CHEMICAL REACTIONSPrinciples of Chemistry and Physics

Chemical Reactions

When one or more substances (reactants) are changed into one or more new substances (products), a CHEMICAL REACTION has occurred and can be represented as a chemical equation

Reactants products

Chemical Equations

Replaces words with chemical formulas

skeleton equations- does not indicate the relative amounts of reactants and

productsHere is the skeleton equation for rusting:

Fe + O2 Fe2O3

Evidence a reaction has occurred: Color change Gas released Precipitate formed Temperature change (endo or exothermic) Odor produced Smoke Light Flames pH change Flammable to nonflammable or vice versa

Types of Reactions

• There are five types of chemical reactions:

1. Synthesis reactions2. Decomposition reactions3. Single displacement reactions4. Double displacement reactions5. Combustion reactions

• Unit objective: identify the type of reaction and predict the product(s).

1. Synthesis reactions

• Synthesis reactions occur when two substances (generally elements) combine to form a compound.

reactant + reactant 1 product

• Basically: A + B AB• Example: 2H2 + O2 2H2O • Example: C + O2 CO2

Synthesis Reactions• Another example of synthesis:

2. Decomposition Reactions

• Decomposition reactions occur when a compound breaks up into the elements or into a few simpler compounds

• 1 Reactant Product + Product • In general: AB A + B• Example: 2 H2O 2H2 + O2

• Example: 2 HgO 2Hg + O2

Decomposition Reactions

• Another view of a decomposition reaction:

Decomposition Reaction

AB A + B

3. Single Replacement Reactions

• Single Replacement Reactions occur when one element replaces another in a compound.

• A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-).

• element + compound element + compound A + BC AC + B (if A is a metal) ORA + BC BA + C (if A is a nonmetal) (remember the cation always goes first!)

When H2O splits into ions, it splits intoH+ and OH- (not H+ and O-2 !!)

Single Replacement Reactions• Another view:

Single Replacement Example

Example: Zn + CuCl2

ZnClCl

Cu +

General: AB + C AC + B

ClCl Zn Cu+

LIKE replaces LIKE

Activity Serieslists metals in order of decreasing reactivity.

As a general rule, more reactive metals replace less reactive metals in a compound

LiKBaCaNaMgAlZnFeCdNiSnPbHCuHgAgAu

4. Double Replacement Reactions

• Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound

• Compound + compound compound+ compound

• AB + CD AD + CB

Decomposition reactions cont.. Solubility rules-

The formation of a precipitate is a driving force of a double replacement reaction

5. Combustion Reactions

• Combustion reactions occur when a fuel reacts with oxygen gas, which produces heat!

Fuel + O2 (+ Heat) Product

Hydrocarbon Combustion Reactions

• Hydrocarbon Combustion: CxHy + O2 CO2 + H2O

• Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some by-products like carbon monoxide)

• Combustion is used to heat homes (CH4)and run automobiles (octane: C8H18)

Carbon Monoxide Effects

Edgar Allen Poe’s drooping eyes and mouth are potential signs of CO poisoning.

Exothermic process – a process that results in the evolution of heat- energy flows out of the system

Endothermic process- a process that absorbs energy from the surroundings- energy flows into the system

Exothermic or endothermic? 1. Your hand gets cold when you

touch ice 2. ice melts when you touch it 3. Ice cream melts 4. Propane is burning in a propane

torch. 5. Water drops on your skin

evaporate after swimming 6. Two chemicals mixing in a beaker

give off heat

Exothermic Endothermic Endothermic Exothermic Endthermic exothermic

Writing Chemical Equations Word Equations

Names of reactants on the left of an arrow separated by plus signs

Names of products to the right of the arrow separated by plus signs

Ex: flour + water + yeast + salt bread

- Ex: carbon + oxygen carbon dioxide

Diatomic Elements

Some elements exist naturally in pairs, as diatomic molecules. You will be expected to memorize these: Br2, I2, N2, Cl2, H2, O2, F2.

Law of Conservation of Mass Mass is never created or destroyed-

ALL must be conserved and accounted for during a chemical reaction

The same number of atoms of reactant elements must equal the atoms of product elements

Rules for balancing equations: Write correct skeleton formula Determine number of atoms of each element of

reactants and products. COUNT POLYATOMIC ION AS A SINGLE UNIT if it appears unchanged on both sides of the equation

Balance elements one at a time by using coefficients-never change subscripts

Begin with the easiest elements first Check both sides to see if they match Make sure coefficients are in the lowest

possible ratio

Counting with Moles

Chemists use the unit mole to measure the amounts of small particles

1 mole = 6.02 x 1023 particles (atoms, molecules)

6.02 x 1023 is known as Avogadro’s number

The molar mass of any two elements contain the same number of atoms

Ex: a dozen apples – 12 apples a dozen oranges – 12 oranges

Molar mass

Molar mass is the amount of one mole of that element or compound (use the periodic table)

Once you know the molar mass of the compound, you can convert moles of that substance into moles

Molar Mass

Ex: the molar mass of one mole of CO2 is 44 g

this means that 44 g CO2 1 mole

1 mole 44 g CO2

So, if I have 55 g of CO2, how many moles do I have?