Ch. 5 - Chemical ReactionsI. Chemical Changes

in Matter

• Chemical Reactions

• Law of Conservation of Mass

• Chemical Equations

Chemical Reaction

• A change in which one or more substances are converted to different substances.

A+BC+DREACTANTS PRODUCTS

Chemical Reaction

Another name for a chemical change

(1)New properties when you are done

(2)No new atoms are made

(3)Atoms are rearranged and new compounds can be made

(4)Old bonds are broken; New bonds are formed

Indications of Chemical Reactions

(1) Fizzing or bubbling (formation of a (1) Fizzing or bubbling (formation of a gas)gas)(2)Change in color, odor, or properties(2)Change in color, odor, or properties(3) Temperature change( colder or (3) Temperature change( colder or hotter)hotter)(4)Formation of a precipitate(4)Formation of a precipitate

• PrecipitatePrecipitate- solid formed in a liquid- solid formed in a liquid ( turns cloudy)( turns cloudy)

Starting a Reaction

• Always takes a little energy

• Energy goes into breaking bonds in the reactants

• Can use different forms of energy

(1) Heat

(2)Electricity

(3)Light

Chemical reactions can be represented in two ways:

(1) Word equations

Liquid water yields gaseous oxygen and gaseous hydrogen

(2) Chemical equation (skeletal)

2H20(l) O2(g) + 2H2(g)

Chemical EquationsAqueous lead(II) nitrate plus two units of

aqueous potassium iodide produces solid

lead(II) iodide and two units of aqueous

potassium nitrate.

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

Coefficient - # of units of each substance

Chemical Equations• Describing Coefficients:

• individual atom = “atom”

2Mg 2 atoms of magnesium

• covalent substance = “molecule”

3CO2 3 molecules of carbon dioxide

• ionic substance = “unit”

4MgO 4 units of magnesium oxide

Parts of a chemical equation

Reactants Products

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

coefficient subscript yields state(1)(1)ReactantsReactants- the substances you start with- the substances you start with(2)(2)ProductsProducts- the substances you make- the substances you make (3)(3)coefficientcoefficient-number in front of formula that changes the -number in front of formula that changes the number of atoms,number of atoms,(4)Subscript(4)Subscript- number below symbol that indicates the number of - number below symbol that indicates the number of atoms.atoms.

C. Chemical Equations

Law of Conservation of Mass

Matter is not created or destroyed.

• Atoms can only rearrange.

• Discovered by Lavoisier.

4H

2O

4H

2O

Balancing Equations

You must have the same number of each element on each side of the equation (must obey the law of conservation of mass- matter can not be created or destroyed.)

A. Balancing Steps1. Write the unbalanced equation.

2. Count atoms on each side.

3. Add coefficients to make #s equal.

Coefficient subscript = # atoms

4. Reduce coefficients to lowest possible

ratio, if necessary.

5. Double check atom balance!!!

1 1

1 1

2 3

2 2

3

6 6

3

2 3 3 2 3 2 Al + CuCl2 Cu + AlCl3

Al

Cu

Cl

2 3 2

B. Balancing ExampleAluminum and copper(II) chloride form copper and

aluminum chloride.

Moles and ReactionsThe coefficients of balanced equations tell

how many particles react and how many moles of particles

• We can make ratios with those moles

• 2 Mg + O2 2 MgO

• If 2 moles of Mg react, 1 mole of O2 will be required

• 2 mol Mg or 1 mol O2

1 mol O2 2 mol Mg

III. Types of ReactionsThere are five types of reactions:

*1.Synthesis

*2. Decomposition

*3.Double displacement

*4. Single displacement

5. Combustion.

1. Synthesis (Composition)• the combination of 2 or more

substances to form a compound

• only one product forms

A + B AB2H2 + O2 2H2O

2. Decomposition• a compound breaks down into 2 or

more simpler substances

• only one reactant

AB A + B2NaCl 2Na + Cl2

3. Single Replacement

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

A + BC AC + B

Zn + CuSO4 ZnSO4 + Cu

4. Double Replacement• ions in two compounds “change partners”

• cation(+) of one compound combines with anion(-) of the other

AB + CD AD + CB

MgO + BaCl2 MgCl2 + BaO

CxHy+O2 CO2+H2O

Combustion

CH4 + 2O2 CO2 + 2H2O

• the burning of a substance in O2 to produce

heat

• hydrocarbons (CxHy) always form CO2 and

H2O

Energy & Chemical Reactions

• Energy Changes

• Exothermic

Reactions

• Endothermic

Reactions

Energy Changes• During a chemical reaction…

–energy is used to break bonds

–energy is released when new bonds are formed

breakingbonds

makingbonds

(1)Exothermic Reaction• reaction that releases energy in the form of

light or heat.

• energy released by making new bonds outweighs energy required to break old bonds

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

– reaction that powers the space shuttle lift-off– Reaction by animals that glow, jellyfish,

lightning bugs ( bioluminescence)

Examples:

C. Endothermic Reaction• reaction that absorbs energy, may feel

cold.

• energy required to break old bonds outweighs energy released by making new bonds

–process used to obtain aluminum from aluminum ore

–Photosynthesis– CO2 +H20 + energy C6H12O6 + O2

2Al2O3 + energy 4Al + 3O2

examples:

Substances that speed up a reactions are catalysts.

Our bodies have catalysts called enzymes.

Substances that slow down reactions are called inhibitors.Preservatives are inhibitors.

Molecular and Formula Mass

• The mass of all atoms in the formula of a compound or molecule.

• To calculate: 1. Write correct formula 2. Identify the number of different atoms in the

formula 3. Multiply the number of atoms by their atomic

mass. 4. Add all the masses together

Example:

• Magnesium Nitrate

• Mg(NO3)2

• Mg = 1 x 24 = 24

• N = 2 x 14 = 28

• O = 6 x 16 = 96

148 amu