Chromatography of chlorophyll

The chromatography of chlorophyll is an experiment often carried out at school. An extract from grass was made by grinding up some grass in a pestle and mortar with a 50:50 mixture of propanone and petroleum ether (60-80 Celsius b.p. range).

Pestle and mortar, grass, scissors and solvent.

Pestle and mortar, grass, scissors and solvent.


Fold up for cutting

Fold up for cutting


Careful of those fingers!

Careful of those fingers!


A close cut ...

A close cut …


... saves time when grinding.

… saves time when grinding.


Ground up extract.

Ground up extract.


Capillary action draws up the liquid.

Capillary action draws up the liquid.


Spotting samples on a 5 x 10cm silica TLC plate

Spotting samples on a 5 x 10cm silica TLC plate


Ready for elution

Ready for elution


The solvent system used was a 70:30 mixture of diethyl ether : petroleum ether (bp 60-80).
Away we go

Away we go


End result

End result


Here’s an animation showing another plate running.
Chromatography of chlorophyll

Chromatography of chlorophyll


A video of the experiment can be viewed here

Glowing things

Quinine gives a distinctive bitter taste to tonic water and also fluoresces brightly under uv light.

Quinine

Quinine

Using tonic water in place of water when making jelly, you can make jelly that “glows in the dark” under uv light.
Left cup - jelly with tonic water

Left cup – jelly with tonic water

Chlorophyll glows bright red under uv light and it’s easy to make an extract containing chlorophyll from green leafy vegetables like spinach.
Spinach juice containing chlorophyll

Spinach juice containing chlorophyll

By combining the two – tonic water jelly and spinach extract, we were able to make a spooky vampire glowing thing.
Fancy a bite?

Fancy a bite?

It’s good enough to eat.

2014 is the International Year of Crystallography.

Commercial tonic water typically contains about 67mg of quinine per litre. It may be possible to separate quinine from tonic water in the school laboratory and use this an International Year of Crystallography project.

Snow from brine

Models of fir trees covered in snow are fun and easy to make.

Fir trees covered in snow

Fir trees covered in snow

We followed the recipe in CLEAPSS GL 131 “Snow scene crystals” and used 20 cm3 of saturated sodium chloride solution (brine) and 2 drops of 0.1M potassium hexacyanoferrate (II) solution per model.

First you need to cut out a shape from cardboard (or other porous material). It is best if this is free standing as shown below. Water soluble colour can also be applied at this stage. We used felt tipped pens.

Cardboard cut-out

Cardboard cut-out

We found shapes with wide bases less than 10cm tall were less likely to fall over.

Once the salt solution is added as described above it takes time to soak up through the cardboard. Evaporation of the water leads to stunning crystal ‘snow’ formation, especially along the edges of the shape.

Fir tree model in sunlight

Fir tree model in sunlight

Crystals close-up

Crystals close-up

With colours

With colours

Snow man?

Snow man?


The salt crystals seen here are very small and (according to the CLEAPSS sheet) due to the effect of the potassium hexacyanoferrate (II), which produces multiple nucleation points around which the crystals form.

Such small crystals are exactly the opposite of the large crystals one would need to carry out X-ray crystallography in order to determine the structure of a crystal. The cubic structure of sodium chloride is however, well known (and the ones in the snow scenes are assumed to be the same).

We made a model of the structure of sodium chloride using modelling clay.

Modelling clay

Modelling clay

Sodium chloride crystal model - cubic structure

Sodium chloride crystal model – cubic structure

Round and round

Round and round


What is 2014 the International Year of?

Here are the answers to the questions posed in the December 13th blog:
1 The gas which inflated the balloons was carbon dioxide.
2 Symbol equations for the two chemical reactions are:
NaHCO3 + HCl -> NaCl + H2O + CO2
CaCO3 + 2HCl -> CaCl2 + H2O + CO2
3 Calculations to show how much gas would be produced by 0.5g of each carbonate:
For NaHCO3
n=m/Mr
where n = number of moles in mol
m = mass in g
Mr = relative molar mass in g mol-1
n = 0.5/84 = 0.006 mol
For CaCO3
n = 0.5/100 = 0.005 mol
It can be seen from the equations that the number of moles of CO2 produced in each case is the same as the number of moles of carbonate used.
4 Since the HCl is in 6.67 to 8 times molar excess the NaHCO3 might be expected to inflate the balloon slightly faster. In the lab it was found that the result was too close to call with both balloons inflating in a matter of seconds.

Finally, at least one other source described making snow scenes using potassium hexacyanoferrate (III) instead of (II). This might be worth investigating from a safety point of view if making up a large volume of stock solution and one would need to consult the relevant MSDS sheets.

Rudolph the red nosed boiling tube!

Here’s some festive fun with a couple of red balloons, some cotton wool and two boiling tubes.

Santa’s challenge is to find out which special reindeer food makes Rudolph’s nose grow big the fastest.

Rudolph ready to blow!

Rudolph ready to blow!

1. Place 20ml 2M hydrochloric acid in a boiling tube and add Universal Indicator solution to give a deep red colour.
2. Push a small amount of cotton wool into the boiling tube so that it forms a platform above the acid. (Don’t use too much cotton wool, the least amount possible the better).
3. Add 0.5g sodium hydrogencarbonate to the tube so that it rests on the cotton wool.
4. Place a red balloon over the end of the tube.
5. Repeat steps 1 to 4 using 0.5g calcium carbonate instead of the sodium hydrogencarbonate.

When ready tilt the tubes so that the acid soaks through the cotton wool and comes in contact with the carbonates. Start a stopwatch and see which tube inflates Rudolph’s red nose the quickest.

Which food made the biggest nose fastest?

Which food made the biggest nose fastest?

What is the name of the gas which inflates the balloon?
Can you write symbol equations for the two chemical reactions?
Can you show by calculation how much gas should be produced by 0.5g of each carbonate?
All other things being equal, which carbonate should produce most gas and inflate the balloon quickest?

Here are some cut out antlers which can be used to decorate the boiling tubes.

Cut out antlers

Cut out antlers

B/Z recipes

For the Belousov–Zhabotinsky reaction in a Petri-dish (3rd December 2013 post)
I used the following recipe:

6 cm3 0.5M potassium bromate(V), KBrO3
0.6 cm3 6M sulfuric(VI) acid, H2SO4
1 cm3 0.5M potassium bromide, KBr
2.5 cm3 0.5M malonic acid
1 cm3 Ferroin indicator solution (Merck commercial solution for waste water analysis)
I read in another source the Ferroin should be about 0.025M.

One mixes the ingredients in a boiling tube or small beaker sequentially in the order listed above.

Videos for the preparation of this mixture can be found here:
Preparation of a Belousov–Zhabotinsky reaction for use in a Petri-dish Part 1
and
Preparation of a Belousov–Zhabotinsky reaction for use in a Petri-dish Part 2

For the Belousov–Zhabotinsky reaction in a beaker (8th December 2013 post)
I used the following recipe:

100 cm3 1M sulfuric(VI) acid, H2SO4
2.86g malonic acid
1.04g potassium bromate(V), KBrO3
0.11g cerium (IV) sulfate, Ce(SO4)2.4H2O
0.5 cm3 Ferroin indicator solution (Merck commercial solution for waste water analysis)

A video for the preparation of this mixture can be found here:
Preparation of a Belousov-Zapotinsky reaction in a beaker

I have another recipe for this reaction in a beaker which is slightly different and I am going to try it out. In the animated .gif of the beaker reaction posted on the 8th December there was no green colour visible during the transitions and I’m hoping this second recipe will show a green colour.

B/Z in a beaker

Here’s a B/Z reaction in a beaker:
2013_12_08_B_Z_beaker_020_75
The small.gif should load quickly, but you many have to download the larger .gif below in order for it to play.

How many colours can you see?

How many colours can you see?

In this post and in the one before it we have been looking at Belousov–Zhabotinsky reactions. What are the ingredients used in these B/Z reactions?

B/Z reactions

B/Z is shorthand for a Belousov–Zhabotinsky reaction.

These produce interesting patterns when carried out in a Petri-dish.
We carried out some B/Z reactions in our Chemistry Club the other day.
Here are some .gif animations of our results.2013_11_21_BZ_01

2013_12_01_B_Z_03_odds_selection_020

2013_12_01_B_Z_05_selected_005

2013_12_01_B_Z_09_020

2013_12_01_B_Z_14_selection_015

What happens when a B/Z reaction is carried out in a beaker?

Pop test

Last time I asked if you could write a symbol equation for the reaction between ammonia and hydrogen chloride gases.

The equation is:
NH3 (g) + HCl (g) –> NH4Cl (s)

I also asked why fluorescent inks in highlighter pens glow. The answer is that certain molecules within the ink absorb energy of one wavelengths (e.g. u.v.) and emit it at a longer wavelength, as visible light. Here are the inks extracted for the Halloween display in bright sunlight.

Highlighter pen inks in sunlight

Highlighter pen inks in sunlight


Finally, I also talked about a mixture of hydrogen and oxygen gases being unlikely to react until a spark or flame is introduced to the mixture. The spark or flame provides the necessary activation energy needed to get the reaction started between a few molecules, and then the two gases react explosively.

A small hydrogen and oxygen explosion

A small hydrogen and oxygen explosion

That this explosive reaction is exothermic is self evident. A lot of energy is given out as heat, light and sound. “Pop!”

Here are some .gif animations of hydrogen pop tests:

Hydrogen and oxygen bubbles ignited with a burning splint

Hydrogen and oxygen bubbles ignited with a burning splint

"Pop!" Testing hydrogen gas with a lighted splint.

“Pop!” Testing hydrogen gas with a lighted splint.

A value for the energy released (the enthalpy of reaction), can be calculated by using bond energies.
Calculating the enthalpy of combustion of hydrogen using bond energy values

Calculating the enthalpy of combustion of hydrogen using bond energy values

The bond energy calculation above uses the values listed in my September 29th blog.

However, this value is unlikely to be as accurate as the value obtained from direct calorimetry experiments.

Of course, it is possible for a mixture of hydrogen and oxygen gases to react spontaneously on its own, without a flame being supplied. It could happen if molecules of hydrogen and oxygen collided with enough energy (the activation energy) by chance. It is just rather unlikely at room temperature.

What is a B/Z reaction? And what does it show?

Halloween chemistry 2

Fluorescent ink is easy to extract from highlighter pens and makes a great halloween chemistry display when illuminated under uv light.
Ghoulish glowGhoulish glow 2Ghoulish glow 3Ghoulish glow 4
Last time I asked if you could calculate the enthalpy of reaction for the reaction between hydrogen and fluorine using the bond energies supplied. Here is how you do it:

The bond energy approach to calculating an enthalpy change for a reaction

The bond energy approach to calculating an enthalpy change for a reaction

Hydrogen and fluorine react spontaneously, but a mixture of hydrogen and oxygen needs a spark in order to make it explode. The reason for this is that the spark provides the activation energy needed by the molecules of hydrogen and oxygen immediately surrounding it to react. Since the reaction between these molecules is exothermic, they in turn provide the activation energy to cause further molecules of hydrogen and oxygen in close proximity to also react. In an instant, these processes extend throughout the mixture until all of the hydrogen and oxygen molecules have reacted.

Finally, when molecules ammonia and hydrogen chloride gases are brought together they react spontaneously to form solid ammonium chloride. In a diffusion tube experiment this is seen as a cloud of white smoke at the point where the two gases meet. Ammonium chloride is an ionic solid and the formation of the ionic bonds is an exothermic process as illustrated by the images below.

Ammonia and hydrogen chloride diffusion experiment at the start

Ammonia and hydrogen chloride diffusion experiment at the start

Ammonium chloride produced as a white cloud

Ammonium chloride produced as a white cloud

Temperature probe moved into the cloud of ammonium chloride being formed

Temperature probe moved into the cloud of ammonium chloride being formed

Maximum temperature increase - an exothermic reaction

Maximum temperature increase – an exothermic reaction

Smaller animated gif which should show motion within the blog

Smaller animated gif which should show motion within the blog


Large animated gif of the experiment - download to view

Large animated gif of the experiment – download to view


Can you write a balanced symbol equation for the reaction between ammonia and hydrogen chloride?
And why do fluorescent inks glow?

Flash, bang, wallop

Hydrogen and fluorine react explosively on meeting. The reaction is instantaneous. According to school textbooks this happens even when the gases are mixed at very low temperatures in the dark.

H2 + F2 –> 2HF

Hydrogen will also react explosively with oxygen, but this reaction is very unlikely to occur spontaneously. To get these two gases to react some energy needs to be supplied, typically in the form of a spark or a small flame, as in the hydrogen ‘pop’ test. Once started however, hydrogen and oxygen react just as rapidly as do hydrogen and fluorine.

2H2 + O2 -> 2H2O

Hydrogen and oxygen combustion

Hydrogen and oxygen explosion
The yellow colour is from the detergent used to trap the bubbles of gas

That hydrogen and fluorine react spontaneously on mixing, whilst hydrogen and oxygen do not, perhaps suggests something in particular about the reactivity of fluorine.

The reaction of hydrogen and fluorine using Lewis diagrams

The reaction between hydrogen and fluorine illustrated using Lewis diagrams

A chemical reaction can be seen as a rearrangement of the order in which the atoms are bonded together in the reactants to form products.

For a reaction to occur the molecules involved in the reaction must first meet; they must collide. This is quite easy to imagine for gases. Gases mix immediately and in all proportions. Their molecules are moving around at random and at relatively high speeds. Many collisions take place. But not all collisions between molecules lead to chemical reactions, for a collision to be successful and result in a reaction the molecules must collide with enough energy and in the correct orientation (that is, in the right way).

The sequence of events by which certain bonds in the reactants are broken and new bonds in the products are formed is described as the reaction mechanism. And without knowing what the actual chain of events for the reaction between hydrogen and fluorine is, by comparing the relative strengths of the bonds in the reactants and products, we can still get some idea as to why this reaction is both spontaneous and explosive.

Table of some bond energies

F-F 159 kJ mol-1
H-H 435
O=O 498
H-F 569
O-H 465

The covalent bond in fluorine has a lower value than the other bond energies in the table. This means a smaller amount of energy needs to be put in to break the fluorine fluorine bond, than the bonds in hydrogen or oxygen. Indeed fluorine is a very reactive gas that reacts with just about everything it comes into contact with and cannot be used in schools.

When molecules of fluorine collide with molecules of hydrogen less energy is needed to initiate a reaction during the collision, compared to the energy required for molecules of hydrogen and oxygen to react. It would appear that the energy needed to start the reaction between hydrogen and fluorine, the so-called activation energy, is particularly low.

Part of the answer to the question I asked last time as to why hydrogen and fluorine react so readily, is because the covalent bond in fluorine is so weak and easy to break. The reaction between hydrogen and fluorine is explosive and gives out energy. It is exothermic. It gives out energy because the product molecules (hydrogen fluoride) are more stable than the reactant molecules (hydrogen and fluorine). Can you calculate a value for the energy change for this reaction (the enthalpy change of reaction) using the bond energies in the table?

And why does a small spark or flame provide enough energy to initiate an explosive reaction between hydrogen and oxygen?

Another spontaneous reaction that is commonly encountered at school is the reaction between ammonia and hydrogen chloride. Even though this reaction is not explosive it does provide some interesting insights into the nature of chemical reactions and the way we describe them as chemists.

Ammonia and hydrogen chloride diffusion experiment

Ammonia and hydrogen chloride diffusion experiment

What is the product formed when ammonia reacts with hydrogen chloride?