major classes of reactions if you can classify a reaction into one of five major categories by...

Major Classes of Reactions

• If you can classify a reaction into one of five major categories by recognizing patterns that occur, you already know a lot about the reaction.

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Major Classes of Reactions

• In one type of reaction, two substances—either elements or compounds—combine to form a compound.

• Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction.

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A Synthesis Reaction

• When iron rusts, iron metal and oxygen gas combine to form one new substance, iron(III) oxide.

• The balanced equation for this synthesis reaction shows that there is more than one reactant but only one product.

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A Synthesis Reaction

Chemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsTopic 8Topic 8

Major Classes of Reactions

• In a decomposition reaction, a compound breaks down into two or more simpler substances.

• The compound may break down into individual elements, such as when mercury(II) oxide decomposes into mercury and oxygen.

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Major Classes of Reactions

• The products may be an element and a compound, such as when hydrogen peroxide decomposes into water and oxygen.

• The compound may break down into simpler compounds.

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A Decomposition Reaction

• When ammonium nitrate is heated to a high temperature, it explosively breaks down into dinitrogen monoxide and water.

• The decomposition reaction taking place is represented by a balanced equation that shows one reactant and more than one product.

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A Decomposition Reaction

Chemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsTopic 8Topic 8

Major Classes of Reactions

• In a single-displacement reaction, one element takes the place of another in a compound.

• The element can replace the first part of a compound, or it can replace the last part of a compound.

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Single Displacement

• If an iron nail is placed into an aqueous solution of copper(II) sulfate, the iron displaces the copper ions in solution, and copper metal forms on the nail.

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Single Displacement

Chemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsChemical Reactions and Equations: Basic ConceptsTopic 8Topic 8

Major Classes of Reactions• In double-displacement

reactions, the positive portions of two ionic compounds are interchanged.

• For a double-displacement reaction to take place, at least one of the products must be a precipitate, gas or water.

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Double Displacement

• When clear aqueous solutions of lead(II) nitrate and potassium iodine are mixed, a double-displacement reaction takes place and a yellow solid appears in the mixture.

• This solid is lead(II) iodine, and it precipitates out because it is insoluble in water, unlike the two reactants and the other product.

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Double Displacement

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Reactions in Aqueous Solutions

• Another example of a double-replacement reaction that produces a precipitate occurs when aqueous solutions of sodium chloride and silver nitrate are mixed to form a precipitate of solid silver chloride.

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Reactions that form water or a gas• Some double-replacement reactions in

aqueous solution produce water or a gas (or both) rather than a precipitate.

• In such cases, the water or gas is shown as a product in the net chemical equation, as are the ions that produced it.

• The following example problem illustrates this concept.

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Reactions that form water or a gas

• When hydrochloric acid and potassium hydroxide solutions are mixed, water results, together with an aqueous solution of potassium chloride.

• Write the balanced chemical equation, for this reaction.

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Reactions that form water or a gas

• The balanced chemical equation is the same as the skeleton equation.

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Major Classes of Reactions

• A combustion reaction is one in which a substance rapidly combines with oxygen to form one or more oxides.

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Combustion

• When welding is done with an acetylene torch, acetylene combines with oxygen to form carbon dioxide and water.

• This combustion reaction is exothermic, and enough energy is released to melt metal.

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Combustion

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Basic Assessment QuestionsBasic Assessment Questions

Question 3

Identify each of the following skeleton equations as representing a synthesis reaction, a combustion reaction, both synthesis and combustion, or decomposition reaction. Balance each equation if necessary.

Topic 8Topic 8

Basic Assessment QuestionsBasic Assessment Questions

synthesis;

Answer 3a

Question 3a Topic 8Topic 8

Basic Assessment QuestionsBasic Assessment Questions

decomposition; already balanced

Answer 3b

Question 3b Topic 8Topic 8