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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:

  1. 6×2=12

  2. 15÷3=5

  3. 3.1415926

  4. 7+3≠11

  5. C6H12O6

  6. CH4 +2O2 →CO2 +2H2O

  7. 2H2O2(aq) →2H2O(l) +O2(g)

  8. 648

  9. 42, 63

  10. 14C

  11. 35°C

11

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