UNIT I: MATTER AND ENERGY

Matter is considered to be anything which hasmass and occupies space.

There are two types of matter, pure substances,and mixtures.

Pure substances consist of elements andcompounds. Pure substances have a definitecomposition and distinct properties.

Mixtures are a combination of two or moresubstances in which each substance retains itsproperties. There are two types of mixtures,homogeneous mixtures and heterogeneousmixtures. Mixtures can be separated byphysical means into pure substances.

PURE SUBSTANCES

All samples of a particular pure substance havethe same melting point, boiling point, and otherproperties related to composition which can beused for identification.

The two types of pure substances are:

Elements - Matter which is composed of only onekind of atoms. The smallest particle of anelement is an atom. Elements can not bedecomposed by chemical means.

Compounds - Matter that is composed of two ormore different kinds of atoms chemicallycombined in definite proportions. Compoundscan be decomposed by chemical means intotheir elements.

Binary compounds - Compounds consisting ofonly two elements regardless of their ratio arecalled binary compounds.

NaCl, NH3, MgCl2, or Al2O3 are examples ofbinary compounds.

Law of Definite Proportions.

Any sample of a pure compound always consists ofthe same elements combined in the sameproportions by mass. This observation is known asthe “Law of Constant Composition” or the “Lawof Definite Proportions.”

For example, every sample of water collectedanywhere in the universe is composed of 11%hydrogen and 89% oxygen. Every water moleculecontains two atoms of hydrogen to one atom ofoxygen to give the formula, H2O.

If the formula of the compound is not H2O (11%hydrogen and 89% oxygen), the compound is notwater but some other substance.

MIXTURES

A mixture consists of two or more distinctsubstances differing in properties andcomposition. The composition of a mixture canbe varied.

The two types of mixtures are:

Homogeneous mixtures - Are mixtures in whichthe composition is the same throughout. Allthe substances in a homogeneous mixture areuniformly dispersed in each other. Solutionsare the best examples of homogeneousmixtures. A solution has exactly the samecomposition regardless of where the sample istaken.

Heterogeneous mixtures - A mixture in which theparticles of each component remain separatedand can be observed as individual substances. Earth (dirt) and concrete are both goodexamples of heterogeneous mixtures.

Kinetic Molecular Theory

To understand the physical properties of matter, weneed a model that helps us picture these particles. Such a model, known as the kinetic moleculartheory, was developed over 100 years but wasfinalized by Rudolf Clausius (1822-1885) in 1857. The kinetic molecular theory is summarized below.

â All matter is made up of a large number of tinyparticles. These particles may be atoms,molecules, or ions.

ã These molecules are always moving in acontinuous, random motion except at “absolutezero” where all molecular motion ceases.

ä The collisions between molecules are perfectlyelastic. While molecules may exchange energywhen they collide, the total amount of energypossessed by all the particles remains constant.

Liquid

Phases or States of Matter

The term “phase” or “state” is used to refer to thegas, liquid, or solid forms of matter.

Solids - Solids have a definite shape and a definitevolume. All true solids have a crystallinestructure or regular geometricpattern. The particles in asolid are constantly vibratingin a fixed position.

Liquids - Liquids have a definitevolume but take the shape oftheir container. The particles inthe liquid phase have no regulararrangement and are in constantmotion moving freely abouttheir container.

Gases - Gases take both the shape and volume oftheir container. The particles in asample of a gas are constantlymoving freely and randomlyabout their containment vessel.

Symbols for Some Different Forms of Matter

Solid (s)

Liquid (R)

Gas (g)

Aqueous or water solution (aq)

What information is given by the symbols below?

NaCl(s) H2O(R) CO2(g)

KCl(aq) Fe(s) CH3OH(R)

ALLOTROPES

Allotropes are different forms of the same elementthat exist in the same physical state under thesame temperature and pressure.

Diamond and graphite are one of the more commonexamples of allotropes. Some properties of theseallotropes are

Property Diamond GraphiteDensity 3.51 g/ml 2.22 g/mlColor colorless blackHardness extremely hard very softConductivity nonconductive semiconductorPhysical Form crystalline flat sheets

Molecular oxygen, O2, and ozone, O3, are anotherallotropic pair. Molecular oxygen, O2, is a colorless,diatomic gas with a boiling point of -183.0°C andmelting point of -218.4°C. Ozone, O3, is a blue gaswith a strong irritating odor. Ozone has a boilingpoint of -111.9°C and melting point of -192.5°C.

PHYSICAL AND CHEMICAL CHANGES

Chemical Change - A chemical change results in apermanent change in the physical properties as aresult of a change in chemical composition.

Physical Changes - A change in the physical form(solid, liquid, or gas) of a substance withoutaltering the composition of the substance.

PHASE CHANGES

As a sample of matter is heated from below itsmelting point to above its boiling point, it readilychanges from one phase to another.

Phase changes are always accompanied by either theabsorption or release of energy.

The terms for the absorption of energy during the twophase changes are called the Heat of Fusion and theHeat of Vaporization.

The heat of fusion is the energy required to melt agiven of amount of solid. This energy is needed tobreak down the rigid solid structure producing therandom liquid phase.

The heat of vaporization is the energy required toboil a given amount of liquid. This energy is neededto completely separate the particles in the liquid phaseforming the scattered gaseous phase.

Melting - The change from the solid phase to theliquid phase is called “melting.”

When melting occurs, the substance absorbs energyresulting in a break down of the regular geometricarrangement of the particles. The result is a collectionof particles with no orderly pattern.

The reverse process to melting is called “freezing.” There is a release of energy during freezing.

Solid Liquid

Boiling - The phase change from the liquid phase tothe gaseous phase is called “boiling.”

When boiling occurs, the particles absorb energy andseparate completely leaving particles which arescattered and random.

The opposite process to boiling is called“condensation.” There is a release of energy duringcondensation.

Liquid Gas

SolidGas

Sublimation - The phase change from the solid statedirectly to the gaseous state without passing throughthe liquid state is called “sublimation.”

Typical solids which sublime are dry ice (carbondioxide), paradichlorobenzene (moth balls), andiodine. The odor of some solids is attributed tosublimation.

The opposite process to sublimation is called“deposition.”

Identify which of the following equations are phasechanges and which are chemical reactions. Name thephase changes where applicable.

â NaCl(R) 6 NaCl(s)

ã H2O(s) 6 H2O(g)

ä NH3(g) + HCl(g) 6 NH4Cl(s)

å CO2(g) 6 CO2(s)

æ H2(g) + Cl2(g) 6 2HCl(g)

ç C2H5OH(R) 6 C2H5OH(g)

è I2(s) 6 I2(g)

é 2H2O(R) 6 2H2(g) + O2(g)

ê Ar(g) 6 Ar(R)

ë C6H4Cl2(s) 6 C6H4Cl2(g)

The Heating Curve for Water

When solid water (ice) is heated at a constant ratefrom -25°C to 125°C, its phase changes to the first theliquid phase and then to the gaseous phase.

Each of the two plateaus on a heating curve representa phase change. The lower plateau always representsthe solid-liquid transition (melting) and the upperplateau always represents the liquid-gaseoustransition (boiling).

Cooling Curve for Water

As water vapor is cooled from 125°C to -25°C on thecooling curve below, the gaseous water firstcondenses into liquid water and finally freezes toform ice. As in a heating curve, a phase changealways occur at a plateau.

1. What is the melting point and boiling point forthe substance given in the heating curve above?

2. On which portion of the curve does the substancehave a definite shape and a definite volume?

3. Which process requires more energy, melting orboiling?

ENTROPY

The entropy of a collection of particles is ameasure of the degree of disorder or randomnessof the particles.

Solids have a regulararrangement of particles andtherefore have a low entropy.

Liquids have a randomcollection of particles andtherefore have a higherentropy than solids.

Gas have the most disorderedcollection of particles andtherefore have the highestentropy.

Solid 6 liquid 6 gasincreasing entropy 6

Which phase change represents an increase inentropy?

1. CO2(g) 6 CO2(R)

2. NaCl(s) 6 NaCl(R)

3. H2O(R) 6 H2O(g)

4. Sn(R) 6 Sn(s)

5. KBr(s) 6 KBr(aq)

6. I2(s) 6 I2(g)