Introduction to Introduction to Chemical ReactionsChemical Reactions

Making new substances Making new substances

Main IdeasMain IdeasChemical Reactions are represented by Chemical Reactions are represented by Chemical Equations.Chemical Equations.

Chemical Equations are balanced to show Chemical Equations are balanced to show the same number of atoms of each the same number of atoms of each element on each side.element on each side.

The Law of Conservation of Mass says The Law of Conservation of Mass says that atoms won’t be created or destroyed that atoms won’t be created or destroyed in a chemical reaction. That is why you in a chemical reaction. That is why you have to balance chemical equations!have to balance chemical equations!

Chemical Chemical Reactions are Reactions are EverywhereEverywhereCookingCooking RespirationRespiration

Chemical Chemical Reactions are Reactions are EverywhereEverywhereHair DyeHair Dye Auto FuelAuto Fuel

DemoDemoUniversal Indicator – Universal Indicator –

shows pHshows pH

Dilute ammonia + Dilute ammonia + effervescent tablet effervescent tablet (e.g. Alka Seltzer)(e.g. Alka Seltzer)

What do you think is What do you think is going to happen?going to happen?

What Happened during What Happened during Demo?Demo?

What did you observe during demonstration?What did you observe during demonstration?

Share your observations with someone sitting next Share your observations with someone sitting next to youto you

How do you know How do you know when a chemical when a chemical

reaction takes place?reaction takes place?Color ChangeColor Change Precipitate FormationPrecipitate Formation

How do you know when How do you know when a chemical reaction a chemical reaction

takes placetakes place??Gas FormationGas Formation OdorOdor

How do you know when How do you know when a chemical reaction a chemical reaction

takes place?takes place?Temperature ChangeTemperature Change Change in AcidityChange in Acidity

Representing Representing Chemical Chemical ReactionsReactionsChemists observe chemical reactions and Chemists observe chemical reactions and

have come up with a way to represent or have come up with a way to represent or model what is happening.model what is happening.

Making NaCl

Solid Sodium combines with Chlorine gas Solid Sodium combines with Chlorine gas to make solid Sodium Chloride:to make solid Sodium Chloride:

2Na (s) + Cl2 (g) 2Na (s) + Cl2 (g) 2NaCl2NaCl

Chemical Equations are Chemical Equations are different from Numerical different from Numerical

EquationsEquationsNumerical Equation: 3x + 2y = 47Numerical Equation: 3x + 2y = 47

Chemical Equation 2Na + Cl2 Chemical Equation 2Na + Cl2 2NaCl 2NaCl

ReactantA + Reactant B ReactantA + Reactant B Product Product

The reactants are used up in forming The reactants are used up in forming the productthe product

The arrow The arrow shows the direction of the shows the direction of the reactionreaction

Symbols used in Symbols used in Chemical Chemical EquationsEquationsSymbol Purpose

+ Separates more than one reactant or product

Separates reactants from products. Indicates direction of

reaction

(s) Identifies a solid state

(aq) Identifies that something is dissolved in water

(l) Identifies liquid state

(g) Identifies gaseous state

Law of Law of Conservation of Conservation of

MassMassIn a chemical reaction, matter is neither In a chemical reaction, matter is neither created nor destroyed. created nor destroyed.

Atoms won’t change their identity (e.g. a Atoms won’t change their identity (e.g. a Carbon atom can’t become an Iron atom)Carbon atom can’t become an Iron atom)

This means that you have to have the This means that you have to have the same number of each type of atom on same number of each type of atom on each side of the chemical equation. each side of the chemical equation.

Conservation of Mass Video

Balancing EquationsBalancing EquationsAfter you write a chemical equation you have to After you write a chemical equation you have to balance it to make sure that the same number balance it to make sure that the same number

of atoms of each element are on each side.of atoms of each element are on each side.

How would you balance this equation?How would you balance this equation?

Li + H2O Li + H2O H2 + LiOH H2 + LiOH

Steps to Balancing a Chemical Steps to Balancing a Chemical EquationEquation

Another Example Another Example CH4 (methane gas) + O2 CH4 (methane gas) + O2 CO2 + CO2 + H2O H2O

7 ≠ 6!7 ≠ 6!Where did our atoms go?Where did our atoms go?

Reactants Products

# of Carbons = 1 # of Carbons = 1

# of Hydrogens = 4 # of Hydrogens = 2

# of Oxygens = 2 # of Oxygens = 3

Total atoms = 7 Total atoms = 6

Example Example ContinuedContinued

Change the Coefficients to make the Change the Coefficients to make the number of atoms of each element equalnumber of atoms of each element equal

Balance the Hydrogens:Balance the Hydrogens:

CH4 + O2 CH4 + O2 CO2 + CO2 + 2 2 H2O H2O

Balance the Oxygens: Balance the Oxygens:

CH4 + CH4 + 2 2 O2 O2 CO2 CO2 + + 22 H2OH2O

Example Example ContinuedContinued

CH4 + CH4 + 2 2 O2 O2 CO2 CO2 + + 22 H2OH2O

Are your coefficients in their simplest ratio?Are your coefficients in their simplest ratio?

Count your atoms again to check your work: Count your atoms again to check your work:

Reactants Products

# of Carbons = 1 # of Carbons = 1

# of Hydrogens = 4 # of Hydrogens = 4

# of Oxygens = 4 # of Oxygens = 4

Total atoms = 9 Total atoms = 9

Try These!Try These!CC22HH66 + O + O22 CO CO22 + H+ H22O O

FeFe22OO33 + H + H22SOSO44 Fe Fe22(SO(SO44))33 + H + H22O O

Hint : balance the polyatomic ion first!Hint : balance the polyatomic ion first!

CaClCaCl22 + AgNO + AgNO33 AgCl + Ca(NO AgCl + Ca(NO33))22

Think – Pair - Share

ReviewReviewMatter is not destroyed Matter is not destroyed

or createdor created

Atoms are rearranged in Atoms are rearranged in chemical reactionschemical reactions

Chemical equations Chemical equations represent chemical represent chemical

reactionsreactions

You have to have the You have to have the same number of each same number of each

type of atom on the left type of atom on the left and right hand side of a and right hand side of a

chemical equationchemical equation

WARNING!WARNING!Don’t mess with the insides of polyatomic ions – Don’t mess with the insides of polyatomic ions – put a square around them, or label them as X – put a square around them, or label them as X – treat the WHOLE polyatomic ion as though it treat the WHOLE polyatomic ion as though it

were an element!were an element!

Don’t ever play around with subscripts (those Don’t ever play around with subscripts (those little numbers that tell you how many atoms are little numbers that tell you how many atoms are

in a molecule) e.g. in a molecule) e.g. CC66HH2222OO1111