Chemical ReactionsChemical Reactions

Chemical ChangeChemical ChangeHow do you know if a How do you know if a chemical change has taken chemical change has taken place?place?

All chemical changes are All chemical changes are results of chemical results of chemical reactions.reactions.

Chemical ReactionsChemical Reactions

Chemical Reactions: occur Chemical Reactions: occur when bonds are broken and a when bonds are broken and a new product is formed. new product is formed. (Chemical Change)(Chemical Change)

Involve the rearrangement of Involve the rearrangement of atomsatoms

Chemical reactionsChemical reactionsChemical formulas are used to write Chemical formulas are used to write

chemical equations.chemical equations.The chemical compounds used to The chemical compounds used to

start the reaction are start the reaction are ReactantsReactants..The chemical compounds at the end The chemical compounds at the end

of the reaction are of the reaction are ProductsProducts..

Reaction SymbolsReaction Symbols There are several types of There are several types of Reaction Reaction

symbols used. symbols used. + plus (added to)+ plus (added to) forms (produces or yields)forms (produces or yields)g – gasg – gass – solids – solidl – liquidl – liquidaq—aqueous solutionaq—aqueous solution

Law of Conservation of MassLaw of Conservation of MassMatter cannot be created or destroyed. It Matter cannot be created or destroyed. It

is always conserved in a reaction. is always conserved in a reaction. Atoms cannot simply appear or Atoms cannot simply appear or

disappear.disappear.The amount used at the start of the The amount used at the start of the

reaction is equal to the amount at the reaction is equal to the amount at the end of the reaction.end of the reaction.

# of atoms of REACTANTS = # of atoms of REACTANTS = # of atoms of PRODUCTS# of atoms of PRODUCTS

Take a look at this…Take a look at this…Does it look balanced?Does it look balanced?

We must the get the OWe must the get the O22 to balance.. to balance..Is this balanced now?Is this balanced now?

And now….Is it balanced?And now….Is it balanced?

YES!YES!

Practice Counting AtomsPractice Counting Atoms

MgCl2 Mg= 1 atom of magnesiumCl= 2 atoms of chlorine

H2SO4 S= 1O= 4

H= 2

More counting!More counting!

NH4Cl N= 1H= 4

Mg(NO3)2 Mg= 1N= 2O= 6

Cl= 1

(NH4)3PO4 N= 3H= 12

P= 1O= 4

Balanced EquationsBalanced EquationsIn order to conserve atoms, we In order to conserve atoms, we

must must balancebalance the number and type the number and type of atoms.of atoms.

CAN’T CHANGE SUBSCRIPTS!CAN’T CHANGE SUBSCRIPTS!Can only change the numbers of atoms on Can only change the numbers of atoms on

each side of the equation by adding each side of the equation by adding COEFFICIENTS!COEFFICIENTS!

CoefficientsCoefficients: the numbers that come in : the numbers that come in front of a chemical formula.front of a chemical formula.

Let’s Try to Figure out CoefficientsLet’s Try to Figure out Coefficients

How many do you have How many do you have of each?of each?

3 H3 H2200 2 H2 H22OO33

4 Fe4 Fe22OO33 6 NaCl6 NaCl

NONO22

Steps to BalancingSteps to BalancingFirst, count the number and First, count the number and

type of each atom on both type of each atom on both sidessides

If there are different numbers of If there are different numbers of atoms for an element on both atoms for an element on both sidessides, you must add , you must add coefficientscoefficients to compounds to to compounds to change the number of atomschange the number of atoms

Second, figure out what Second, figure out what coefficient is needed in order to coefficient is needed in order to make the numbers of atoms make the numbers of atoms equal. equal.

REMEMBER: IF YOU CHANGE REMEMBER: IF YOU CHANGE A COEFFICIENT, ALL A COEFFICIENT, ALL ELEMENTS IN THAT ELEMENTS IN THAT COMPOUND ARE AFFECTED.COMPOUND ARE AFFECTED.

Magnesium + Oxygen (g) Magnesium + Oxygen (g) Magnesium Oxide Magnesium Oxide

Mg + 0Mg + 022 MgOMgO

Now try on your own…Now try on your own…

C + ClC + Cl22 CCl CCl44

AlAl22OO33 Al + O Al + O22

Helpful Hints to BalancingHelpful Hints to BalancingIf an element appears in If an element appears in more than one compound more than one compound on the same side of the on the same side of the equation, leave that element equation, leave that element until LAST to balanceuntil LAST to balance

Other Helpful HintsOther Helpful HintsIf there is an If there is an eveneven number of number of

atoms of an element on one atoms of an element on one side of the equation and an side of the equation and an oddodd number on the other, place a number on the other, place a coefficient of coefficient of 22 in front of the in front of the ODD compoundODD compound

Energy in ReactionsEnergy in Reactions

Energy is either Energy is either released or absorbedreleased or absorbed

Exothermic or Exothermic or endothermicendothermic

Exothermic v. Exothermic v. EndothermicEndothermic

Exothermic rxns produce Exothermic rxns produce energy (RELEASE!) so the energy (RELEASE!) so the temperature rises!temperature rises!

Endothermic rxns require a lot Endothermic rxns require a lot of energy (ABSORBS!) so the of energy (ABSORBS!) so the temperature drops!temperature drops!

Exothermic ReactionsExothermic Reactions

LESS energy is required to LESS energy is required to break the bonds in the break the bonds in the reactants than is released when reactants than is released when bonds are formed to make new bonds are formed to make new productsproducts

More energy is released when More energy is released when new products formnew products form

EndothermicEndothermicMORE energy is required to MORE energy is required to

break the bonds of the break the bonds of the reactants than is released from reactants than is released from the formation of new bondsthe formation of new bonds

Energy is being pulled into the Energy is being pulled into the reaction from outsidereaction from outside

THE END! :0)THE END! :0)