I II III IV V

I. Intro to Reactions

Ch. 7 – Chemical Reactions

What is a chemical reaction?

It is a process in which substances undergo a change to form completely new substances

During this process chemical bonds break and reform, thus changing the substances

Endo vs. Exo-thermic

When chemical bonds break and reform, the net bond energy determines if heat is released or absorbed.

If new bonds formed don’t take as much energy to form, then the excess energy is released. You feel it get hot. Thus the term, Exo-thermic!

How do you know a chemical reaction has taken place?

The new substance is altered so much that it cannot be changed back to the original substance without undergoing another chemical change.

Other ways to know a chemical reaction…

Heat and/or light is produced.

Other ways to know a chemical reaction…

Color change.

Other ways to know a chemical reaction…

Gas is produced (Sometimes you can smell it even if you can’t see it…think burning toast)

Other ways to know a chemical reaction…

A precipitate is formed. PRECIPITATE – a solid that forms

when two liquids are combined

B. Law of Conservation of Mass mass is neither created nor destroyed

in a chemical reaction

4 H

2 O

4 H

2 O4 g 32 g

36 g

total mass stays the same atoms can only rearrange

C. Chemical Equations

A+B C+DREACTANTS PRODUCTS

2NaOH + CaBr2 2NaBr + Ca(OH)2

REACTANTS – the starting materials of the reaction.

PRODUCTS – the ending materials of the reaction.

(the arrow) – means “yields.” Shows the direction from the reactants to products.

2NaOH + CaBr2 2NaBr + Ca(OH)2

COEFFICIENTS – numbers placed in front of a compound to indicate how many molecules are present. Can be changed to balance the equation.

SUBSCRIPTS – numbers that indicate how many atoms are present. May not be changed to balance the chemical equation.

You may also see…

Words or symbols over the arrow.

Δ – heat is required

hv – light is required

elec – electricity is required

C. Chemical Equations

I II III IV V

D. Chemical Reactions

Types of Chemical Reactions

1. Combustion

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

the burning of any substance in O2 to produce heat

A + O2 B

Na(s)+ O2(g)

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

1. Combustion

contain oxygen as a reactant hydrocarbons form CO2 + H2O

CO2(g)+ H2O(g)

Na2O(s) 4 2

2. Synthesis

the combination of 2 or more substances to form a compound

only one product

A + B AB

2. Synthesis

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

3. Decomposition

a compound breaks down into 2 or more simpler substances

only one reactant

AB A + B

3. Decomposition

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

4. Single Replacement

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

A + BC B + AC

4. Single Replacement

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

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

5. Single Replacement

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

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

AB + CD AD + CB

5. Double Replacement

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

anion of the other

5. Double Replacement

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

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

5. Double Replacement

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

solubility table)

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

2 2

6. Acid – Base Reactions

This reaction takes place between an Acid, HA and a Base B(OH).

The products are always water and the “left-overs” BA.

HA + B(OH) = H2O + BA