Chapter 21: Chemical Reactions
Describing Chemical Reactions
Chemical Reaction: a change in which one or more substances are converted into new substances.
Reactants: The substances that react.
Products: The new substances produced.
Conservation of Mass
The total mass of the products always equals the total mass of the reactants.
Writing Equations
Chemical Equation: a way to describe a chemical reaction using chemical formulas and other symbols.
Chemical equations make it easier to calculate the quantities of reactants that are needed and the quantities of products that are formed.
Unit Managers
Atoms are rearranged but never lost or destroyed.
Coefficients: represent the number of units of each substance taking part in a reaction.
Knowing the number of units of reactants enables chemists to add the correct amounts of reactants to a reaction.
Rust can seriously damage iron structures because it crumbles and exposes more iron to the air.
Unlike rust, aluminum oxide adheres to the aluminum surface, forming an extremely thin layer that protects the aluminum from further attack.
Copper is another metal that corrodes when it is exposed to air, forming a blue-green coating called a patina.
Balanced Equations
The formulas in a chemical equation must accurately represent the compounds that react.
Balancing an equation doesn’t change what happens in a reaction—it simply changes the way the reaction is represented.
Balanced Chemical Equation: has the same number of atoms of each element on both sides of the equation.
Types of Reactions
Chemists have defined five main categories of chemical reactions: combustion, synthesis, decomposition, single displacement, and double displacement.
Combustion Reaction: occurs when a substance reacts with oxygen to produce energy in the form of heat and light.
Combustion reactions also produce one or more products that contain the elements in the reactants.
Synthesis Reaction: two or more substances combine to form another substance.
A decomposition reaction is just the reverse of a synthesis.
Decomposition Reaction: occurs when one substance breaks down, or decomposes, into two or more substances.
Most decomposition reactions require the use of heat, light, or electricity.
Single-Displacement Reaction: When one element replaces another element in a compound.
A metal will replace any less active metal.
Double-Displacement Reaction: the positive ion of one compound replaces the positive ion of the other to form two new compounds.
A double-displacement reaction takes place if a precipitate, water, or a gas forms when two ionic compounds in solution are combined.
Precipitate: an insoluble compound that comes out of solution during this type of reaction.
One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons.
Oxidation: describes the loss of electrons
Reduction: describes the gain of electrons.
Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements.
The cause and effect of oxidation and reduction can be taken one step further by describing the substances after the electron transfer.
Chemical Reactions—Energy Exchanges
All chemical reactions release or absorb energy.
When most chemical reactions take place, some chemical bonds in the reactants are broken, which requires energy.
Bond formation releases energy.
More Energy Out
Exergonic Reactions: Chemical reactions that release energy.
Exergonic reaction produces visible light.
Exothermic Reaction: When the energy given off in a reaction is primarily in the form of heat.
Exothermic reactions provide most of the power used in homes and industries.
More Energy In
Endergonic Reactions: A chemical reaction that requires more energy to break bonds than is released when new ones are formed.
Electricity is often used to supply energy to endergonic reactions.
Endothermic Reaction: When the energy needed is in the form of heat
Catalyst: a substance that speeds up a chemical reaction without being permanently changed itself.
When you add a catalyst to a reaction, the mass of the product that is formed remains the same, but it will form more rapidly. The catalyst remains unchanged and often is recovered and reused.
Inhibitors: used to slow down a chemical reaction.
One thing to remember when thinking about catalysts and inhibitors is that they do not change the amount of product produced. They only change the rate of production.
Describing Chemical Reactions
Chemical Reaction: a change in which one or more substances are converted into new substances.
Reactants: The substances that react.
Products: The new substances produced.
Conservation of Mass
The total mass of the products always equals the total mass of the reactants.
Writing Equations
Chemical Equation: a way to describe a chemical reaction using chemical formulas and other symbols.
Chemical equations make it easier to calculate the quantities of reactants that are needed and the quantities of products that are formed.
Unit Managers
Atoms are rearranged but never lost or destroyed.
Coefficients: represent the number of units of each substance taking part in a reaction.
Knowing the number of units of reactants enables chemists to add the correct amounts of reactants to a reaction.
Rust can seriously damage iron structures because it crumbles and exposes more iron to the air.
Unlike rust, aluminum oxide adheres to the aluminum surface, forming an extremely thin layer that protects the aluminum from further attack.
Copper is another metal that corrodes when it is exposed to air, forming a blue-green coating called a patina.
Balanced Equations
The formulas in a chemical equation must accurately represent the compounds that react.
Balancing an equation doesn’t change what happens in a reaction—it simply changes the way the reaction is represented.
Balanced Chemical Equation: has the same number of atoms of each element on both sides of the equation.
Types of Reactions
Chemists have defined five main categories of chemical reactions: combustion, synthesis, decomposition, single displacement, and double displacement.
Combustion Reaction: occurs when a substance reacts with oxygen to produce energy in the form of heat and light.
Combustion reactions also produce one or more products that contain the elements in the reactants.
Synthesis Reaction: two or more substances combine to form another substance.
A decomposition reaction is just the reverse of a synthesis.
Decomposition Reaction: occurs when one substance breaks down, or decomposes, into two or more substances.
Most decomposition reactions require the use of heat, light, or electricity.
Single-Displacement Reaction: When one element replaces another element in a compound.
A metal will replace any less active metal.
Double-Displacement Reaction: the positive ion of one compound replaces the positive ion of the other to form two new compounds.
A double-displacement reaction takes place if a precipitate, water, or a gas forms when two ionic compounds in solution are combined.
Precipitate: an insoluble compound that comes out of solution during this type of reaction.
One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons.
Oxidation: describes the loss of electrons
Reduction: describes the gain of electrons.
Chemical reactions involving electron transfer of this sort often involve oxygen, which is very reactive, pulling electrons from metallic elements.
The cause and effect of oxidation and reduction can be taken one step further by describing the substances after the electron transfer.
Chemical Reactions—Energy Exchanges
All chemical reactions release or absorb energy.
When most chemical reactions take place, some chemical bonds in the reactants are broken, which requires energy.
Bond formation releases energy.
More Energy Out
Exergonic Reactions: Chemical reactions that release energy.
Exergonic reaction produces visible light.
Exothermic Reaction: When the energy given off in a reaction is primarily in the form of heat.
Exothermic reactions provide most of the power used in homes and industries.
More Energy In
Endergonic Reactions: A chemical reaction that requires more energy to break bonds than is released when new ones are formed.
Electricity is often used to supply energy to endergonic reactions.
Endothermic Reaction: When the energy needed is in the form of heat
Catalyst: a substance that speeds up a chemical reaction without being permanently changed itself.
When you add a catalyst to a reaction, the mass of the product that is formed remains the same, but it will form more rapidly. The catalyst remains unchanged and often is recovered and reused.
Inhibitors: used to slow down a chemical reaction.
One thing to remember when thinking about catalysts and inhibitors is that they do not change the amount of product produced. They only change the rate of production.
