I II III IV V

Ch. 10 – Chemical Ch. 10 – Chemical ReactionsReactions

A. A. Signs of a Chemical ReactionSigns of a Chemical Reaction

Evolution of Evolution of heatheat and and lightlight

Formation of a Formation of a gasgas

Formation of a Formation of a precipitateprecipitate (solid) (solid)

ColorColor changechange

B. B. Chemical EquationsChemical Equations

Have two parts:Have two parts:1.1. ReactantsReactants = the substances you start with = the substances you start with2.2. ProductsProducts = the substances you end up = the substances you end up

withwith

The reactants will turn into the The reactants will turn into the products.products.

A + B A + B C + D C + DReactantsReactants ProductsProducts

B. B. Chemical EquationsChemical Equations

Symbols used in equationsSymbols used in equations

■ Special conditions other than heat Special conditions other than heat can be written over the arrow also.can be written over the arrow also.

■ Such as “high pressure”Such as “high pressure”■ Or a catalyst – which makes the Or a catalyst – which makes the

reaction go faster, but is not changed reaction go faster, but is not changed in the reaction.in the reaction.

Hi Pressure MnO2

Example: Translating a reaction into an Example: Translating a reaction into an equationequation

In the catalytic converter of a car, carbon In the catalytic converter of a car, carbon monoxide gas and oxygen gas become monoxide gas and oxygen gas become carbon dioxide in the carbon dioxide in the presence of Platinum presence of Platinum metalmetal

** special condition= catalyst!** special condition= catalyst!

CO(g) + OCO(g) + O22(g) CO(g) CO22(g)(g)Pt

Law of Conservation of MassLaw of Conservation of Massmass is neither created nor destroyed in mass is neither created nor destroyed in a chemical reactiona chemical reaction

4 H4 H

2 O2 O

4 H4 H

2 O2 O

4 g 32 g

36 g

total mass stays the total mass stays the samesame

atoms can only atoms can only rearrangerearrange

Describing Balanced EquationsDescribing Balanced Equations

to produceto produce

• How many?• Of what?• In what state?

Zn(Zn(ss) + 2HCl() + 2HCl(aqaq) ) ZnCl ZnCl22((aqaq) + H) + H22((gg))

One atom of solid zincOne atom of solid zinc reacts withreacts with

two molecules of aqueous two molecules of aqueous hydrochloric acidhydrochloric acid one unitone unit

of aqueous zinc chlorideof aqueous zinc chloride and oneand onemolecule of hydrogen gas.molecule of hydrogen gas.

Describing EquationsDescribing EquationsDescribing CoefficientsDescribing Coefficients::

– individual atom = “atom”individual atom = “atom”

– covalent substance = “molecule”covalent substance = “molecule”

– ionic substance = “unit”ionic substance = “unit”

3 molecules of carbon dioxide3 molecules of carbon dioxide

2 atoms of magnesium2 atoms of magnesium

4 units of magnesium oxide4 units of magnesium oxide

3CO3CO22

2Mg 2Mg

4MgO 4MgO

Counting AtomsCounting Atoms

To determine if an equation is balanced or To determine if an equation is balanced or not, you need to be able to count atoms in not, you need to be able to count atoms in a compound.a compound.

Cu(NOCu(NO33))22 has . . . . has . . . .

1 copper atom1 copper atom

2 nitrogen atoms2 nitrogen atoms

6 oxygen atoms6 oxygen atoms

I II III IV V

Chapter 10Chapter 10

“Chemical “Chemical Reactions”Reactions”

Balancing ReactionsBalancing Reactions

Balanced Chemical EquationsBalanced Chemical Equations

Atoms can’t be created or Atoms can’t be created or destroyed in an ordinary reaction:destroyed in an ordinary reaction:–All the atoms we All the atoms we start withstart with we must we must

end up withend up with (meaning: balanced!) (meaning: balanced!)

A balanced equation has the same A balanced equation has the same number of each element number of each element on both on both sidessides of the equation. of the equation.

Examples of balanced chemical Examples of balanced chemical EquationsEquations

2 Ag + S 2 Ag + S Ag Ag22SS

CHCH44 + 2 O + 2 O22 CO CO22 + 2 H + 2 H22O O

+

++

How to Balance Chemical How to Balance Chemical EquationsEquations

You can only balance equations using You can only balance equations using COEFFICIENTS COEFFICIENTS in front of a reactant or product!!!in front of a reactant or product!!!

Never change the Never change the SUBSCRIPTSSUBSCRIPTS in a reactant or in a reactant or product!!!!product!!!!– If you change the subscript (formula) you are If you change the subscript (formula) you are

describing a different chemical.describing a different chemical.– HH22O is a different compound than HO is a different compound than H22OO22

NeverNever put a coefficient in the put a coefficient in the middlemiddle of a of a formula; they must go only in the formula; they must go only in the frontfront

22NaCl is okay, but NaNaCl is okay, but Na22Cl is not.Cl is not.

Tips for Balancing EquationsTips for Balancing Equations

Start with elements that only appear once Start with elements that only appear once on BOTH sides of the equationon BOTH sides of the equation

CCO + OO + O22 CCOO22

Balance Oxygen and Hydrogen last… they Balance Oxygen and Hydrogen last… they usually appear more than once in an usually appear more than once in an equationequation

22CCOO + + OO22 22CCOO22

If a polyatomic ion appears unchanged on both If a polyatomic ion appears unchanged on both sides of the equation, treat it as a single unit, sides of the equation, treat it as a single unit, instead of counting individual atomsinstead of counting individual atoms

CuCuSOSO44 + NaCl + NaCl Na Na22SOSO44 + CuCl + CuCl22CuCuSOSO44 + + 22NaCl NaCl Na Na22SOSO44 + CuCl + CuCl22

Check to see if the coefficients used give equal Check to see if the coefficients used give equal numbers of atoms on both sides of arrow. If not, numbers of atoms on both sides of arrow. If not, go back and re-balance!!! go back and re-balance!!!

Last, if all the coefficients are multiples of each Last, if all the coefficients are multiples of each other, reduce to the least common multiple.other, reduce to the least common multiple.

Practice Balancing ExamplesPractice Balancing Examples

_AgNO_AgNO33 + _Cu + _Cu _Cu(NO _Cu(NO33))22 + _Ag + _Ag

_Mg + _N_Mg + _N22 _Mg _Mg33NN2 2

_P + _O_P + _O22 _P _P44OO1010

__CaH__CaH22 + __H + __H22O O _H _H22 + __Ca(OH) + __Ca(OH)22

_CH_CH44 + _O + _O22 _CO _CO22 + _H + _H22OO

2 2

3

4 5

2 2

22

More ExamplesMore Examples

__ZnS + __O__ZnS + __O22 __ZnO + ___SO __ZnO + ___SO22

__C__C22HH66 + __ O + __ O22 __CO __CO22 + __H + __H22OO

__ NH__ NH33 + __O + __O22 __ N __ N22 + __H + __H22OO