1. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

2. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

3. Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

4. A Synthesis Reaction Topic 8
Chemical Reactions and Equations: Basic Concepts
Topic 8
A Synthesis Reaction

5. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 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.

6. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

7. A Decomposition Reaction
Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

8. A Decomposition Reaction
Chemical Reactions and Equations: Basic Concepts
Topic 8
A Decomposition Reaction

9. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 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.

10. Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

11. Single Displacement Topic 8
Chemical Reactions and Equations: Basic Concepts
Topic 8
Single Displacement

12. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 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.

13. Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

14. Double Displacement Topic 8
Chemical Reactions and Equations: Basic Concepts
Topic 8
Double Displacement

15. Reactions in Aqueous Solutions
Chemical Reactions and Equations: Additional Concepts
Topic 8
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.

16. Reactions that form water or a gas
Chemical Reactions and Equations: Additional Concepts
Topic 8
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.

17. Reactions that form water or a gas
Chemical Reactions and Equations: Additional Concepts
Topic 8
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.

18. Reactions that form water or a gas
Chemical Reactions and Equations: Additional Concepts
Topic 8
Reactions that form water or a gas
The balanced chemical equation is the same as the skeleton equation.

19. Major Classes of Reactions
Chemical Reactions and Equations: Basic Concepts
Topic 8
Major Classes of Reactions
A combustion reaction is one in which a substance rapidly combines with oxygen to form one or more oxides.

20. Chemical Reactions and Equations: Basic Concepts
Topic 8
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.

21. Chemical Reactions and Equations: Basic Concepts
Topic 8
Combustion

22. Basic Assessment Questions
Topic 8
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.

23. Basic Assessment Questions
Topic 8
Question 3a
Answer 3a
synthesis;

24. Question 3b Answer 3b decomposition; already balanced Topic 8
Basic Assessment Questions
Topic 8
Question 3b
Answer 3b
decomposition; already balanced