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Section 1: The Nature of Chemical Reactions
Section 2: Reaction Types
Section 3: Balancing Chemical Equations
Section 4: Rates of Change
Key Terms
Reactant
Product
Chemical Energy
Exothermic Reaction
Endothermic Reaction
The Nature of Chemical Reactions
What are some chemical reactions that we see everyday?
Growing, Decaying, or Burning
Chemical reactions change substances (pg 184)
Chemical reactions occur when substances undergo chemical changes to form new substances
Production of gas and change of color are signs of chemical reactions chemical reactions rearrange atoms.
Reactants Products
New product atoms are not formed and old reactant atoms are not formed (Conservation of Matter). The atoms are rearranged.
2C8H18 + 25O2 16CO2 + 18H2O + Energy
Energy and Reaction (pg 185)
In order for a reaction to occur energy must be added.
The energy that is added is used to break bonds
Energy may be added in different ways
Ex. Heat, Electricity, Sound, or Light
The formation of bonds release energy
Remember conservation of energy
In chemical reactions, energy is conserved
Stored energy is called chemical energy
The total energy before the reaction is equal to the total energy of the products and their surroundings
Reactions that release energy are exothermic
This happens because more energy is released as the products form than is absorbed to break the bonds in the reactants. (The surrounding becomes warmer)
Reactants that absorb energy are endothermic
More energy is needed to break the bonds in the reactants than is given off by forming bonds in the products.
Key Terms
Synthesis Reaction Decomposition Reaction Electrolysis
Combustion Reaction Radical
Single-displacement Reaction
Double-displacement Reaction
Oxidation-reduction Reaction
Reaction Types
Classifying Reactions (pg 190)
Patterns can be used to identify the general types of chemical reactions and to predict the products of the chemical reaction.
Synthesis reactions combine substances
In this type of reaction at least two reactants join to form a product
Examples: A + B AB
2Na + Cl2 2NaCl
Decomposition reactions break substances apart
Examples: AB A + B
2H2O 2H2 + O2
Electrolysis of water is an example of a decomposition reaction that involves passing an electric current through water to get hydrogen and oxygen gas.
Combustion reactions use oxygen as a reactant
Since oxygen is always used in combustion reactions, oxygen is always a product.
In combustion the products depend on the amount of oxygen present
In single-displacement reactions, elements trade places
Examples: AX + B BX + A
3CuCl2 + 2Al 2AlCl3 + 3Cu
Usually more reactive elements take the place of less reactive ones
Double-displacement reactions, ions appear to be exchanged between compounds
Examples: AX + BY AY + BX
Pb(NO3)2 + K2CrO4 PbCrO4 + 2KNO3
Electrons and Chemical Reactions (pg 196)
Oxidation-reduction reaction (redox)
Electrons are transferred in redox reactions
Substances that accept electrons are said to be reduced
Substances that give up electrons are said to be oxidized
In all redox reactions, one or more reactants in reduced and one or more is oxidized
Some redox reactions deal with the loss or gain of hydrogen and oxygen
Oxidation is a gain of oxygen and a loss of hydrogen
Reduction is a gain of hydrogen and a loss of oxygen
Example: Respiration and combustion
Radicals have electrons available for bonding
Key Terms
Chemical Equation
Mole Ratio
Balancing Chemical Equations
Chemical equations summarize reactions
To write a chemical equation we use chemical formulas
The arrow () in an equation means “gives” or “yields”
Balancing chemical equations account for the conservation of mass.
How do you balance a chemical equation?
Remember you cannot balance an equation by changing the chemical formula. You have to leave the subscripts in the formulas alone.
An equation can only be balanced by placing numbers, called coefficients, in front of the chemical formula.
Information from a balanced equation
Balanced chemical equations are the standard way chemist write about reaction to describe both the substances in the reactions an the amounts involved.
Determining Mole Rations
A coefficient of 1 is never written
Balancing equations show conservation of mass
The law of definite proportions states;
A compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed.
Mole ratios can be derived from balanced equations
What do mole ratios allow us to do?
They allow us to calculate the mass of the reactants. We can determine the relative mass of the substance need to react completely
Key Terms
Catalyst
Enzyme
Substrate
Chemical Equilibrium
Rates of Change
Chemical reactions can occur at different speeds or rates
Factors Affecting Reaction Rates
Most reactions go faster at higher temperatures
Large surface area speeds up reactions
Concentrated solutions react faster
Reactions are faster at higher pressure
Massive, bulky molecules react slower
Catalysts change the rates of chemical reactions
Catalyst is a substance that changes the rate of a chemical reaction without being consumed or changed significantly.
Enzymes are biological catalyst they are proteins that are catalysts for chemical reactions in living things
Equilibrium Systems
Most reactions go until completion. Most of the reactants are converted to products, and the amount that is not converted is not noticeable and usually is not important
Some changes are reversible
Reactions that do not go to completion are said to be reversible
How do we represent a reversible reaction?
Equilibrium results when rates balance
Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reversible reaction and the concentration of products and reactants remain unchanged.
Le Châtelier’s principle predicts changes in equilibrium
If a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached
What can we use Le Châtelier’sprinciple for?
To control reactions
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