Chemical Equations & Reactions

Chemical Reactions

You should be able to

Classify reactions by type.

Write a balanced molecular equation, complete ionic equation, and a net ionic equation.

Balance oxidation-reduction reactions.

Predict if a precipitate will form using the solubility rules.

Predict products of reactions given the chemical names of thereactants.

Organize Your Thoughts

Chemicalreactions

Chemicalequations

Chemicalequations

• Writing equations • Balancing equations

• Predicting products from reactants

• Synthesis• Decomposition• Single replacement• Double replacement• Combustion• Ox-Redox

Describing a Chemical Reaction

Indications of a Chemical Reaction

– Evolution of heat, light, and/or sound

– Production of a gas

– Formation of a precipitate

– Color change

Signs of Chemical Reactions

There are five main signs that indicate a chemical reaction has taken place:

change in color change in odor production of newgases or vapor

input or releaseof energy

difficult to reverse

rele

ase

inpu

t

Chemical Equations

Depict the kind of reactants and products and their relative amounts in a reaction.

4 Al(s) + 3 O2(g) 2 Al2O3(s)

The numbers in the front are called stoichiometric coefficients.

The letters (s), (g), and (l) are the physical states of compounds.

reactants product

aluminum oxide

Chemical Equations

This equation means:

4 Al(s) + 3 O2(g) 2 Al2O3(s)

4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3

4 Al moles + 3 O2 moles yield 2 moles of Al2O3

or

4 g Al + 3 g O2 yield 2 g Al2O3

4 mol Al@27g/mol 3 mol O2@32g/mol 2 mol Al2O3@102g/mol

108 g + 96 g = 204 g

aluminum oxide sandpaper

Chemical Equations

Because the same atoms are present in a reaction at the beginning (reactants)and at the end (products), the amount of matter in a system does not change.

The Law of Conservation of Matter

Kotz web

Chemical Factory

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Chemical Equations

Because of the principle of the conservation of matter,

An equation must be balanced.

It must have the same number of atoms of the same kind on both sides.

Lavoisier, 1788

Characteristics of Chemical Equations

• The equation must represent known facts.

• The equation must contain the correct formulas for the reactants and products.

• The law of conservation of mass must be satisfied.

Chemical Equations

• Reactants – the substances that exist before a chemical change (or reaction) takes place.

• Products – the new substance(s) that are formed during the chemical changes.

• CHEMICAL EQUATION indicates the reactants and products of a reaction.

REACTANTS PRODUCTS

Additional Symbols Used in Chemical Equations

“Yields”; indicates result of reaction

Used to indicate a reversible reaction

A reactant or product in the solid state; also used to indicate a precipitate

Alternative to (s), but used only to indicate a precipitate

A reactant or product in the liquid state

A reactant or product in an aqueous solution (dissolved in water)

A reactant or product in the gaseous state

(s)

(l)

(aq)

(g)

Additional Symbols Used in Chemical Equations

Alternative to (g), but used only to indicate a gaseous product

Reactants are heated

Pressure at which reaction is carried out, in this case 2 atm

Pressure at which reaction is carried out exceeds normalatmospheric pressure

Temperature at which reaction is carried out, in this case 0 oC

Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction

2 atm

pressure

0 oC

MnO2

D

Word Equations

• A WORD EQUATION describes chemical change using the names of the reactants and products.

Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water.

methane + oxygen

Reactant Product

CH4 O2 CO2 H2O+ + 22

carbon dioxide + water

Cl

Cl

ClH

H

H

ClClCl

Cl HHH

H

H2 + Cl2 HCl H2 + Cl2 2 HCl

reactants products

H

Cl

reactants products

H

Cl

22

2 22 2

11

(unbalanced) (balanced)

Unbalanced and Balanced Equations

Balancing Chemical Equations

Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product

What is the relationship between conservation of mass andthe fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation?

Determine whether the following equation is balanced.

2 Na + H2O 2 NaOH + H2

2 Na + 2 H2O 2 NaOH + H2

Balancing Chemical Equations

• Write a word equation for the reaction.

• Write the correct formulas for all reactants and products.

• Determine the coefficients that make the equation balance.

Balancing Chemical Equations

An important point to remember

2 NO(g) + O2(g) 2NO2(g)

The 2 to the left of NO(g) and NO2(g) refers to the number of molecules present in the balanced equation.

It is a “multiplier” for every atom in the molecule.

The subscript 2 in O2 (g) and NO2(g) refers to the number of atoms of this type that are present in each molecules (or ionic compound).

(NH4)3PO4 + Mg(OH)2 Mg3(PO4)2 NH4OH?

ammonium phosphate magnesium hydroxide magnesium phosphate ammonium hydroxide

NH41+ OH1-

+ 62 3

Now you try…

AlCl3 + Li2CO3 Al2(CO3)3 + LiCl32 6

1) Write a word equation for the reaction.

Write a balanced equation for the reaction between chlorineand sodium bromide to produce bromine and sodium chloride.

2) Write the correct formulas for all reactants and products.

3) Determine the coefficients that make the equation balance.

chlorine + sodium bromide bromine + sodium chloride

Cl2 + NaBr Br2 + NaCl

Cl2 + 2 NaBr Br2 + 2 NaCl

1) Write a word equation for the reaction.

2) Write the correct formulas for all reactants and products.

3) Determine the coefficients that make the equation balance.

aluminum sulfate + calcium chloride calcium sulfate

Al2(SO4)3 + CaCl2 CaSO4 + AlCl3

Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate.

Al2(SO4)3 + 3 CaCl2 3 CaSO4 + 2 AlCl3

+ aluminum chloride? ?

CH4 + 2 O2 CO2 + 2 H2O

Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms

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

hydrogen oxygen water

Reactants Products

balanced

catalyst – speeds up reaction

C(s) + O2(g) CO2(g)

carbon oxygen carbon dioxideReactants Product1 carbon atom 1 carbon atom2 oxygen atoms 2 oxygen atoms

Reactants Product2 hydrogen atoms 2 hydrogen atoms2 oxygen atoms 1 oxygen atoms

Pt

Pt

+

+

2 2

Un

Reactants Product2 hydrogen atoms 4 hydrogen atoms2 oxygen atoms 2 oxygen atoms

Reactants Product4 hydrogen atoms 4 hydrogen atoms2 oxygen atoms 2 oxygen atoms