1. Chapter 9: Chemical Reactions

2. 9.1 Reactions and Equations

3. Chemical Reactions
The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical reaction
Another name for a chemical change

4. Many reactions provide physical evidence that they have occurred
Temperature change
Release energy in the form of heat and light
Absorb heat
Color change
Odor
Gas bubbles
Formation of a solid

5. Representing Chemical Reactions
Chemists use statements called equations to represent chemical reactions
Reactants are the starting substances
Products are the substances formed during the reaction
Chemical equations show the direction in which the reaction progresses
An arrow, rather than an equal sign, is used to separate the reactants on the left and products on the right
When there are two or more reactants, or when there are two or more products, a plus sign separates each reactant or each product

6. Symbols are used to show the physical states of the reactants and products
Solid (s)
Liquid (l)
Gas (g)
Aqueous : dissolved in water (aq)

7. Word equations Skeleton equations
Full name is used for reactants and products
Ex: iron(s) + chlorine(g)  iron (III) chloride(s)
Skeleton equations
Uses chemical formulas rather than words to identify the reactants and the products
Ex: Fe(s) + Cl2(g)  FeCl2(s)

8. Chemical equations
In a chemical change, matter is neither created nor destroyed
Chemical equations must show that matter is conserved during a reaction
The equation must show equal numbers of atoms of each reactant and each product on both sides of the arrow
Called a balanced chemical equation
A chemical equation is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.

9. Balancing Chemical Equations
To balance an equation, you must find the correct coefficients for the chemical formulas in the skeleton equation
A coefficient is the whole number written in front of a reactant or product
Not written if the value is one
The coefficients in a balanced equation describe the lowest whole number ratio of the amounts of all of the reactants and products

10. Steps for balancing equations
Step 1: Write the skeleton equation for the reaction
Step 2: Count the atoms of the elements in the reactants
If a reaction involves identical polyatomic ions in the reactants and products, count each polyatomic ion as a single element
Step 3: Count the atoms of the elements in the products.
Step 4: Change the coefficients to make the number of atoms of each element equal on both sides of the equation.
Never change a subscript
Step 5: Write the coefficients in their lowest possible ratio
Step 6: Check your work
Make sure that the chemical formulas are written correctly
Check that the number of atoms of each element is equal on both sides of the equation

11. 9.2 Classifying Chemical Reactions

12. Synthesis Reactions
A chemical reaction in which two or more substances react to produce a single product
A+BAB
Two elements can combine to form a compound, or two compounds can combine to form one compound.
Ex: 2Na(s) + Cl2(g)  2NaCl(s)

13. Combustion Reactions
In a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light.
When coal is burned to produce energy, a combustion reaction occurs
C(s) + O2(g)  CO2(g)
Some combustion reactions are also synthesis reactions

14. Decomposition Reactions
A single compound breaks down into two or more elements or new compounds
AB A+B
Often require an energy source such as heat, light or electricity

15. Replacement Reaction
Involve the replacement of an element in a compound
Two types: single, and double replacement
Single-replacement reactions
A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single-replacement reaction
A+BX­­→ AX +B

16. 3 types A metal replaces a hydrogen in a water molecule
A metal replaces another metal in a compound dissolved in water
A specific metal can replace any metal listed below it on the activity series, but cannot replace any metal listed above it
Figure 13
A nonmetal replaces another nonmetal
Halogens frequently involved
A more reactive halogen replaces a less reactive halogen that is part of a compound dissolved in water

17. Double-replacement reactions
Involves an exchange of ions between two compounds
AX+BY→AY+BX
A and B represent cations
X and Y represent anions
The anions change place and bond to the other cation in the reaction
All double-replacement reactions produce either water, a precipitate, or a gas.
A solid produced during a chemical reaction in a solution is called a precipitate

18. 9.3 Reactions in Aqueous Solutions

19. An aqueous solution is a solution in which the solvent is water
A solution contains one or more substances called solutes dissolved in the solvent
The solvent is the most plentiful substance in a solutions
An aqueous solution is a solution in which the solvent is water

20. Aqueous Solutions
Some solutes are molecular compounds while other solutes are ions dissolved in water
When ionic compounds dissolve in water, their ions can separate – a process called dissociation
When two aqueous solutions that contain ions as solutes are combined, the ions may react with one another
Always double-replacement reactions
In these reactions, the water molecules do not usually react

21. Reactions That Form Precipitates
To show the details of reactions that involve ions in aqueous solutions, chemists use ionic equations
The substances that are ions in solution are written as ions in the equation
Ex: 2Na+ (aq) + 2OH- (aq) instead of 2NaOH(aq)
An ionic equation that shows all of the particles in a solution as they exist is called a complete ionic equation
Ions that do not participate in a reaction are called spectator ions
Ions present in the same form as both a reactant and a product
Usually not shown in ionic equations

22. 2OH-(aq) +Cu2+(aq) → Cu(OH)2(s)
Ionic equations that include only the particles that participate in the reaction are called net ionic equations
Written from complete ionic equations by removing all spectator ions
Ex:
2Na+ (aq) + 2OH-(aq) +Cu2+(aq) + 2Cl-(aq) → 2Na+ (aq) +2Cl-(aq) + Cu(OH)2(s)
2OH-(aq) +Cu2+(aq) → Cu(OH)2(s)

23. Reactions That Form Water
One type of double-replacement reaction that occurs in an aqueous solution produces water molecules
No evidence of a chemical reaction is observable
Ex: hydrobromic acid + sodium hydroxide

24. Reactions That Form Gases
Another type of double-replacement reaction that occurs in aqueous solutions results in the formation of a gas
Commonly produced gases are carbon dioxide, hydrogen cyanide, and hydrogen sulfide
The gas bubbles out of the solution

25. Overall Equations
Two reactions can be combined and represented by one chemical equation
An equation that combines two reactions is called an overall equation
To write an overall equation, the reactants in the two reactions are written on the reactant side of the combined equation, and the products of the two reactions are written on the product side
Any substances that are on both sides of the equation are crossed out

26. Ex: Reaction 1 HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
Reaction 2 H2CO3(aq) → H2O(l) + CO2(g)
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Combined HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)
Overall HCl(aq) + NaHCO3(aq) → H2O(l) + CO2(g)