Chemical Equations&

Chemical Reaction Types

• Chemical equations give information in two major areas.• First, they tell us what substances are reacting (those being

used up) and what substances are products (those being made).

• Second, the coefficients of a balanced equation tell us in what ratio the substances will react or are produced.

• Thirdly they also will represent the number of moles of each substance reacting or produced.

The Meaning of a Chemical Equation

Equation Breakdown• The reactants are on the left side of a chemical

equation and the products are on the right side.• However, you might ask, "On the left and right side

of what?"• Answer - the arrow.• 2 H2 + O2 ---> 2 H2O • On the left side are the reactants - hydrogen and

oxygen. We will ignore the two in front of the hydrogen for a moment.

• On the right side is the product - water. We will also ignore the two in front of the water, but we will soon return to it.

• Please be aware that there can be one, two, three, or more substances on either side of the arrow, as in this more complex equation:

• Ca(H2PO4)2 + CaSO4 + HF ---> Ca10F2(PO4)6 + H2SO4 • Typically the arrow is replaced with "produces" or "yields"

when the equation is said out loud.• Coefficients are the numbers in front of the formulas.• 2 H2 + O2 ---> 2 H2O • Note the presence of a two in front of the hydrogen and also

the water.• These are called the coefficients. These numbers give two

very important pieces of information about the equation.

• First: the coefficients give the number of molecules (or atoms) involved in the reaction.

• Second: the coefficients give the number of moles of each substance involved in the reaction.

• The point just made is CRITICAL.• The coefficients of an equation tell us how many

moles of each reactant are involved as well as how many moles of each product get produced.

• What is the relationship between molecules and moles?

• Yes!, Avogadro's Number is the factor between the two points above.

Quick Review • There are three important things to remember about

reading an equation.

• One: Reactants are on the left and products are on the right. Of What?

• Arrow

• Two: Coefficients are the numbers in front of each formula. If no number is shown, a one is understood.

• Three: The coefficients tell us how many molecules or moles of each reactant used and how many molecules or moles of each product made.

Practice Problems• Identify the reactants, the products and the

coefficients of these equations:

1) Zn + 2 HCl ---> ZnCl2 + H2

2) 2 KClO3 ---> 2 KCl + 3 O2

3) S8 + 24 F2 ---> 8 SF6

4) 4 Fe + 3 O2 ---> 2 Fe2O3

5) 2 C2H6 + 7 O2 ---> 4 CO2 + 6 H2O

Balancing Chemical Equations• Chemical equations do not come already balanced. This

must be done before the equation can be used in a chemically meaningful way.

• All chemical calculations to come must be done with a balanced equation.

• A balanced equation has equal numbers of each type of atom on each side of the equation.

• Why?• The Law of Conservation of Mass, Matter & Energy “Mass, Matter & Energy are neither created nor destroyed." Therefore, we must finish our chemical reaction with as many

atoms of each element as when we started.

• H2 + O2 ---> H2O

• In the example equation, there are two atoms of hydrogen on each side, BUT there are two atoms of oxygen on the left side and only one on the right side.

• Remember this: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.

• An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that only the coefficients can be changed, NEVER a subscript.

Here is the example equation for this lesson:

So, now to balancing the example equation:

• H2 + O2 ---> H2O

• The hydrogen are balanced, but the oxygens are not. We have to get both balanced. We put a two in front of the water and this balances the oxygen.

• H2 + O2 ---> 2 H2O

• However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side.

• 2 H2 + O2 ---> 2 H2O

• This balances the equation.

Balance this equation: H2 + Cl2 ---> HCl

•Remember that the rule is: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.•H2 + Cl2 ---> 2 HCl

•Balance this equation: O2 ---> O3

•3 O2 ---> 2 O3 The LCM tells you how many of each atom

will be needed. Least common multiple.

Reaction Types

• Single Replacement

• Double Replacement or Ionic

• Decomposition or Analysis

• Composition or Synthesis

• Combustion

Single Replacement• During single replacement, one element replaces another element in

a compound. There are two different possibilities:

1. One cation replaces another. Written using generic symbols, it is:

AX + Y ---> YX + A

• Element Y has replaced A (in the compound AX) to form a new compound YX and the free element A. Remember that A and Y are both cations ( positively-charged ions) in this example.

• Examples Rule

• Cu + AgNO3 ---> Ag + Cu(NO3)2 More active metal

• Ca + H2O ---> Ca(OH)2 + H2 Metal replaces Hydrogen in Water

• Zn + HCl ---> ZnCl2 + H2 Metal replaces Hydrogen in an Acid

2. One anion replaces another. Written using generic symbols, it is:

A + XY ---> XA + Y• Element A has replaced Y (in the compound XY) to form a

new compound XA and the free element Y.• Remember that A and Y are both anions

(negatively-charged ions) in these examples.

Examples Rule

Cl2 + NaBr ---> NaCl + Br2 More Active Halogen

Br2 + KI ---> KBr + I2 More Active Halogen

ZnS + O2 ---> ZnO + S8 More Active Non-metal

Reaction Types: Double Replacement• During double replacement, the cations or anions of two

different compounds switch places.• Written using generic symbols, it is:

AB + XY ---> AY + XB• A and X are the cations (postively-charged ions) in this

example, with B and Y being the anions (negatively-charged ions).

• Examples Rule

• KOH + H2SO4 ---> K2SO4 + H2O Water is a Product

• AgNO3 + NaCl ---> AgCl + NaNO3 A precipitate is formed

Use solubility table to see if a precipitate will form

Double Replacement

In order for a double replacement reaction to occur, one of three things must show up as a product.

1. Water

2. Precipitate

3. Gas (Non-metallic oxides, diatomic substances)

Reaction Types: Decomposition

• During decomposition, one compound splits apart into two (or more pieces). These pieces can be elements or simpler compounds.

• Written using generic symbols, it is usually shown as: • AB ---> A + B • There are 6 general decomposition type reactions that can occur, the

1st 5 given here require heat being applied to the reactant side.• 1. Metallic oxide when heated: form a metal and oxygen• Mercuric oxide Mercury and Oxygen• 2. Metallic carbonates when heated : form a metallic oxide and

carbon dioxide (gas)• Calcium carbonate Calcium oxide and carbon dioxide gas.

Decomposition cont.

• 3. Many metallic hydroxides when heated form metallic

oxides and water vapor.• Calcium hydroxide Calcium oxide and water (gas)• 4. Metallic chlorates when heated form metallic chloride

and oxygen (gas).• Potassium chlorate Potassium chloride and oxygen (g)• 5. Some acids when heated decompose into nonmetallic

oxides and water.• Carbonic Acid Carbon dioxide and water• 6. Decomposition by electrolysis (electricity)• Water Hydrogen (g) and oxygen (g) or • Sodium Chloride Sodium (s) and chlorine (g)

Reaction Types: Synthesis or Composition• Synthesis are, at this introductory level, almost always the

reverse of a decomposition reaction. That means that two pieces join together to produce one, a more complex compounds. These pieces can be elements or simpler compounds.

• A + B ---> AB

Reaction Types: Combustion •Combustion, at its most general, can mean the reaction of oxygen gas (O2) with anything. •However, we will understand combustion to mean the reaction of oxygen with a compound containing carbon and hydrogen. A common synonym for combustion is burn. •Written using generic symbols, it is usually shown as: •CxHy + O2 ---> CO2 + H2O

Combustion• C6H12O6 + O2 ---> CO2 + H2O

• CH4 + O2 ---> CO2 + H2O

• Notice that some compounds contain carbon, hydrogen AND oxygen. However, the products are all the same, in every reaction.

• We could vary it a bit by adding nitrogen (burns to form NO2) to the

compound formula or sulfur (burns to form SO2 or SO3). Like these:

• C21H24N2O4 + O2 ---> CO2 + H2O + NO2

• C2H5SH + O2 ---> CO2 + H2O + SO2

Phase Notation & Other forms of notation

• (g) gas (ppt) precipitate or (s) solid

• (l) liquid (s) solid

• (aq) aqueous – dissolved in water or solution of

MnO2 shows as a catalyst

heat is applied