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I II III IV VS.Barry

1. Copy down HW2. Take out HW & COMPARE your work to your table partner’s

BELLWORK


Chemical Reactions & Equations

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Chemical Equations

A + B C + D

REACTANTS PRODUCTS

• Shows what takes place during a chemical reaction

•Reactant : (substances reacting) on left•Arrow - (→) at center ; yields or produces•Products: (new substances formed) on right

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Coefficients: how many? ; # appears in FRONT of the formula “molecules” for covalent substances

“moles“ of atoms “units” for ionic substances

3CO2

2Mg

4MgO

3 molecules of carbon dioxide

2 moles of magnesium

4 units of magnesium oxide

Phases: • solid (s), liquid (l), gas (g), Aqueous (aq)

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Writing Equations

Names of reactants: methane & oxygen Names of products: carbon dioxide & water

CH4 + 2O2 CO2 + 2H2O

How many of each (coefficient):

__CH4 ____O2 ____CO2 ___H2O

Ratio of coefficients: ___: ____: ___: ___

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Expressing Chemical Equations

Word: carbon + oxygen yields carbon

dioxide Remember your diatomic gases:

(BrINClHOF)Formula: C + O2 → CO2

→+Diagram:

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Expressing Equations with words

to produce

• How many?• Of what?• In what state?

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

One atom of solid zinc reacts withtwo molecules of aqueous hydrochloric acid one unitof aqueous zinc chloride and onemolecule of hydrogen gas.

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Exothermic & Endothermic Processes – see RB pg 34 table 2.5

Exothermic: release energy; surrounding temp increases A + B → AB + energy

- CH4 + 3 O2 → CO2 + 2 H2O + energy

Endothermic: require/absorb energy; surrounding temp decreases AB + energy→ A + B

-H2O(s) + energy → H2O (l)

Endothermic video http://www.youtube.com/watch?v=5RJLvQXce4A

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Law of Conservation of Mass/Matter/Energy

Matter is neither created nor destroyed in chemical reactions. In any chemical total numbers & kinds of atoms must remain unchanged in the reaction. This is called a balanced equation.

Charge, energy, mass, and # atoms are conserved (unchanged) in balanced reactions

4 H

2 O

4 H

2 O

4 g 32 g

36 g

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Signs of a Chemical Reaction

Evolution of heat and light Formation of a gas Formation of a precipitate Color change

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Chemical Change – chemical composition changesSigns of a Chemical ReactionEvolution of heat and lightFormation of a gasFormation of a precipitateColor change

Physical Change – chemical composition remains the same

Examples of a physical changeRipping, tearing, breakingBoiling, melting, freezing, vaporing a substance (a phase change)dissolving

Examples of a chemical changeBurning, rusting, oxidizingFlammable, explosive, reacting


II. Balancing Equations

Chemical Reactions

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A. Balancing Steps

1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient subscript = # of atoms

4. Reduce coefficients to lowest possible ratio, if necessary.

5. Double check atom balance!!!

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B. Helpful Tips

Balance one element at a time. Update ALL atom counts after adding

a coefficient. If an element appears more than

once per side, balance it last. Balance polyatomic ions as single

units. “1 SO4” instead of “1 S” and “4 O”

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Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

1 1

1 1

2 3

2

3

6

3

33 2

C. Balancing Example

Aluminum and copper(II) chloride react to form copper and aluminum chloride.

2

2

6


Chemical Reactions

III. Types of Chemical Reactions

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A. Combustion

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

the burning of any substance in O2 to produce heat

A + O2 B

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Na(s)+ O2(g)

C3H8(g)+ O2(g) 5 3 4

A. Combustion

Products: contain oxygen hydrocarbons form CO2 + H2O

CO2(g)+ H2O(g)

Na2O(s) 4 2

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B. Synthesis

the combination of 2 or more substances to form a compound

only one product

A + B AB

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B. Synthesis

H2(g) + Cl2(g) 2 HCl(g)

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Al(s)+ Cl2(g) AlCl3(s)2 3 2

B. Synthesis

Products: ionic - cancel charges

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C. Decomposition

a compound breaks down into 2 or more simpler substances

only one reactant

AB A + B

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C. Decomposition

2 H2O(l) 2 H2(g) + O2(g)

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KBr(l) K(s) + Br2(l) 2 2

C. Decomposition

Products: binary - break into elements

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D. Single Replacement

one element replaces another in a compound metal replaces metal (+) nonmetal replaces nonmetal (-)

A + BC B + AC

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Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

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Fe(s)+ CuSO4(aq) Cu(s)+ FeSO4(aq)


Products: metal metal (+) nonmetal nonmetal (-) free element must be more active (check activity

series-Table J)

Br2(l)+ NaCl(aq) N.R.

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AB + CD AD + CB

E. Double Replacement

ions in two compounds “change partners” cation of one compound combines with

anion of the other

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Pb(NO3)2(aq) + K2CrO4(aq) PbCrO4(s) + 2KNO3(aq)

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Pb(NO3)2(aq)+ KI(aq) PbI2(s)+ KNO3(aq)


Products: switch negative ions one product must be insoluble (check

solubility table)

NaNO3(aq)+ KI(aq) N.R.

2 2