unit a: chemistry - buck mountain

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


• 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


• 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)


• 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

http://michele.usc.edu/105b/electrochemistry/pickle.mov

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)).


• 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


• 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


• Answer 1acehj and 2adg on p.58


• 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.


• 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


• 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





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





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.

unit a: chemistry - buck mountain

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