CHEMICAL REACTIONSCHEMICAL REACTIONS

Reactants: Zn + IReactants: Zn + I22 Product: Zn IProduct: Zn I22

• Introduction– Chemical reactions occur when bonds between the

outermost parts of atoms are formed or broken– Chemical reactions involve changes in matter, the making of

new materials with new properties, or energy changes.– Chemical Reactions are described using a shorthand called

a chemical equation

– One or more substances change into one or more new substances during a chemical reaction.

– A substance present at the start of the reaction is a reactant.

– A substance produced in the reaction is a product.

Recognizing Chemical Changes2.4

• Recognizing Chemical Changes– What are four possible clues that a chemical

change has taken place?

Recognizing Chemical Changes

– Possible clues to chemical change include:• a transfer of energy• a change in color• the production of a gas• the formation of a precipitate.

2.4

– Chemical equations show the conversion of reactantsreactants (the molecules shown on the left of the arrow) into productsproducts (the molecules shown on the right of the arrow).• A + sign separates molecules on the same

side• The arrow is read as “yields”• Example

C + O2 CO2

• This reads “carbon plus oxygen react to yield carbon dioxide”

Parts of a Reaction Parts of a Reaction EquationEquation

• The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O2 CO2, contains the same information as the English sentence but has quantitative meaning as well.

• Solid ___

• Liquid (l)

• Gas ___

• Aqueous solution (aq)

• Catalyst H2SO4

• Escaping gas ()• Change of temperature ()

Symbols Used in Symbols Used in EquationsEquations

Symbols Used in Chemical Equations

Symbols Used in Chemical Equations

Law of Conservation of Mass

• Simply stated: “No matter can be created or destroyed”

• The same number of molecules, atoms, etc. must show up on both sides of the equations.

Na + Cl2 NaCl

• Also true: the same amounts of each atom, molecule, etc must be on each sides of the equation.

• Example:

2 Na + Cl2 2 NaCl 2 NaCl

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

___ H___ H22(g) + ___ O(g) + ___ O22(g) ---> ___ H(g) ---> ___ H22O(l)O(l)2 2

What Happened to the Other Oxygen Atom?????

This equation is not balanced!

Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

– When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but

you may notnot change the subscripts.

• Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

Balancing EquationsBalancing Equations

There are four basic steps to balancing a chemical equation.1. Write the correct formula for the reactants and the

products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!

2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.

4. Check your answer to see if:– The numbers of atoms on both sides of the

equation are now balanced.– The coefficients are in the lowest possible whole

number ratios. (reduced)

Steps to Balancing Steps to Balancing EquationsEquations

Some Suggestions to Help Some Suggestions to Help YouYou

Some of Mrs. Borden’s Helpful Hints for balancing equations:• Take one element at a time, working left to right

except for H and O. Save H for next to last, and O until last.

• IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

• (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

Balancing simple equations

K + Cl2 KCl Fe + O2 Fe2O3

LiCl Li + Cl2 NaN3 Na + N2

NiO + Al Al2O3 + Ni C4H8 + O2 CO2 + H2O

Ag2O Ag + O2

B2O3 + Mg B + MgO

KBr + Cl2 KCl + Br2

Balance:

Cu + AgNO3 Ag + Cu(NO3)2

NaI + Pb(NO3)2 NaNO3 + PbI2

Ca(CH3COO)2+ K2CO3 CaCO3+ KCH3COO

Balancing Balancing Equations EquationsBalancing Balancing Equations Equations

___ Al(s) + ___ Br___ Al(s) + ___ Br22(l) ---> ___ Al(l) ---> ___ Al22BrBr66(s)(s)2 3

Balancing Balancing EquationsEquationsBalancing Balancing EquationsEquations

____C____C33HH88(g) + _____ O(g) + _____ O22(g) ---->(g) ---->

_____CO_____CO22(g) + _____ H(g) + _____ H22O(g)O(g)

____B____B44HH1010(g) + _____ O(g) + _____ O22(g) ---->(g) ---->

___ B___ B22OO33(g) + _____ H(g) + _____ H22O(g)O(g)

Balancing EquationsBalancing EquationsBalancing EquationsBalancing EquationsSodium phosphate + iron (III) oxide Sodium phosphate + iron (III) oxide

sodium oxide + iron (III) phosphatesodium oxide + iron (III) phosphate

NaNa33POPO44 + Fe + Fe22OO33 ----> ---->

NaNa22O + FePOO + FePO44