Regular chemistry Unit 4, Lecture notes of Chemistry

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Unit 4: Chemical Reactions

Unit Outline

 Chemical Reactions:  Evidence of a chemical change  Balancing Chemical Equations Types of Chemical Reactions  Synthesis reactions  Combustion reactions  Decomposition reactions  Replacement reactions  Reactions in Aqueous Solutions (Dissolved in water) Precipitate reactions  Complete ionic equations, spectator ions, net ionic equations

Chemical Reaction : The process of atoms rearranging to form different substances.

Sodium + Chlorine  Sodium chloride

Evidence of a chemical reaction: How do you know that a Chemical Reaction has occurred? 

Change in temperature (sometimes these indicate

only a physical change) 

Release of light



Gas produced



Color change



Formation of a solid

Representing Chemical Changes  Reactants : Starting substances  Products : Substances that are formed

Representing Chemical Changes  All (or some) of the reactants are used up during a chemical reaction to produce the new substances.  So the arrow in a chemical reaction shows the direction that the reaction progresses in.  Symbols are also used to identify the physical state of the chemicals involved in the reaction.  (s) (l) (g) (aq)

****Important things to remember!****  Write metals as a singular atom.  Write non-metal gases in their natural pure form, which is as a diatomic molecule.  Non-metal solids are written as a singular atom.

Representing Chemical Changes  Chemical Reactions : Use chemical formulas to show the reactants and the products, and the relative amounts of the substances involved in the reaction. Eg.  In a chemical reaction, a coefficient is the number written in front of the reactant or product. Coefficients are usually whole numbers, and are not usually written when the value is 1.

Balancing Chemical equations: Steps

1. Write the skeleton equation (the correct chemical formulas of the reactants and the products)
2. Count the atoms of each element in the reactants.
3. Count the atoms of each element in the products.
4. Change the COEFFICIENTS to make the number of each element equal on both sides of the equation. ♦NEVER CHANGE THE SUBSCRIPTS.♦
5. Write coefficients in lowest possible ratio.
6. Check number of atoms of each element are equal on both sides of the equation.

 Example 2: Write the balanced chemical equation for the reaction in which aqueous sodium hydroxide and aqueous calcium bromide react to produce solid calcium hydroxide and aqueous sodium bromide.

Types of Chemical Reactions  Knowing the categories of chemical reactions can help you remember them, and more importantly, you will be able to recognize patterns and predict the products of

1. Synthesis Reactions

 Two (or more) substances react to produce a single product.

A + B  AB

 Examples: 2H 2

+ O

2

 2H

2

O 2 SO

2

+ O

2

2 SO

3 2Na + Cl 2  2NaCl

3. Decomposition Reactions

 One compound breaks down into two or more products.  Opposite of synthesis reactions. Need energy to start.

AB A + B

Examples: NH 4

NO

3 (s)  N 2 O(g) + 2H 2 O(g) Textbook p.292 #18-

4. Replacement Reactions (aka displacement)  Replacement of an element in a compound.  (i) Single replacement reactions

A + BX AX + B

 Examples: 2Li(s) + 2H 2 O(l)  2LiOH(aq) + H 2 (g) Cu(s) + 2AgNO 3 (aq)  2Ag(s) + Cu(NO 3

2 F (g) + 2NaBr(aq)  2NaF(aq) + Br (l)