The Nature of Chemical Reactions

Chemical Reaction• A well defined example of a chemical

change– One or more substances are changed into

a new substance– Ex.

• Grow• Ripen• Decay• burn

Chemical Reaction

You know a chemical reaction has taken place because you will see changes

• Ex.– Change in color– Production of gas– Precipitate

Chemical Equation

• A representation of a chemical reaction that uses symbols to show the relationship between the substances being reacted and the substances being produced

• Reactants– Substances you start with that go into a

reaction

• Products– The new substance(s) that are formed from

the reaction– REACTANTS ----------- PRODUCTS

• Ex. 2Na + Cl2 -------->2NaCl

– Sodium plus Chlorine yields Sodium Chloride

• THE PRODUCTS AND REACTANTS CONTAIN THE SAME TYPES OF ATOMS… THEY ARE JUST RE-ARRANGED… MASS IS ALWAYS CONSERVED

Conservation of mass & energy• In a chemical reaction, matter and

energy cannot be created or destroyed… therefore mass and energy are conserved

Exothermic reactions

• A chemical reaction in which heat is released to the surroundings– Following an exothermic reaction,

the temperature of surroundings rises because energy is released

Exothermic Reaction

Endothermic Reactions

• Chemical reaction that requires heat– More energy is needed to break the

bonds in the reactants than is given off by forming the product

• Following an endothermic reaction the temperature of the surrounding will decrease because energy is needed

Endothermic Reaction

Energy in Reactions• Activation Energy - The total amount

of energy needed for a reaction to take place.

• Catalyst / Enzyme – Reduces the total amount of activation energy needed. ** Speeds up the reaction

• Inhibitor – Increases the total amount of activation energy needed. ** Slows down the reaction.

ACTIVATION ENERGY

Activation Energy with a Catalyst

Activation Energy with an Inhibitor

Reaction Types

Synthesis Reactions• Synthesis Reactions

– A reaction where two or more substances combine to form a new compound (combined substances)

• They have this general formA + B -------- AB

• Example2Na + Cl2 -------- 2NaCl

Synthesis Reaction

+

Decomposition Reaction

• Decomposition Reactions– A reaction in which a single compound

breaks down to form two or more simpler substances (breaks substances apart)

• They have this general formAB -------- A + B

• Example2H2O -------- 2H2 + O2

• Electrolysis – when electric current is used to produce this type of reaction

Decomposition Reaction

Combustion Reaction

• Combustion Reactions– A reaction that uses oxygen and

carbon as reactants and at least one product of the reaction always contains oxygen and/or carbon.

– Water and carbon dioxide are common products of these reactions

• Example2CO + O2 -------- 2CO2

Single Displacement Reaction• Single displacement Reactions

– A reaction in which one element takes the place of another element in the compound

• They have this general formAX + B -------- BX + A

• Example3CuCl2 + 2Al -------- 2AlCl3 + 3Cu

Single Displacement Reaction

Double Displacement Reaction• Double Displacement Reactions

– A reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds

– They have this general form

AX + BY -------- AY + BX

• ExamplePb(NO3)2 + K2CrO4 -------- PbCrO4 + 2KNO3

Double Displacement Reaction

Type of Reaction

Definition Equation

Synthesis

Decomposition

Single Replacement

Double Replacement

A = Red B = Blue C = Green D = Yellow

A + B → AB

AB → A + B

AB + C → AC + B

AB + CD → AC + BD

Two or more elements or compounds combine to make a more complex

substanceCompounds break down into simpler substances

Occurs when one element replaces another one in a

compound

Occurs when different atoms in two different

compounds trade places

Balancing Chemical Equations• Purpose: To show the conservation of

mass in a chemical reaction

• Process: Add COEFFICIENTS to the equation to balance both sides of the equation.

Step 1 to Balancing Equations• 1. Check the equation for balance

by counting the number of atoms of each element on each side of the equation

Example of Step 1• 1. __NaCl + __BeF2 --> __NaF + __BeCl2

Na – 1 Na - 1

Cl – 1 Cl - 2

Be – 1 Be - 1

F – 2 F - 1

Step 2 to Balancing Equations• 2. Choose coefficients that

balance the equation (NEVER change the subscripts)– Coefficients multiply the number of

atoms

Example of Step 2• 1. 2NaCl + __BeF2 --> 2NaF + __BeCl2

Na – 1 2 Na – 1 2

Cl – 1 2 Cl - 2

Be – 1 Be - 1

F – 2 F – 1 2

• ** Chlorine and Fluorine are not balanced…until the coefficient is placed.

• Placing a coefficient in changes all atoms in that compound.

Balancing Equations Tips:• Practice Balancing Equations

• It's Elemental - Balancing Act! (practice quiz)

Practice ProblemsHere are some practice problems. The solutions are in the section below this one.

1. __FeCl3 + __Be3(PO4)2 --> __BeCl2 + __FePO4 2. __AgNO3 + __LiOH --> __AgOH + __LiNO3

3. __CH4 + __O2 --> __CO2 + __H2O 4. __Mg + __Mn2O3 --> __MgO + __Mn

Solutions1. 2 NaCl + 1 BeF2 --> 2 NaF + 1 BeCl2

2. 2 FeCl3 + 1 Be3(PO4)2 --> 3 BeCl2 + 2 FePO4 3. 1 AgNO3 + 1 LiOH --> 1 AgOH + 1 LiNO3

4. 1 CH4+ 2 O2 --> 1 CO2 + 2 H2O 5. 3 Mg + 1 Mn2O3 --> 3 MgO + 2 Mn