Experiment 5.2

Theory

Decomposition reactions (general information)

“A decomposition reaction is a chemical reaction in which two or more products are formed from one reactant.”

A more complete explanation

Decomposition reactions is a process in which on the left-hand side of the arrow there is one reactant and on the right-hand side of the arrow two or more products.

The reactant in these processes is always made up of two or more different kinds of atoms. It is therefore a compound.

A drawing to explain a decomposition reaction more easily:

And this is how a decomposition reaction looks like in a equation:

CaCO3 >>> CaO + CO2

Most decomposition reactions can only take place when energy is supplied;

energy will seldom be released.

An example of an experiment which produces energy is mentioned below

An orange substance, ammonium dichromate ((NH4)2Cr2O7), will disappear during a decomposition reaction and a green powder will be formed instead.

On the decomposition of ammonium dichromate three products are formed.

The green powder contains 1. chromium oxide (Cr2O3)

2. nitrogen (N2)

3. water vapour (H2O)

The word equation: ammonium dichromate (s) >>> chromium oxide (s) + nitrogen (g) + water (l)

During this decomposition, visible fire is present. It proves that this decomposition produces energy.

Reaction temperature

For a decomposition reaction, a certain temperature is needed. This is called the reaction temperature. At a temperature lower than the reaction temperature, no reaction will occur. The energy needed to reach the reaction temperature does not effect the total energy effect of a reaction.

The reaction products of a decomposition reaction

A decomposable substance is always a compound.

When you decompose the compound again, the reaction products are very often elements, but not always!

Different types of decomposition reactions

Like mentioned before, most decomposition reactions need energy.

There a different sources of energy: heat, electricity and light.

Thermal decompositions:

a decomposition reaction using heat as a source of energy.

Electrolysis:

a decomposition reaction using electricity as a source of energy.

Decomposition by light:

a decomposition reaction using light as a source of energy.

These decomposition reactions are going to be explained more deeply in the next pages of this report.

Thermal decomposition reaction

What is a thermal decomposition reaction? This experiment is a thermal decomposition reaction. Thermal decomposition reactions use heat as source of energy. This is a very common decomposing – when you are cooking in the kitchen you also use thermal decomposition reactions! For example when you boil your potatoes or vegetables. Even when you burn your food it is a thermal decomposition reaction!

What happens in a thermal decomposition reaction? A thermal decomposition reaction is a kind of decomposition reaction, obviously. This means that one reactant is changed into multiple products. The products have different characteristics than the reactant has, you are dealing with a chemical reaction. The word thermal in the name ‘thermal decomposition’ has to do with the source of energy used. Thermal means heat. So, a thermal decomposition reaction is a decomposition reaction where heat is used as energy source.

How do you recognize a thermal reaction? Firstly, take a look if you are dealing with a chemical reaction. This is when new substances with different properties are formed from the reactants. Check! Then, search for the kind of reaction. Is there one reactant and are multiple products formed? That’s a decomposition reaction! Lastly, what kind of energy is used? If heat is used to decompose the reactant, then it is a thermal decomposition reaction!

The experiment

Research question

Is it possible to divide the compound sugar into different elements using a thermal decomposition reaction?

Hypothesis

The sugar will heat up and change into another substance. I think that it will stick together because it will melt a bit. The sugar will change color and become black or brown. Some gas will be formed too. I think the sugar will smell burned after heating it. I think that water (or hydrogen and oxygen apart from each other) and carbon will be formed, because the chemical equation of glucose is C6H12O6.

Procedure

List of materials used:

• Test tube

• Dry granulated sugar

• Burner

• Long clothes pin

• A lighter

Procedure:

• The test tube with the sugar was hold above the burner. The flame was small and colourless and therefore the sugar was heated slowly.

• At the moment there was gas coming our of the tube, a lighter was used to lit the substance.

Results

Observations:

In the beginning the sugar looked like ordinary sugar. When it was heated, it slowly turned yellowish and it started to smell like cotton candy. After some more time it became brown and finally black. There was gas coming out of the tube. The gas caught fire when the lighter was hold near it. When the lighter was removed, the flame would still keep burning. When it stopped the sugar turned into a black solid, it filled one third of the tube. This was a lot more than in the beginning; then it filled only a very small part of the tube. The black solid smelled bad and looked like crystals.

Conclusion:

This was a complete thermal decomposition reaction because the properties of the substances change, multiple products were formed (the gas and the black solid) and heat was used as source of energy. It was a complete combustion because there is no sugar left.

Multiple chemical reactions occurred. When the sugar was heated in the beginning it was a transition of phase, it was melting. Then the color changed and it started to effervesce. This was the decomposition reaction. In the decomposition reaction gas and the black solid were formed. The black solid was carbon.

The hypothesis was partly right. Indeed, water and carbon were formed. The sugar melted, it stuck together and changed into black. I didn’t expect the yellow and brown color and the sweet smell, I thought it would smell burnt. The experiment was carried out in the right way. The gas was detected because you could lit it. Because you could lit the gas you also know that the gas is a combustible substance, which makes it easier to identify the substance.

The chemical equation for this experiment is C6H12O6  C + H2O.

When you balance this the equation becomes C6H12O6  6C + 6H2O.

We have found the answer to our research question. Is it possible to divide the compound sugar into different elements using a thermal decomposition reaction? This experiment shows it: yes, it is possible! The elements carbon and water are formed.

So, which types of decomposition reactions are there? After this experiment you know that you can have a decomposition reaction using light as energy source!

Research question

What does silver nitrate and sodium chloride turn into when it is exposed by light?

Hypothesis

I expect that the solution will turn into two other aqueouses and/or solids.

Procedure

List of materials used:

• 2 test tubes

• Water

• Silver nitrate

• Sodium chloride

• (Light)

Procedure:

• The 2 test tubes were both filled with different aqueouses. One was filled with a solution of silver nitrate in water. The other one was filled with a solution of sodium chloride in water.

• Both were split up in two equal amounts and mixed with half the amount of the other aqueous. A precipitate was formed. A precipitate is sensitive to light.

Results

Observations:

Two test tubes were both filled with a part of sodium chloride and a part of silver nitrate.

Once one of the tubes was placed in the light a reaction occurred. The test tube was turned into a clearer solution.

Besides that the solution turned from a turbid compound into a clearer one, it could also be noticed that in the first tube (placed in daylight) solid particles were lying at the bottom.

The color of the compound, which was lying at the bottom of the tube, turned black with a red shade after being exposed with daylight.

Conclusion:

Silver nitrate and sodium chloride absorb radiation (light). The two products are silver chloride and sodium nitrate. Silver chloride can be noticed at the bottom as a solid.

The chemical equation for this experiment is:

Silver nitrate + sodium chloride  silver chloride + sodium nitrate

AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)

Silver nitrate, sodium chloride and sodium nitrate are all aqueous solutions. This means that they are solutions in which the solvent is water.

The decomposition of both silver nitrate and sodium chloride resulted in the new solid silver chloride and the aqueous; sodium nitrate. The change of color of the compound at the bottom can be explained by the existence of the small particles of silver metal.

In this experiment light is used as a catalyst. The light is not mentioned in the formula though it is necessary to speed up the reaction.

Like I expected, there were indeed 2 different phases formed. Silver chloride = a solid and sodium nitrate = an aqueous.

It was necessary to make use of two tubes with the same solution in it. So when we placed in the light, we were able to compare that one with the one which was kept in the dark. Without the second tube we couldn’t prove if this result was due to the light.

Conclusion

This is a general conclusion about the experiments. For detailed conclusions of every experiment, take a look at the different experiments in this report.

We were searching for different ways of a decomposition reaction. After having done the three experiments we have found three ways of decomposition. We found out that it is possible for decomposition to occur using light, heat or electrodes as an energy source.

In the first experiment, the thermal decomposition reaction, a complete decomposition of glucose occurred. All the glucose was transformed into carbon and water.

By doing the second experiment, the decomposition with light as an energy source, we found that silver nitrate and sodium chloride absorb radiation (light). The two products are silver chloride and sodium nitrate. Silver chloride could be noticed at the bottom of the tube as a solid. This was also a decomposition reaction.

And last but not least, we found that the electrolysis.

Our hypothesis was because we thought and .

Aknowledgements

Where did we get our information? Well, mostly from our course book. We also used the notes and the workbook for the details about the experiment. Internet was a small source of information, mostly for the pictures.

• http://chemwiki.ucdavis.edu - title page picture

• www.tutorvista.com - thermal decomposition picture

• www.photographersdirect.com - theory picture of thermal decomposition

• http://www.rustyiron.com - picture theory electrolysis

• http://www.itp.uni-hannover.de - picture theory light decomposition

• http://dl.clackamas.edu - information about decomposition

• Our course book, ‘It’s all chemistry’, EPN - theory and information