Illustrated Guide to Home Chemistry Experiments

162 DIY Science: Illustrated Guide to Home Chemistry Experiments

example, the general form AB → A + B is expanded to ABC →

A + B + C or, more precisely, 2 ABC → A + B + C, where ABC is

sodium bicarbonate, A is sodium carbonate, B is carbon dioxide,

and C is water. The balanced equation for this reaction is:

2 NaHCo 3 → Na 2 Co 3 + Co 2 + H 2 o

displacement reaction
A displacement reaction, also called a replacement reaction,
occurs when two or more reactant substances recombine to
form two or more different products.

Single displacement reaction

In a single displacement reaction, a more active element

displaces a less active element in a compound, forming a new

compound that incorporates the more active element and

releasing the less active element in elemental form. Single

displacement reactions take the general form A + BX → AX

+ B, where A is the more active element, B is the less active

element, and X is an anion (such as chloride, sulfate, or nitrate).

For example, if you react sodium metal (Na) with hydrochloric

acid (HCl), the more active sodium displaces the less active

hydrogen, according to the following balanced equation:

2 Na + 2 HCl → 2 NaCl + H 2

double displacement reaction

In a double displacement reaction, also called a metathesis

reaction, two compounds exchange elements or ionic species

with each other. Double displacement reactions take the

general form AX + BY → AY + BX, where A and B are cations

and X and Y are anions. If AX and BY are both freely soluble and

(for example) AY is insoluble, combining solutions of AX and BY

results in a precipitate.

Any of these reaction types can be represented by
stoichiometrically balanced equations. For example, the
decomposition reaction of sodium hydrogen carbonate
into sodium carbonate, carbon dioxide, and water can be
represented as:

NaHCo 3 (s) → Na 2 Co 3 (s) + Co 2 (g) + H 2 o(g)

or, in balanced form:

2 NaHCo 3 (s) → Na 2 Co 3 (s) + Co 2 (g) + H 2 o(g)

The laboratory sessions in this chapter explore various aspects
of these types of chemical reactions.

dECmpoo SITIoN By STAGES

In practice, the actual products from the decomposition

of sodium hydrogen carbonate depend on the

temperature and duration of heating. At higher

temperatures and longer durations, the products are

indeed anhydrous sodium carbonate, carbon dioxide gas

and water vapor. At lower temperatures and durations,

some or all of the water may be retained by the sodium

carbonate as water of hydration.

This variation in products is common to decomposition

reactions, particularly if the reactant is a complex

organic compound. For example, the products of the

decomposition of sucrose (table sugar) by heating depend

on the temperature and duration. Heat sucrose gently and

you get caramel. Heat sucrose intensely (or decompose

it with concentrated sulfuric acid), and you end up with

nothing more than carbon, carbon dioxide, and water.

EvdRE y Ay CHEmICAL REACTIoNS

Chemical reactions are so much a part of everyday life

that it’s impossible to provide even a representative

list in this space. Nearly everything with which we have

daily contact is the product of a chemical reaction or

reactions, and we’re constantly surrounded by chemical

reactions in progress.

If you make bread, for example, it rises because of

chemical reactions, bakes because of other chemical

reactions, and grows stale because of still other chemical

reactions. When you eat the bread, chemical reactions

in your body convert the starches it contains to glucose,

which your cells then metabolize via other chemical

reactions to provide the energy you use to complete the

labs in this book. The carbon dioxide waste from those

metabolic chemical reactions enters your bloodstream

via other chemical reactions, and is eventually converted

back into carbon dioxide gas by other chemical reactions

and disposed of when you exhale. Our bodies are, in

effect, chemical reaction vessels.

Civilization itself is fundamentally based on chemical

reactions under human control. The two pillars of

civilization are human manipulation of metals and of

energy, both of which ultimately devolve to controlled

chemical reactions.