unit a: chemistry - buck mountain
TRANSCRIPT
2 . 2 : E X P L A I N I N G P R O P E R T I E S O F S U B S T A N C E S
UNIT A: CHEMISTRY
IN THIS SECTION YOU WILL LEARN ABOUT...
• General Outcome #2 – Explain, using the periodic
table, how elements combine to form compounds,
follow IUPAC guidelines for naming ionic
compounds and simple molecular compounds • Classify ionic and molecular compounds, on the basis of their
properties
• Predict whether an ionic compound is relatively soluble in water,
using a solubility chart
• Relate the molecular structure of simple substances to their
properties: conductivity, pH, solubility and state
WHY IS THIS IMPORTANT??
• Both ionic and molecular compounds form crystals.
Possibly the most important molecular crystal
studied in the last century was DNA. DNA is the
molecule responsible for passing genetic
information from one generation to the next in all
living things. An X-ray photograph of DNA taken by
Rosalind Franklin in 1951 led to the understanding
that DNA is composed of two long chains twisted
into a helix.
IDENTIFYING IONIC COMPOUNDS
• Recall that ionic compounds always contain
positive and negative ions (the first is always a
cation and the second is always an anion)
• You can recognize an ionic compound by
inspecting it’s formula, remember that it always
begins with a metal (while molecular contain two
non-metallic atoms)
PROPERTIES OF IONIC COMPOUNDS
• High Melting Point
• all have high melting point therefore solids at room temperature (above 300 oC)
• because attractions between cations and anions in solids
are so strong, they have a tight, highly organized crystal
lattice
• large amount of energy must be added in the form of heat
before ions can move into the liquid state
PROPERTIES OF IONIC COMPOUNDS
• Retention of Crystal Shape
• each crystal is made of perfectly flat surfaces
• before being hit, all ions alternate precisely (recall that
opposites attract); however after being hit, the positive ions
momentarily move close to the positive ions and negative ions momentarily move close to the negative ions, causing
the crystal to split.
• ionic compounds still retain crystal shape after being hit
PROPERTIES OF IONIC COMPOUNDS
• Solubility in Water
• all ionic compounds dissolve in water to some extent
because both cations and anions are strongly attracted to
water molecules
• this occurs because each water molecule is polar (one end has a slightly positive electric charge and one has a slightly
negative charge)
PROPERTIES OF IONIC COMPOUNDS
• Conductivity in Solution
• solutions containing ionic compounds are good
conductors of electricity, unlike pure water which has
almost no conductivity
• the greater the concentration of ions, the greater the conductivity (eg. 2g of NaCl in 1L of water is twice as
conductive as 1g in 1L)
• solutions of ionic compounds are considered to be
excellent electrolytes (any solution that can conduct
electricity)
• some molecular compounds, such as HCl(g) become ionic
in solution, which is a general characteristic of acids
SOLUBILITY OF IONIC COMPOUNDS
• The properties of solubility and conductivity are of
great importance to chemists. Chemists often
check the solubility or conductivity of a compound
when analyzing it. Scientists test compounds for
solubility many times and then put the results into a
table of solubility. A table of solubility shows how
soluble common ionic compounds are in water.
• The solubility is rated as very soluble (dissolves well;
recall when a chemical formula has the subscript
(aq) it dissolves well) or slightly soluble (its chemical
formula is followed by (s)).
SOLUBILITY OF IONIC COMPOUNDS
• Group 1 contains mostly alkali metals, how soluble is
this group?
• very soluble
• How soluble are chloride ions? Are there any
exceptions?
• most are very soluble; the exceptions are silver chloride, lead (II) chloride, mercury (I) chloride, copper (I) chloride
and thallium (I) chloride
SOLUBILITY OF IONIC COMPOUNDS
• sometimes when ionic compounds are mixed, they
form a precipitate (a solid with low solubility that
forms from a solution)
• precipitates may also form from mixing two
solutions; the process of forming a precipitate is
called precipitation
SOLUBILITY OF IONIC COMPOUNDS
• Answer 1acehj and 2adg on p.58
SOLUBILITY OF IONIC COMPOUNDS
• 1a)(NH4)2S (aq)
• 1c) AgCl (s)
• 1e) Fe(OH)3 (s) • 1h) Na3PO 4 (aq)
• 1j) AgCH3COO(s)
• 2a) very soluble
• 2d) very soluble
• 2g) very soluble
PROPERTIES OF MOLECULAR COMPOUNDS
• Recall: molecules are non-metallic atoms held
together by covalent bonds (sharing of electrons)
• Molecular compounds exhibit special properties
due to the type of bonding that exists between
individual atoms and between molecules of the
compound. In molecular compounds, often the
bonds between the individual atoms are very
strong; but the bonds between molecules are
weak. A property, such as a low melting point (less
that 300 oC), is a direct result of this weak bond
between molecules.
PROPERTIES OF MOLECULAR COMPOUNDS
• Only a small amount of energy is required for the
molecules to begin sliding past one another; this
does not mean that the molecule breaks up during
melting therefore melting is not considered a
chemical change
PROPERTIES OF MOLECULAR COMPOUNDS
• In general, the properties of molecular compounds
are
• most are colorless or white
• most are liquid or gas however some are solid
• low melting point and boiling point
• generally insoluble in water, some may be slightly soluble
• covalent bonds are neutral atoms therefore they are non-
electrolytes and do not conduct electricity
• not malleable (beaten or rolled into sheets)
SPECIAL PROPERTIES OF WATER
• one of water’s most important properties is that it is
polar (each water molecule has a positive and
negative end)
• water molecules are more attracted to each other,
making it easier for water to form a liquid
• What if water was not polar??
• oceans would boil away in cold winters
• life on earth would either be non-existent or would be
different than we know it today!
• Practice Investigation 2-B pg. 58
SPECIAL PROPERTIES OF WATER
• Why does ice float?
• In most cases as a substance changes from gas to liquid to solid, the substance becomes denser. If the solid state is the
densest, it should sink; yet, ice floats.
As liquid water turns to ice, the molecules spread out and
form a three dimensional array that contains six-sided rings therefore the ordered structure of molecules in ice takes
more space. So, there are fewer molecules in ice than in
water for a given volume. Fewer molecules per unit volume
results in a lower density hence Ice floats because it is less
dense than liquid water!
2 . 3 : A C I D S A N D B A S E S
UNIT A: CHEMISTRY
IN THIS SECTION YOU WILL LEARN ABOUT...
• General Outcome #2 – Explain, using the periodic
table, how elements combine to form compounds,
follow IUPAC guidelines for naming ionic
compounds and simple molecular compounds • Classify acids and bases on the basis of their properties
THIS IS WHY WE DON’T PLAY WITH ACIDS OR BASES...
RECALL FROM SCIENCE 9...
• Acid – a compound that dissolves in water to form
a solution with a pH lower than 7
• Base – a compound that dissolves in water to form
a solution with a pH higher than 7
ACIDS AND BASES IN YOUR BODY
• Acids and bases are present in the foods you eat,
the products you use, and the fluids in your body
systems. Your stomach, for example, contains strong
hydrochloric acid; other organs in your body
produce bases to neutralize stomach acid at
certain points in the digestive process.
PROPERTIES OF ACIDS AND BASES
• Acids
• tastes sour (lemons)
• do not feel slippery
• react with metals
• pH lower than 7
• conducts electricity
• Bases
• tastes bitter (soap)
• feels slippery
(bleach)
• do not react with
metals
• pH higher than 7
• conducts electricity
INDICATORS
• Acids – turns blue litmus paper red
• Bases – turns red litmus paper blue
• In a neutral solution litmus paper does not change
• Universal Indicator – a mixture of several indicators
that change color as the acidity changes
THE PH SCALE
• Every increase of 1 indicates an increase of 10 in
how basic a substance is; similarly, a decrease of 1
indicated 10 times increase in acidity
• Here are some common acidic and basic solutions
NAMING ACIDS
• Acids have a hydrogen symbol (H) on the left side
of the compound’s formula. Organic acids, like
CH3COOH, are the only exception. For these acids,
the hydrogen symbol (H) is on the right.
• Acids that contain hydrogen and one other non-
metallic element can be written with the prefix
“hydro-” and suffix “-ic” (eg. HCl – hydrochloric
acid)
NAMING ACIDS
• Acids that contain oxygen, such as H2SO4 (aq) follow
a different set of rules
• If the anion ends in the suffix “-ate” then use the first
element of the anion (in this case sulfur) and the
suffix with “-ic”; therefore H2SO4 (aq) is sulfuric acid
RECOGNIZING BASES BY THEIR FORMULAS
• The presence of the hydroxide ion (OH-) indicates a
basic solution
• This isn’t always the case though, for example
ammonia (NH3 (aq)) doesn’t contain (OH-) however
for now you only need to use the basic solutions
that end in (OH-)
NEUTRALIZATION
• When acids and bases react together, both acidic
and basic properties disappear, this is called
neutralization
• This reaction will produce water and a salt
Practice 20-23 pg. 70
A 2 . 5 : O U R C H E M I C A L S O C I E T Y
UNIT A: CHEMISTRY
IN THIS SECTION YOU WILL LEARN ABOUT...
• General Outcome #2 – Explain, using the periodic
table, how elements combine to form compounds,
follow IUPAC guidelines for naming ionic
compounds and simple molecular compounds • Outline the issues related to personal and societal use of
potentially toxic or hazardous compounds
TOPICS
• Issues Related to Chemicals
• Introduction
• Environmental Effects
• Health Concerns
• Introduction
• Alcohol
• Nicotine and Other Tobacco-Related Chemicals
TOPICS
• Benzene – A Regulated Substance
• Introduction
• Safe Transportation and Handling of Benzene
• Chemistry Related Careers
• Food Technologist
• Cosmetics Formulator
• Working With Chemistry
• Introduction
YOUR MISSION...
• There are 10 topics, you may work with two or three
other people to summarize each topic and give a 2
minute presentation about each. Every person in
your group must say something in front of the class.