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CHAPTER 6

CHEMICAL REACTIONS

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Chemical reaction represents the phenomenon through

which one or more substances are transformed in other

substances, without affecting the nature of the constituent

atoms of the transformed substances.

In the environment several reactions can be

observed, although most of them have a slow rate. Some

examples in this way are rusting of the steel pieces,

alcoholic fermentation, green turning of leaves due to the

forming of chlorophyll, the ignition of fuels. Chemical

reactions can be emphasized through the next

manifestations:

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a) Evolution of gas bubblesIf we introduce a

piece of zinc in a

hydrochloric acid

solution, we may

observe the

hydrogen evolution

reaction.

Zn + 2HCl ZnCl2 + H2

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A more violent reaction

occurs between

sodium and water. The

reaction product is also

hydrogen.

2Na + 2H2O 2NaOH + H2

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b) Forming of precipitates By pouring sodium

dichromate solution in a

lead nitrate solution we

observe the appearance

of a yellow-coloured

precipitate consisting of

slightly soluble lead

dichromate.

Pb(NO3)2 + Na2Cr2O7 2NaNO3 + PbCr2O7

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c) Changing of colour

Substances absorb light of different wave lengths, so

they appear differently coloured. Changing the nature

of a substance through a chemical reaction can sometimes

lead to color modifications. So, if in a colourless solution of

ammonium thiocyanate we pour an iron (III) and ammonia

sulphate solution we observe the colouring of the solution in

deep red because of the forming of the iron (III) tiocyanate.

Sometimes the modifying of colour can be the sign of a

physical process, not necessary chemical.

3NH4SCN + FeNH4(SO4)2 Fe(SCN)3 + 2(NH4)2SO4

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d) Appearance of flame

This is another sign that a

chemical reaction takes place.

An example is the ignition

reactions of hydrocarbons.

The flame that appears at the

Bunsen bulb is the sign of the

oxidation reaction of methane

with oxygen from air.

CH4 + 2 O2 CO2 + 2 H2O

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e) Modification of physical properties of solutions

This is another proof of a chemical reaction. Such kind of

property is conductivity.

If in a vessel with hydrochloric acid solution we add a

sodium hydroxide solution, with the help of a conductivity

meter one can measure the decreasing of the solution’s

conductivity until the complete neutralization of the acid.