Chemical Equations

Review

A + B ABReactant Side Product side

Definition

1. Chemical reaction: a change in composition. This means that one or more substances changes into a new a different substance

2. Ex: vinegar and baking sode water and carbon dioxide

Properties of a Chemical Reaction1. Reactions are usually irreversible

2. Physical and chemical properties are changed.

3. New products are formed.

Five Types of Reactions1. Synthesis

Means to put together.

A. Two substances combine to form one new substance

B. Sample equationi. A + X AX

2. DecompositionA. One substance decomposes into two or more new

substances.

B. Sample equationi. AX A + X

3. Single ReplacementA. A single element reacts with a compound creating a new

compound and a new element.B. Sample equation

i. A + BX B + AX

C. If the lone element (A on the reactant side) is a cation, then it can only switch with another cation (B) if cation A has a higher activity according to the activity series of metals.

Activity Series of Metals

Name SymbolLithium LiPotassium KCalcium CaSodium NaMagnesium MgAluminum AlZinc ZnIron Fe(Hydrogen) (H)Copper CuMercury HgSilver Ag

d. Predict what would happen in the following equation:

Li(s) + NaCl(aq)

e. Predict what would happen in the following equation when solid silver is mixed with copper(II) nitrate.

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

Why is there no change?

LiCl(aq) + Na(s)

No Change

f. Anions also follow a reactivity rule. Anions higher on the periodic table will replace anions lower on the periodic table

4. Double displacementA. Two compounds react and trade partners to form two new

compounds.

B. Sample Equation:i. AX + BY AY + BX

5. CombustionA. A substance that reacts with oxygen gas to form

water and carbon dioxide OR an oxide (rust).

Sample Problems

Identify the type of reaction. 2Mg + FeO2 Fe + 2MgO

NaCl + AgNO3 NaNO3 + AgCl

CaCO3 CaO + CO2

2 C6H6 + 15 O2 6 H2O + 12 CO2

F2 + 2NaCl 2NaF + Cl22 C5H5 + Fe Fe(C5H5)2

Chemical Equation

1. Defined as the amount of compounds and elements taking part in a reaction.

A. Show with an arrow separating the reactants from the products.

B. Remember: reactants are found on the left side in the chemical equation while products are found on the right.

Phases of Chemicals in Equations

Symbol Phase Definition

(s) Solid Molecules touching and

organized. Has definite shape and volume.

Symbol Phase Definition

(l) liquid Molecules touching but not organized. Has a definite volume

but takes the shape of its container.

Symbol Phase Definition

(g) Gas Space between molecules.

Have no definite shape and no

volume.

Symbol Phase Definition

(aq) Aqueous Molecules are dissolved in

water. Has a definite volume but takes the shape of its container.

Common Gases and Acids

1. Certain gases have a special formula because they are diatomic.

a. These gases include:

Hydrogen (H2)

Nitrogen (N2)

Oxygen (O2)

Fluorine (F2)

Chlorine (Cl2)

Bromine (Br2)

Iodine (I2)

2. Also, be familiar with certain acids, though you will be given their formula on the test.

A. These acids include

Hydrochloric acid (HCl)

Phosphoric acid (H3PO4)

Sulfuric acid (H2SO4)

Nitric acid (HNO3)

Carbonic acid (H2CO3)

Numbers and Formulas

2Mg + O2 2MgO

Coefficient

Coefficients tell the relative amount of a substance in an equation

Subscripts

Tell the number of atoms you have.

Side lesson

Why write 2MgO? Why not just write it as Mg2O2?

2MgO tells us we have 2 separate MgO compounds

Mg-O Mg-O

Mg2O2 tells us we have 1 compound made of 2 magnesium atoms and 2 oxygen atoms

Mg-O-O-Mg . .. .. . .. ..

This is actually impossible because Mg does not have 8 valence electrons around it

Naming Rules

1. Remember: gases of certain elements must be written in diatomic form.

A. Example: oxygen gas = O2; not O

2. Names of acids and how they are written in chemical equations can be found on your periodic table. Acids normally begin with a hydrogen atom.

A. Example: hydrochloric acid = HCl

Law of Conservation in Action1. All chemical equations must be balanced.2. Only coefficients can be changed when

balancing equations.3. Ex: Magnesium reacts with oxygen gas to

form magnesium oxide. What is the chemical equation?

Mg + O2 MgO

A. Notice that the number of oxygen atoms on the reactant side is different than the number of oxygen atoms on the product side

B. This is impossible due to the law of conservation of mass, which states that matter cannot be created nor destroyed.

4. The equation needs to be balanced

Balancing Chemical Equations1. All chemical equations must be balanced.2. Start by writing the proper formulas for the reactant

side and product side 3. Pick an element to start (usually leave oxygen and

hydrogen for last).4. Count the number for each type of atom on the

reactant side and be sure they match the same number of atoms for each type of atom on the product side.

A. For example: if there are 3 oxygen atoms on reactant side, then there must be 3 oxygens on the product side.

5. If the number of atoms are different, then only the coefficients can be changed.

Mg + O2 MgONumber of oxygen atoms are different

Mg + O2 2MgONumber of magnesium atoms are different

2Mg + O2 2MgO

The number of Mg and O atoms are the same on both side

Practice Problems

Al2O3 Al + O2

Al Al O O

2

2223

3

6

2

46

4

4

____As + ____I2 ____AsI2

As AsI I

____CH4 + ____O2 ____CO2 + ____H2O

C CH HO O

2

1

4

4

2

1

4

4

Exothermic and Endothermic Reactions in Equations1. Exothermic reactions: reactions that release

energy. In the chemical equation the energy is written in the product side. Ex: combustions reactions have exothermic reactions

2. Endothermic reactions: reactions that absorb energy. These reactions feel cold. In the chemical equation the energy is written in the reactant side.

3. The change in energy can be written as ΔHA. The triangle (delta) stands for “change in” and

the H stands for enthalpy or heat.

B. If ΔH is positive, then the reaction is endothermic

C. If ΔH is negative, then the reaction is exothermic.

Practice

Determine if the reaction is exothermic or endothermic. State what side of the reaction the energy should be written on.

2HCl(g) H2(g) + Cl2(g) ΔH = +185 kJ/mol

Endothermic. Energy written on the reactant side

H2(g) + Cl2(g) 2HCl(g) ΔH = -185 kJ/mol

Exothermic. Energy written on the product side.