Page 314 - (DK) The Ultimate Visual Dictionary 2nd Ed.

PHYSICS AND CHEMISTR Y

Chemical reactions SALT FORMATION (ACID ON METAL)

Glass
A CHEMICAL REACTION TAKES PLACE whenever bonds between
beaker
atoms are broken or made. In each case, atoms or groups of Hydrogen gas
(H 2 ) given off
atoms rearrange, making new substances (products) from the
original ones (reactants). Reactions happen naturally, or can be Zinc (Zn) replaces
hydrogen in acid
made to happen; they may take years, or only an instant. Some
(HCl) to form zinc
of the main types are shown here. A reaction usually involves chloride solution
a change in energy (see pp. 314-315). In a burning reaction, for (ZnCl 2 )
example, the making of new bonds between atoms releases energy Hydrogen in acid
driven off when Hydrochloric
as heat and light. This type of reaction, in which heat is given
acid meets a acid (HCl)
off, is an exothermic reaction. Many reactions, like burning, are reactive metal
irreversible, but some can take place in either direction, and are Effervescence
said to be reversible. Reactions can be used to form solids from Zinc metal
chippings Zinc metal
solutions: in a double decomposition reaction, two compounds (Zn) chippings (Zn)
in solution break down and re-form into two new substances,
THE REACTION
often creating a precipitate (insoluble solid); in displacement, Hydrochloric acid added to zinc
an element (e.g., copper) displaces another element (e.g., silver) produces zinc chloride and hydrogen.
from a solution. The rate (speed) of a reaction is determined by Zn + 2HCl → ZnCl 2 + H 2
many different factors, such as temperature, and the size and DISPLACEMENT
shape of the reactants. To describe and keep track of reactions,
internationally recognized chemical symbols and equations
are used. Reactions are also used in the laboratory to identify Copper metal Copper (Cu)
matter. An experiment with candle wax, for example, (Cu) displaces silver
ions (Ag 2+ ) from
demonstrates that it contains carbon and hydrogen.
silver nitrate
Silver nitrate solution (AgNO 3 )
BURNING MATTER solution
In this burning reaction, (AgNO 3 ) Blue solution of
Ammonium dichromate atoms form simpler copper nitrate
((NH 4 ) 2 Cr 2 O 7 ) substances and give Two metals compete (Cu(NO 3 ) 2 )
off heat and light for nitrate ions forms
Flame Ammonium dichromate Needles of silver
((NH 4 ) 2 Cr 2 O 7 ) converts to Glass flask metal (Ag) form
chromium oxide (Cr 2 O 3 )
THE REACTION Nitrogen monoxide (NO) THE REACTION
When lit, ammonium and water vapor (H 2 O) Copper metal added to silver nitrate solution
dichromate combines with oxygen from air. given off as colorless gases produces copper nitrate and silver metal.
(NH 4 ) 2 Cr 2 O 7 + O 2 → Cr 2 O 3 + 4H 2 O + 2NO Cu + 2AgNO 3 → Cu(NO 3 ) 2 + 2Ag
A REVERSIBLE REACTION Pipette Pipette Sodium
hydroxide
Flat-bottomed Hydrochloric acid (NaOH)
glass flask (HCl) added in drops Sodium hydroxide (NaOH) added in
neutralizes the acid drops
Potassium Acid causes reaction
Chromate solution to take place Solution turns to
(K 2 CrO 4 ) bright orange of Solution returns
Chromate ions potassium dichromate to original
converted to orange bright yellow
Bright yellow dichromate ions Potassium color
solution contains Potassium dichromate (KCr O 7 )
2
potassium and dichromate re-forms to potassium
chromate ions chromate (K 2 CrO 4 )
(KCr 2 O 7 ) forms
1. THE REACTANT 2. THE REACTION 3. REVERSING
Potassium chromate dissolves in water to Addition of hydrochloric acid changes Addition of sodium hydroxide changes
form potassium ions and chromate ions. chromate ions into dichromate ions. dichromate ions back into chromate ions.
2- 2- 2- Cr 2 O 7 → 2CrO 4 2-
2-
K 2 CrO 4 → 2K + + CrO 4 2CrO 4 → Cr 2 O 7
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