ChemistryChemistry

Combining Elements, Chemical Reactions

Why Do Elements Why Do Elements Combine?Combine?

•Elements combine to become more stablemore stable.–Elements become stable by filling their outermost level of electrons.

What is a Compound?What is a Compound?

A compoundcompound is what results when two or more elementstwo or more elements combine.

A binarybinary compoundcompound is a compound formed from two elementstwo elements.Example: CaF2 , Calcium fluoride.

How Do Elements Combine?How Do Elements Combine?

• Elements combine through chemical bonding.

• There are 2 types of chemical bonding:– Ionic BondingIonic Bonding– Covalent BondingCovalent Bonding

Ionic BondingIonic BondingIonic bondingIonic bonding happens when elements elements

combinecombine by gaining or losing electrons.gaining or losing electrons.ExampleExample: : Na + ClNa + Cl

Atoms become charged as electrons are transferred from Na to Cl.

NaNa++ClCl--

Opposite charged atoms attract.Attraction force forms an ionic bondionic bond

between elements and a new compound is formed. NaClNaCl

This type compound is called an ionic ionic compound.compound.

Note: the compound as a whole is neutral.

Elements Bond in Elements Bond in Predictable WaysPredictable Ways

MetalsMetals will loselose electrons electrons and become “+”“+” charged.

Non-metalsNon-metals will gain gain electronselectrons and become “-““-“ charged.

Covalent bondingCovalent bonding happens when elements combine by sharing elements combine by sharing

electronselectrons..

For example: Cl + ClCl + ClEach atom has the same number of protons and electrons,

therefore each have a neutral charge.:: Cl :. .: Cl :::: Cl :. .: Cl ::

Each atom is looking for one electron to fill outer energy level (8) so each atom shares one electron with the other atom.

:: Cl ::: Cl :::: Cl ::: Cl ::Since electrons are shared, no atom gains or loses

electrons. The result is a new molecule with a neutral charge.

ClCl22

Note: the force of attractionforce of attraction between the “ –” electrons and both “+” nucleus’ is what holds the molecule holds the molecule togethertogether.

What is a Molecule?

• A moleculeA molecule is the neutral particle formed as a result of atoms sharing electrons.

• Example: HH.. + .H + .H H:H H:H• Two hydrogen atoms each share

one electron to form a more stable hydrogen moleculemolecule, or “HH22”.

Polar and Non-polar Polar and Non-polar MoleculesMolecules

• Molecules can be classified as polar or non-polar.

Polar moleculesPolar molecules are molecules that have two opposite charged ends, or poles.

Why Do Poles Form?Why Do Poles Form?

• The nucleus of the atom containing The nucleus of the atom containing more protons (here Oxygen with +8 more protons (here Oxygen with +8 nucleus) has a greater force of nucleus) has a greater force of attraction for the “-” electrons than attraction for the “-” electrons than the less attractive Hydrogen’s with +1 the less attractive Hydrogen’s with +1 in each nucleus.in each nucleus.

• As a result, the oxygen end of a water As a result, the oxygen end of a water molecule has a slight negative charge molecule has a slight negative charge while the hydrogen end has a slight while the hydrogen end has a slight positive charge. The result is a polar positive charge. The result is a polar molecule.molecule.

Non-polar MoleculesNon-polar Molecules

Non-polar molecules are molecules that do not have charged ends.

Example: ClCl22 ::Cl:::Cl:: ::Cl:::Cl::

The equal number of protons in both nuclei results in an equal force of attraction for the shared electrons, therefore, no poles are created.

Non-polar MoleculesNon-polar Molecules

Writing Chemical Writing Chemical FormulasFormulas

“Nomenclature” is the system of shorthand used for writing chemical formulas and equations.

Nomenclature TermsNomenclature Terms - - SubscriptSubscript

A subscriptsubscript is a number placed below and to the right of an elements symbol. This number indicates how many atoms of that indicates how many atoms of that elementelement are present. (the number 1 does not need to be written since the symbol already indicates one atom is present).Examples: OO22 = = 22 oxygen atoms oxygen atoms

ClCl22 = = 22 chlorine atoms chlorine atoms

HH22O= O= 22 hydrogen atoms and hydrogen atoms and 11 oxygen atom oxygen atom

CC66HH1212OO66 = = 66 carbon atoms and carbon atoms and 1212 hydrogen hydrogen atoms and atoms and 66 oxygen atoms oxygen atoms

Nomenclature TermsNomenclature Terms - - SuperscriptSuperscript

A superscriptsuperscript is a number placed above and to the right of an elements symbol. This number indicates the chargeindicates the charge of an atom, or the atom’s oxidation numberoxidation number.Examples: OO-2-2 means that the oxygen gained 2 “-”electrons and now has a -2 charge.

KK+1+1 means that the potassium atom has lost one electron and has a +1 charge.

Nomenclature Terms -Nomenclature Terms - Oxidation NumberOxidation Number

An oxidationoxidation n numberumber is the number of electrons that an atom gains, loses or shares when bonding with another atom.Example: when Lithium (Li) gives away an electron it becomes LiLi+1+1. The +1 is the oxidation number; it indicates that lithium has lost one “-” electron and the result is a +1 charge on the atom.

Nomenclature TermsNomenclature Terms - - CoefficientCoefficient

A coefficientcoefficient is the number in front of each “item” in a chemical equation. The coefficient shows how many of each “item” there is in the equation.Example: 66 HH22 ++ 66 O O 66 HH22OO

How Do We Write Chemical How Do We Write Chemical Formulas?Formulas?

• Using a series of steps allows us to correctly write chemical formulas.

Steps For Writing The Chemical Formula For Binary Compounds

Write the symbolsymbol of the element with “+”“+” oxidation number (all metals and hydrogen). Example: CaCa

Next write the symbolsymbol of the element with the “-”“-” oxidation number (all nonmetals). Example: Ca FF

Write in the oxidation numbers for each elementoxidation numbers for each element. Example: Ca+2+2 F-1-1

BalanceBalance the formula. There must be an equal number of “+” and “-“ charges in the completed formula to have a neutral compound. Example: Ca+2 22F-1

Put in subscriptssubscripts so the sum of the charges is equal to sum of the charges is equal to zerozero. Example: Ca1F2 , drop the unnecessary “1” and get CaF2.

Criss-Cross MethodCriss-Cross Method

A Short CutA Short Cut is to use the criss-cross criss-cross methodmethod.

Change the oxidation number to a subscript for the other element Example: Ca+2+2 F-1-1 becomes Ca11F22

Drop the unnecessary “1” and get CaFCaF22.

Naming Binary Naming Binary CompoundsCompounds

• Write the name of the first elementname of the first element. Example: CalciumCalcium.

• Write the rootroot ofof the name of the second elementsecond element.

Ex: Calcium FlourFlour.• Add the suffix “ideide” to the root.

Example: Calcium Fluorideide.

How Do Elements How Do Elements Combine?Combine?

Elements combine through chemical bonding.

Chemical bonding involves chemical reactions.

What Is Involved In Chemical Reactions?

The theory of Conservation Conservation of Matterof Matter states that matter can not be created nor destroyed.

What Is Involved In What Is Involved In Chemical Reactions?Chemical Reactions?

• ReactantsReactants are the elements or molecules involved at the start of a chemical reaction.

• ProductsProducts are the results of the chemical reaction. What we are left with after the reaction.

How Do We Know What How Do We Know What Happens During Chemical Happens During Chemical

Reactions?Reactions?

Chemical equations are a way of showing what is taking place during a chemical reaction by using numbers and symbols.Ex: Ag + HAg + H22S S Ag Ag22S + HS + H22

What is a Balanced Equation?What is a Balanced Equation?

A balanced equationbalanced equation is when there are the same number of atoms of each element on both sides of the chemical equation.

What we start with = what we end withWhat we start with = what we end with.Ex: 2Ag + H2Ag + H22S S Ag Ag22S + HS + H22

Balancing Chemical Balancing Chemical EquationsEquations

1. Describe the chemical reaction in words.

Ex: silver nitrate plus sodium chloride yields silver chloride plus sodium nitrate.

2. Write the chemical equation using formulas and symbols.

Ex: AgNO3 + NaCl AgCl + NaNO3

3. Check for balance. (make a table)4. Determine the coefficients (if

necessary).

Balance The Chemical Equation For Balance The Chemical Equation For How The Human Body Makes How The Human Body Makes

EnergyEnergyGlucose plus oxygen produces carbon dioxide and

water.C6H12O6 + O2 CO2 + H2O

Balanced = C6H12O6 + 6O2 6CO2 + 6H2O

Why Are Chemical Reactions Why Are Chemical Reactions Important?Important?

Life could not exist without chemical reactions.

In chemical reactions atoms rearrange themselves forming all of life’s substances and compounds.

Types of Chemical Types of Chemical ReactionsReactions

• There are four (4) types of chemical reactions, all are based on the way atoms rearrange themselves during the reaction.

• These 4 are synthesis synthesis reactions, decompositiondecomposition reactions, singlesingle replacementreplacement, doubledouble displacementdisplacement.

Synthesis ReactionSynthesis Reaction• Synthesis reactionsSynthesis reactions are when 2 or more

substances combinesubstances combine to form another substance.

• Most synthesis reactions give off energygive off energy in the form of heat and light.

• General formula for synthesis reactions:A + B A + B AB AB

• Example: Aluminum + oxygen produces aluminum oxide

Al + O2 Al2O3

Balanced: 4 Al + 3O2 2 Al2O3

Decomposition ReactionDecomposition Reaction• Decomposition reactionsDecomposition reactions are when one

substance breaks downsubstance breaks down into simpler substances.

• Most decomposition reactions require the require the addition of energyaddition of energy (need energy)

• General formula for decomposition reactions:AB AB A + B A + B

• Example: Carbonation (bubbles) in sodaDecomposition of carbonic acid yields

water and carbon dioxide.H2CO3 H2O + CO2

Single Replacement ReactionSingle Replacement Reaction

• Single replacement reactionsSingle replacement reactions are when one one element replaces anotherelement replaces another element in a compound.

• General formulas for single replacement reactions:

A + BC A + BC AC + B AC + B(the positive ion (B) is replaced (by A))

D + BC D + BC BD + C BD + C(the negative ion (C) is replaced (by D))

Example: TarnishAluminum plus silver sulfide yields aluminum sulfide

plus silver

2Al + 3AgS 2Al + 3AgS Al Al22SS33 + 3Ag + 3Ag

Double Displacement Double Displacement ReactionsReactions

• Double displacementDouble displacement reactions reactions are when the positive ion of one compound replaces the positive positive ion of one compound replaces the positive ion of another compoundion of another compound forming two new compounds.

• General formula for double displacement reactions:

AB + CD AB + CD AD + CB AD + CB• Most acid-base reactions are double displacement

reactions.• Often a precipitate forms in these reactions.Example: Antacid plus stomach acid yields precipitate

plus waterMagnesium hydroxide (antacid) plus hydrochloric acid

(stomach acid) yields magnesium chloride (precipitate) plus water.

Mg(OH)2 + 2HCl MgCl2 + 2H2O