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