Upload
kelley-phelps
View
235
Download
0
Embed Size (px)
Citation preview
Chapter 19: Chemical Reactions
• Section 1: Chemical Change
• Section 2: Classifying Chemical Reactions
• Section 3: Chemical Reactions and Energy
• Section 4: Reaction Rates and Equilibrium
Section 1: Chemical Change
Chemical Reaction – a change in which one or more substances are converted into new substances• A chemical reaction consists of two parts:
Reactants – the substances that react Products – the new substances produced
• Chemical reactions obey the conservation laws The mass of the reactants always equal the mass of the
products
• Examples of chemical reactions: Digestion. Internal combustion engines. Cooking – sometimes
Section 1: Chemical ChangeChemical equations• The exact nature of chemical reactions are difficult to
describe using words Example: Aluminum metal can react with iron oxide to
give us two new substances, iron and aluminum oxide How much aluminum is needed to react with how
much iron oxide? Which iron oxide is involved in the reaction? What form –solid, liquid, gas—are the reactants and
products?• Chemists have developed a shorthand way to describe
chemical reactions Chemical equation – a way to describe a chemical
reaction using chemical formulas and other symbols General form:
Example: The equation tells us the 2 units of aluminum will react
with 1 unit of iron (III) oxide to form 2 units of iron and 1 unit of aluminum (III) oxide
The equation also tells us that the reactants and products are in the solid state during the reaction
3 2 322Al(s)+Fe O (s) 2Fe(s)+Al O (s)
Section 2: Classifying Chemical Reactions
Types of Chemical Reactions
Most reactions may be grouped into three basic types: synthesis, decomposition, and replacement reactions• Synthesis reaction – a reaction where two or more
substances combine to form one new substance General form: Example:
• Decomposition reaction – involves breaking a compound into two or more substances General form: Example:
• Replacement reaction – one atom or group of atoms in a compound is replaced with another atom or group of atoms A single replacement reaction is when on atom or group
of atoms is replaced General form: Example:
A double replacement reaction occurs when two different types of atoms or groups of atoms exchange place General form: Example: A precipitate is a solid that is formed in a double
replacement reaction
Section 2: Classifying Chemical Reactions
Balancing Chemical Equations• Chemical equations obey the Law of Conservation of Mass
A balanced chemical equation has the same number of atoms of each element of both sides of the equation
To balance a chemical equation, coefficients must be used until the number of atoms of each element is balanced.
Coefficients – represent the number of units of each substance taking part in the reaction
Important: You cannot balance a chemical equation by changing subscripts. If you change the subscript you change the composition of the substance
Section 2: Classifying Chemical Reactions
Ex.: Balance the following equation: Step 1: Check the equation for atom balance:
The Mg atoms are balanced but the O atoms are not, therefore the equation is not balanced Step 2: Choose coefficients that balance the equation Try putting a coefficient of 2 in front of the MgO
Step 3: Recheck the number of atoms on each side of the equation and adjust if necessary
Now, there are two Mg’s on the right side but only one of the left, so a coefficient of 2 is needed for the Mg also
The equation is now balanced
2Mg(s) O (g) MgO(s)
Atom Mg + O2 MgOMg 1 1O 2 1
2Mg(s) O (g) 2MgO
Atom Mg + O2 MgOMg 1 2O 2 2
22Mg(s) O (g) 2MgO Atom Mg + O2 MgOMg 2 2O 2 2
Section 3: Chemical Reactions and Energy
All chemical reactions release or absorb energy• Exergonic reactions – less energy is required to break the
original bonds than is released when new bonds form
• Exothermic reaction – a reaction where the energy given off in the reaction is primarily in the form of heat
• Endergonic reactions – a chemical reaction that requires more energy to break bonds than is released when new bonds are formed
• Endothermic reaction – a reaction where the energy needed is in the form of heat
• Catalyst – a substance that speeds up a chemical reaction without being permanently changed itself
• Inhibitors – substances used to prevent certain reactions from occurring
Section 4: Reaction Rates and Equilibrium
Reaction rate – the rate at which reactants change into products• Factors that affect reaction rate include:
Temperature at which reaction occurs Concentration of the reactants Volume and pressure (in reactions that involve gases) Surface area over which the reaction occurs The presence of catalysts or inhibitors
Equilibrium – a state in which forward and reverse reactions or process proceed at equal rates• LeChatelier’s principle – if a stress is applied to a system at
equilibrium, the equilibrium shifts in the direction that opposes the stress Common stresses include: changing the concentration
of the reactants, changing the temperature, and changing volume and pressure
The Mole and Molar Mass
Because atoms are so small and it requires a tremendous number of atoms to make any one thing, it is very difficult to count individual atoms.• A counting unit is used to count out a very large number of
a very small thing Example 1: When you order popcorn at the movies you do
not buy a specific number of popcorn kernels; you buy a size of container for the popcorn. The size of the container—small, medium, and large—are the counting units for popcorn.
Example 2: You do not go to the grocery store and buy one egg; you buy a dozen (12) eggs. The dozen is the counting unit for buying eggs
Mole – the counting unit chemists use for counting atoms and other small particles.• One mole is a collection of a very large number of particle• One mole is equal to 6.022 x 1023 particles• The mole is usually written as 6.022 x 1023/mole, and is
known as Avogadro’s constant
The Mole and Molar Mass
Molar mass – the mass in grams of one mole of a substance• The molar mass of an element in grams is the same as its
atomic mass listed in the periodic table Example: The average atomic mass of carbon is 12.01, so
one mole of carbon has a mass of 12.01 grams Calculations involving molar mass
• Finding number of atoms if you know massExample: 85.5-g sample of Hg in a barometer
• Finding mass if know the number of atomsExample: Determine the mass of a piece of titanium if it contains 2.24x1028 atoms
( . )x( ) .85 5 2 57 1023g)x(1 mole
200.59g
6.022x10 atoms
1 mol Hgatoms of Hg
23
x
( . )x( ) .2 24 10 178 1028 6x xatoms)x(1mol Ti
6.022x10 atoms
47.88g
1 mol Tig Ti
23