Unreactive metals

One way to make a salt is to react a metal with an acid.

acid + metal -> salt + hydrogen

However, not all metals react with acids and those that do often react at different rates.

GCSE level students learn a reactivity series of metals. This lists a number of common metals in order of decreasing reactivity in chemical reactions.

One such list (in order of decreasing reactivity) would be:

Potassium
Sodium
Lithium
Calcium
Magnesium
Aluminium
Zinc
Iron
Tin
Lead
Copper
Silver
Gold

Potassium is the most reactive metal in this list and gold is the least reactive.

At school students frequently carry out reactions on a test-tube scale to illustrate the reactivity series of metals. One such reaction is the reaction of metals with hydrochloric acid.

In the pictures below magnesium, zinc, iron* and copper are shown reacting with 2M hydrochloric acid.

Four metals used at school to illustrate the reactivity series of metals

Four metals used at school to illustrate the reactivity series of metals

What is the order of reactivity of the four metals with 2M hydrochloric acid?

What is the order of reactivity of the four metals with 2M hydrochloric acid?


After about a minute magnesium is seen reacting the most vigourously of the four, followed by zinc which is the second most reactive.
After a few tens of seconds

After a few tens of seconds

*The ‘iron’ nail is almost certainly not pure iron, but probably a form of steel.

Under these conditions the magnesium completely reacts in a couple of minutes. After about an hour the zinc is seen reacting steadily and bubbles of hydrogen gas can be seen on the surface of the nail.

After about an hour

After about an hour

Thus, the order of reactivity as illustrated by this experiment is:
Magnesium
Zinc
Iron
Copper

Copper is sometimes described as being unreactive in this experiment, but the word unreactive must not be taken to mean ‘does not react at all’. Copper metal does react with 2M hydrochloric acid, only it reacts rather slowly. This is illustrated by the animation below (apologies for the wobbly nature of the timelapse photography here).

Iron and copper with 2M HCl for 1 week

Iron and copper with 2M HCl for 1 week


In this experiment, at the end of one week the rather thin piece of copper foil had virtually all gone, whilst the much bigger iron nail, or its remnants, were still seen bubbling away.
Copper foil after one week in 75ml 2M HCl

Copper foil after one week in 75ml 2M HCl

Iron nail after one week in 75ml 2M HCl

Iron nail after one week in 75ml 2M HCl

Thus, copper can be described as being relatively unreactive in this reaction with 2M hydrochloric acid, but it does react slowly. Can you suggest any metals which would not react with 2M hydrochloric acid?

Last time quiz crystal 1 was quartz and quiz crystal 2 was iron pyrite or Fool’s Gold. The answer to the third quiz question was neutralisation.

What does the word ‘salt’ mean?

The word salt has two meanings in chemistry, a general meaning and a specific meaning.

General meaning
Salt = a chemical compound (made in a chemical reaction, for example, by reacting an acid with a base).

Specific meaning
Salt = sodium chloride (‘common salt’)

So, one way to make a salt is to react an acid with a base. The word equation for this is:
acid + base -> salt + water (general meaning of the word salt)

An example of this kind of reaction would be to react hydrochloric acid with sodium hydroxide:
hydrochloric acid + sodium hydroxide -> sodium chloride + water (and in this example the salt made is salt! So you could say the word salt has both the general and the specific meaning here. Two meanings in one go.)

At school, students often make salts by chemical reactions. And most of the salts featured in the June 2014 post are examples of salts commonly made by students. However, at GCSE level students are typically only required to learn the structure of one salt and the structure that they learn is that of salt (sodium chloride) itself.

Sodium chloride has a cubic structure.

Here is an animation illustrating the build up of a cubic structure from sodium ions, Na+ shown as yellow spheres and chloride ions, Cl- shown as green spheres.

Building a sodium chloride crystal lattice

Building a sodium chloride crystal lattice

NaCl in 2D

NaCl in 2D

NaCl in 3D

NaCl in 3D

Our students also learn three other crystal structures at GCSE, but none of them are salts. They are the structures of diamond, quartz and graphite.

Can you identify the crystals in the two pictures below? One of them appears as golden cubes, but it is neither salt nor gold.

Quiz crystal 1

Quiz crystal 1

Quiz crystal 2

Quiz crystal 2

Third quiz question: what is the name of the type of reaction that takes place when an acid reacts with a base? Answers next post.

Iycr2014

2014 is The International Year of Crystallography and here are some crystals to celebrate the event.

Sodium chloride
NaCl_01

Sodium chloride crystals in an evaporating basin

Sodium chloride crystals in an evaporating basin

Sodium chloride crystals close up

Sodium chloride crystals close up

Sodium chloride crystals

Sodium chloride crystals

Sodium bromide
NaBr 01NaBr 02

Sodium bromide crystals on a watch glass

Sodium bromide crystals on a watch glass

Sodium bromide crystals close up

Sodium bromide crystals close up

Sodium iodide
NaI 01NaI 02

Sodium iodide crystallising in a Petri dish under the microscope

Sodium iodide crystallising in a Petri dish under the microscope

Sodium iodide crystals

Sodium iodide crystals

Sodium iodide in an evaporating basin

Sodium iodide in an evaporating basin

Sodium iodide crystal chunks

Sodium iodide crystal chunks

Sodium iodide on a watch glass

Sodium iodide on a watch glass

Potassium chloride
KCl 01KCl 02

Potassium chloride crystallising on a microscope slide

Potassium chloride crystallising on a microscope slide

Potassium chloride crystals

Potassium chloride crystals

KCl 05
Large potassium chloride crystals forming

Large potassium chloride crystals forming

Large crystals on a microscope slide

Large crystals on a microscope slide

Potassium bromide
KBr 01

Potassium bromide on a microscope slide

Potassium bromide on a microscope slide

Potassium bromide in an evaporating basin showing crystal creep

Potassium bromide in an evaporating basin showing crystal creep

Potassium bromide crystals

Potassium bromide crystals

Potassium iodide
KI 01

Potassium iodide crystals

Potassium iodide crystals

Potassium iodide crystals

Potassium iodide crystals

Potassium iodide crystal creep

Potassium iodide crystal creep

Ammonium sulfate
ammonium sulfate 01

Ammonium sulfate crystallising in an evaporating basin

Ammonium sulfate crystallising in an evaporating basin

Ammonium sulfate crystals

Ammonium sulfate crystals

Magnesium sulfate
MgSO4 01

Magnesium sulfate crystallising on a microscope slide

Magnesium sulfate crystallising on a microscope slide

Magnesium sulfate in an evaporating basin

Magnesium sulfate in an evaporating basin

Magnesium sulfate crystals

Magnesium sulfate crystals

Copper (II) sulfate
CuSO4 01

Copper sulfate in an evaporating basin

Copper sulfate in an evaporating basin

Copper sulfate crystals - blue diamond shaped

Copper sulfate crystals – blue diamond shaped

Last time I asked what would be the colours of rubidium and caesium in flame tests. The flame colours can be seen on this Royal Society of Chemistry video. I can see blue, purple and red colours in the rubidium and blue and a little purple in the caesium.

Violet is another word used to describe the colours which I have called purple.

LiNaKCa flame tests

Group 1 metals react with water to produce hydroxides. For example, sodium reacts to produce sodium hydroxide and potassium would give potassium hydroxide, as shown in the equations below:

sodium + water -> sodium hydroxide + hydrogen
2Na + 2H2O -> 2NaOH + H2
potassium + water -> potassium hydroxide + hydrogen
2K + 2H2O -> 2KOH + H2

These hydroxides dissolve in the water as they are produced and the water becomes alkaline. Hence the Group 1 metals are called The Alkali metals.

Most simple alkali metal compounds are white, crystalline solids which are soluble in water. The alkali metal chlorides, LiCl, NaCl and KCl are good examples, which appear as identical white, crystalline salts when encountered in the school laboratory.

One way to tell the difference between these Group 1 chlorides would be to carry out flame tests on them.

Here a small sample of a each salt is introduced into a blue Bunsen burner flame. Can you identify the metal in each of the flame tests on the basis of its colour? And which of the four samples is the odd one out?

Answers are given in the second set of four pictures, which follow in the same sample order.

Crimson red

Crimson red

Yellow

Yellow

Lilac

Lilac

Orange red

Orange red

Of course, the colours that we see in these images depend on a number of things including the camera used to record the images and computer screen used to view them. The colours we actually see with our eyes during such experiments may differ in shade and intensity from those shown here.

The four animated .gif images above were assembled from images taken with a Casio FH100 digital camera (which has a 10.1MP 1/2.3-inch backlit CMOS sensor).

By way of contrast the still images below were recorded using the same samples under the same conditions with a Ricoh Caplio GX100 digital camera (10.01 MP CCD 1/1.75-inch primary-color sensor).

Lithium chloride flame test

Lithium chloride flame test

Sodium chloride flame test

Sodium chloride flame test

Potassium chloride flame test

Potassium chloride flame test

Calcium nitrate flame test

Calcium nitrate flame test

The differences between the two image sets are quite striking.

In school textbooks the colours are recorded as; lithium = crimson red, sodium = yellow, potassium = lilac and calcium = red (sometimes as orange red or brick red).

Rubidium chloride and caesium chloride also produce characteristic flame test colours. What are they?

The Group 1 alkali metals all react vigorously with cold water. Calcium reacts a little less vigorously and is in Group 2 of the Periodic Table, The Alkaline Earth metals.

As such is the odd one out of the four flame tests shown above.

Alkali metals in water

All together now!

All together now!

The reactions of the alkali metals with water are amongst the more spectacular demonstrations carried out at school. Lithium, sodium and potassium are commonly used, rubidium and caesium are not because they are too reactive.

Lithium, sodium and potassium are all stored in oil to prevent them reacting with air. Even so, a thick layer of oxide accumulates with time as shown in the pictures here:

Lithium

Lithium

Sodium

Sodium

Potassium

Potassium

Being in the same Group of the Periodic Table they all react in the same way. For example, when lithium reacts with water the products are lithium hydroxide and hydrogen gas.
lithium + water -> lithium hydroxide + hydrogen
2Li + 2H2O -> 2LiOH + H2

Lithium at the top of Group 1 is the least reactive alkali metal. As for the others reactivity increases with each successive member as you go down the Group with Francium the most reactive at the bottom.

Lithium with water

Lithium with water

Sodium with water

Sodium with water

Potassium with water

Potassium with water

Experimental observations are often recorded in a table like this:Snap 2014-05-16 at 16.27.59All three metals disappear as they react, forming soluble hydroxides as products.
Lithium, sodium and potassium added to water

Lithium, sodium and potassium added to water

Rubidium and caesium are even more reactive. Videos showing rubidium and caesium reacting with water can found on the internet, but not francium as it is too reactive to be isolated.

Here are some questions students are often called upon to answer at school:
1. Why are the Group 1 metals called the alkali metals?
2. What colours do lithium, sodium and potassium compounds produce in a Bunsen burner flame?
3. Write equations for the reactions of sodium and potassium with water.

Great snakes!

A popular experiment in our Chemistry Club is one that is widely known as ‘Black snakes’. We use a┬árecipe from Anne Marie Helmenstine at About.com Chemistry, where details about how to carry out this activity can be found.

The snaking trails of ash formed by the burning reaction mixture are exciting to watch as they grow.

Here is an animation of one such experiment:

Black snakes burning

Black snakes burning

We even got a dragon once!

Dragon

Dragon

What creature will you get?

 

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