The Nature of Chemical Reactions
Dec 27, 2015
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
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.
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
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
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
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
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