1 chapter 6 chemical reactions. 2 chemical reaction represents the phenomenon through which one or...
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1
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.
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