
Summary Reactions and Equations
Some chemical reactions release energy in the form of heat and light, and some absorb energy.
Changes in temperature, color, odor, and physical state are all types of evidence that indicate a chemical reaction has occurred.
Word and skeleton equations provide important information about a chemical reaction, such as the
reactants and products involved in the reaction and their physical states.
A chemical equation gives the identities and relative amounts of the reactants and products that are involved in a chemical reaction. Chemical equations are balanced.
Balancing an equation involves adjusting the coefficients of the chemical formulas in the skeleton
equation until the number of atoms of each element is equal on both sides of the equation.
Classifying Chemical Reactions
Classifying chemical reactions makes them easier to understand, remember, and recognize.
Synthesis, combustion, decomposition, single-replacement, and double-replacement reactions are five classes of chemical reactions.
A synthesis reaction occurs when two substances react to yield a single product. The substances that react can be two elements, a compound and an element, or two compounds.
A combustion reaction occurs when a substance reacts with oxygen, producing heat and light.
A decomposition reaction occurs when a single compound breaks down into two or more elements or new compounds.
A single-replacement reaction occurs when the atoms of one element replace the atoms of another element in a compound.
In single-replacement reactions, a metal may replace hydrogen in water, a metal may replace another metal in a compound dissolved in water, and a non-metal may replace another nonmetal in a compound.
Metals and halogens can be ordered according to their reactivities. These listings, which are called activity series, can be used to predict if single- replacement reactions will occur.
A double-replacement reaction involves the exchange of positive ions between two compounds.
Exercise 1. Study the following mathematical symbols and their meanings:
Symbol |
Name |
Example |
Meaning |
. |
point |
8.5689 |
eight point five six eight nine |
+ |
plus [ples] |
R1+R2 |
R one plus R two |
- |
minus ['mainəs] |
V-V1 |
V minus V1 |
= |
equal sign ['i:kwəl] [sain] |
R=R1+R2 |
R equals/ is equal to R one plus R two |
≠ |
inequality sign [ini'kwɔləti] |
V≠V1+V2 |
V doesn’t equal/ isn’t equal to V one plus V two |
× |
multiplication sign |
f×120 |
f times/ multiplied by one hundred and twenty |
÷ |
division sign |
36÷5=7.2 |
thirty six divided by five equals to seven point two |
° |
degree |
100°C |
hundred degrees celsius |
2 |
square |
R2 |
R squared |
3 |
cube |
R3 |
R cubed |
4 |
power |
104 |
ten to the power four |
4He |
superscript |
4He |
helium four |
H2O |
subscript |
H2O |
H two O |
→ |
arrow |
A+B →AB AB→ A+B |
A and B react to produce/ yield AB AB decomposes into A and B |
Exercise 2. Write the following equations and characters in words:
6×2=12
15÷3=5
3.1415926
7+3≠11
C6H12O6
CH4 +2O2 →CO2 +2H2O
2H2O2(aq) →2H2O(l) +O2(g)
648
42, 63
14C
35°C