LECTURE 2- SCIENVC

Matter is anything that occupies space and has mass.

A substance is a form of matter that has a definite composition and distinct properties.

Chemistry is the study of matter and thechanges it undergoes

liquid nitrogen gold ingots silicon crystals

Classifying Matter All Matter

Pure Substances Mixtures

Can it be separated by a physical process?

CompoundsElements

YESNO

Can it be broken down into simpler ones by chemical means?

NO YES

An element is a substance that cannot be separated into simpler substances by chemical means.

• 114 elements have been identified

• 82 elements occur naturally on Earth

gold, aluminum, lead, oxygen, carbon, sulfur

• 32 elements have been created by scientists

technetium, americium, seaborgium

6

The Modern Periodic Table

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

A compound is a substance composed of atoms of two or more elements chemically united in fixed proportions.

Compounds can only be separated into their pure components (elements) by chemical means.

lithium fluoride quartz dry ice – carbon dioxide

A mixture is a combination of two or more substances in which the substances retain their distinct identities.

1. Homogenous mixture – composition of the mixture is the same throughout.

2. Heterogeneous mixture – composition is not uniform throughout.

soft drink, milk, solder

cement, iron filings in sand

Physical means can be used to separate a mixture into its pure components.

magnet

distillation

A Comparison: The Three States of Matter

A physical change does not alter the composition or identity of a substance.

A chemical change alters the composition or identity of the substance(s) involved.

ice meltingsugar dissolving

in water

hydrogen burns in air to form water

Types of Changes

12

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Atomic number, Mass number and Isotopes

Electron configuration is how the electrons are distributed among the various atomic orbitals in an atom.

1s1

principal quantumnumber n

angular momentumquantum number l

number of electronsin the orbital or subshell

Orbital diagram

H

1s1

What is the electron configuration of Mg?

Mg 12 electrons

1s < 2s < 2p < 3s < 3p < 4s

1s22s22p63s2 2 + 2 + 6 + 2 = 12 electrons

Abbreviated as [Ne]3s2 [Ne] 1s22s22p6

What are the possible quantum numbers for the last (outermost) electron in Cl?

Cl 17 electrons 1s < 2s < 2p < 3s < 3p < 4s

1s22s22p63s23p5 2 + 2 + 6 + 2 + 5 = 17 electrons

Last electron added to 3p orbital

n = 3 l = 1 ml = -1, 0, or +1 ms = ½ or -½

15

The Modern Periodic Table

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

The electrons in the outermost energy levels are called valence electrons.

The group number (of the representative elements) on the periodic table tells you the number of valence electrons.

1A

2A 3A 4A 5A 6A 7A

8A

Group 1A: 1 valence electron

Group 3A: 3 valence electrons

What is the electron configuration of C?

C 6 electrons

1s < 2s < 2p

1s22s22p2

2 + 2 = 4 valence electrons C

How many valence electrons are there in O? N? H?

O N H

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Why should two atoms share electrons?

F F+

7e- 7e-

F F

8e- 8e-

F F

F F

Lewis structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

8e-

H HO+ + OH H O HHor

2e- 2e-

Lewis structure of water

Double bond – two atoms share two pairs of electrons

single covalent bonds

O C O or O C O

8e- 8e-8e-double bonds double bonds

Triple bond – two atoms share three pairs of electrons

N N8e-8e-

N N

triple bondtriple bond

or

1. Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center.

2. Count total number of valence e-. Add 1 for each negative charge. Subtract 1 for each positive charge.

3. Complete an octet for all atoms except hydrogen

4. If structure contains too many electrons, form double and triple bonds on central atom as needed.

Writing Lewis Structures

Write the Lewis structure of nitrogen trifluoride (NF3).

Step 1 – N is less electronegative than F, put N in center

F N F

F

Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)

5 + (3 x 7) = 26 valence electrons

Step 3 – Draw single bonds between N and F atoms and complete octets on N and F atoms.

Step 4 - Check, are # of e- in structure equal to number of valence e- ?

3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons

Lewis structure of neutral molecules

O

N

H

C 4 bonds

3 bonds

2 bonds

1 bond

3 ways of representing the reaction of H2 with O2 to form H2O

A process in which one or more substances is changed into one or more new substances is a chemical reaction

A chemical equation uses chemical symbols to show what happens during a chemical reaction

reactants products

How to “Read” Chemical Equations

2 Mg + O2 2 MgO

2 atoms Mg + 1 molecule O2 makes 2 formula units MgO

2 moles Mg + 1 mole O2 makes 2 moles MgO

48.6 grams Mg + 32.0 grams O2 makes 80.6 g MgO

NOT

2 grams Mg + 1 gram O2 makes 2 g MgO

Balancing Chemical Equations

1. Write the correct formula(s) for the reactants on the left side and the correct formula(s) for the product(s) on the right side of the equation.

Ethane reacts with oxygen to form carbon dioxide and water

C2H6 + O2 CO2 + H2O

2. Change the numbers in front of the formulas (coefficients) to make the number of atoms of each element the same on both sides of the equation. Do not change the subscripts.

2C2H6 NOT C4H12

Balancing Chemical Equations

3. Start by balancing those elements that appear in only one reactant and one product.

C2H6 + O2 CO2 + H2O start with C or H but not O

2 carbonon left

1 carbonon right

multiply CO2 by 2

C2H6 + O2 2CO2 + H2O

6 hydrogenon left

2 hydrogenon right

multiply H2O by 3

C2H6 + O2 2CO2 + 3H2O

Balancing Chemical Equations

4. Balance those elements that appear in two or more reactants or products.

2 oxygenon left

4 oxygen(2x2)

C2H6 + O2 2CO2 + 3H2O

+ 3 oxygen(3x1)

multiply O2 by 72

= 7 oxygenon right

C2H6 + O2 2CO2 + 3H2O72

remove fractionmultiply both sides by 2

2C2H6 + 7O2 4CO2 + 6H2O

Balancing Chemical Equations

5. Check to make sure that you have the same number of each type of atom on both sides of the equation.

2C2H6 + 7O2 4CO2 + 6H2O

Reactants Products

4 C12 H14 O

4 C12 H14 O

4 C (2 x 2) 4 C

12 H (2 x 6) 12 H (6 x 2)

14 O (7 x 2) 14 O (4 x 2 + 6)

C18H36O2 + O2 CO2 + H2O

3.60 Balance the following equations:

a. N2O5 → N2O4 + O2

b. KNO3 → KNO2 + O2

c. NH4NO3 → N2O + H2O

d. NH4NO2 → N2 + H2O

e. NaHCO3 → Na2CO3 + H2O + CO2

f. P4O10 + H2O → H3PO4

g. HCl + CaCO3 → CaCl2 + H2O + CO2

h. Al + H2SO4 → Al2(SO4)3 + H2

i. CO2 + KOH → K2CO3 + H2O

3.60 Balance the following equations (continuation):

j. CH4 + O2 → CO2 + H2O

k. Be2C + H2O → Be(OH)2 + CH4

l. Cu + HNO3 → Cu(NO3)2 + NO + H2O

m. S + HNO3 → H2SO4 + NO2 + H2O

n. NH3 + CuO → Cu + N2 + H2O

How do we relate balancedchemical reactions to energy?

Energy is the capacity to do work.

• Radiant energy comes from the sun and is earth’s primary energy source

• Thermal energy is the energy associated with the random motion of atoms and molecules

• Chemical energy is the energy stored within the bonds of chemical substances

• Nuclear energy is the energy stored within the collection of neutrons and protons in the atom

• Potential energy is the energy available by virtue of an object’s position

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

Temperature = Thermal Energy

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.

Endothermic process is any process in which heat has to be supplied to the system from the surroundings.

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

energy + H2O (s) H2O (l)

Schematic of Exothermic and Endothermic Processes

First law of thermodynamics – energy can be converted from one form to another, but cannot be created or destroyed.

Esystem + Esurroundings = 0

or

Esystem = -Esurroundings

C3H8 + 5O2 3CO2 + 4H2O

Exothermic chemical reaction!

Chemical energy lost by combustion = Energy gained by the surroundingssystem surroundings

Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.

H = H (products) – H (reactants)

H = heat given off or absorbed during a reaction at constant pressure

Hproducts < Hreactants

H < 0Hproducts > Hreactants

H > 0

Thermochemical Equations

H2O (s) H2O (l) H = 6.01 kJ/mol

Is H negative or positive?

System absorbs heat

Endothermic

H > 0

6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.

Thermochemical Equations

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H = -890.4 kJ/mol

Is H negative or positive?

System gives off heat

Exothermic

H < 0

890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.

H2O (s) H2O (l) H = 6.01 kJ/mol

• The stoichiometric coefficients always refer to the number of moles of a substance

Thermochemical Equations

• If you reverse a reaction, the sign of H changes

H2O (l) H2O (s) H = -6.01 kJ/mol

• If you multiply both sides of the equation by a factor n, then H must change by the same factor n.

2H2O (s) 2H2O (l) H = 2 x 6.01 = 12.0 kJ

H2O (s) H2O (l) H = 6.01 kJ/mol

• The physical states of all reactants and products must be specified in thermochemical equations.

Thermochemical Equations

H2O (l) H2O (g) H = 44.0 kJ/mol

Entropy (S) is a measure of the randomness or disorder of a system.

order SdisorderS

Processes that lead to an increase in entropy (S > 0)

Efficiency = X 100%Th - Tc

Th

Chemistry In Action: The Efficiency of Heat Engines

Some of the work is wasted as heat !!!

Different forms of energy are converted to heat.

Heat are dispersed in the surroundings.

Increases the entropy of the universe.

“Chaotic” World???

Can you imagine how life would beIf there were no entropy?Or, making matters even worse,The law of entropy were reversed?Books would get straighter on their shelves,And children's rooms would clean themselves!And every rock or stick or treeWould form a crystal, perfectly.There'd be no anarchy or warFor everyone would know the score.Every thing and every faceWould have its certain time and place. Replacing every beach would passAn endless stretch of flawless glass.The sea would be the brightest blue,And every day the sky would too.How beautiful would be our worldIf order did command it.If all were straight and never curled:Perhaps we should demand it.

You'd think a world sans entropyWould be a lovely place to be.I said this recently myself, As all my books fell off their shelf.Yet pondering this ordered bliss,I noticed things that I would miss,Like rolling waves upon the sea,Or sugar for my morning tea:The sugar won't dissolve, it's true,That ant-entropy holds like glue.And after that, I saw with grief,There'd be no fractaled maple leaf:No beauty in the summer wood, Should chaos disappear for good.What a bore, to know each dayWould turn out in the same old way.If entropy would disappearThere'd be no fortune, fate or luckAnd even after many years,Vegas wouldn't make a buck.

Chaos, Keep It Coming!By Heather Ryphemi Stregay