chromatograms spotting smaller samples, as both of the above plates suffered from streaking due to sample overloading.