Chemical Reactions - General Chemistry - Lecture Slides, Slides of Chemistry

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Chemical Reactions:

An Introduction

Chemical Reactions

• Reactions involve chemical changes in

matter resulting in new substances

• Reactions involve rearrangement and

exchange of atoms to produce newmolecules– Elements are not transmuted during a reaction

Reactants



Products

Figure 6.1: Bubbles ofhydrogen and oxygen gasform when an electriccurrent is used todecompose water

Figure 6.2: Hot and cold pack reactions

Figure 6.3 (b):Chemicalreactions

Figure 6.3 (c):Chemicalreactions

Chemical Equations

• Shorthand way of describing a reaction• Provides information about the reaction
  • Formulas of reactants and products– States of reactants and products– Relative numbers of reactant and product

molecules that are required

• Can be used to determine weights of reactants

used and of products that can be made

Figure 6.4: Thereaction betweenmethane andoxygen to givewater and carbondioxide

Combustion of Methane

-^

methane gas burns to produce carbon dioxide gasand liquid water– whenever something burns it combines with O

(g) 2

CH

(g) + O 4

(g)^2

CO

(g) + H 2

O( 2

l )

H

H

C

H

H

O

O

O^ C O

O

H

H

1 C + 4 H

2 O

1 C + 2 O

+ 2 H + O

1 C + 2 H + 3 O

Combustion of Methane

Balanced

-^

to show the reaction obeys the Law ofConservation of Mass it must be

balanced

CH

(g) + 4

^2

O

(g) 2

CO

(g) + 2

^2

H

O( 2

l )

H

H

C

H

H

O

O

O^ C O

O

H

H

O

O

O

H

H

1 C +

4 H +

4 O

1 C + 4 H

+ 4 O

Symbols Used in Equations

• symbols used after chemical formula to

indicate state– (g) = gas; (

l ) = liquid; (s) = solid

• (aq) = aqueous, dissolved in water

Balancing by Inspection

Count atoms of each element• polyatomic ions may be counted as one “element” if it

does not change in the reaction

Al + FeSO

4

Al

(SO 2

3

+ Fe

SO

4

• if an element appears in more than one compound on

the same side, count each separately and add

CO + O

2

CO

2

O

Examples

-^

when magnesium metal burns in air it produces awhite, powdery compound magnesium oxide– burning in air means reacting with O

2

write the equation in words– identify the state of each chemical

magnesium(s) + oxygen(g)

magnesium oxide(s)

write the equation in formulas– identify diatomic elements– identify polyatomic ions– determine formulas

Mg(s) + O

(g) 2

MgO(s)

Examples

-^

when magnesium metal burns in air it produces awhite, powdery compound magnesium oxide– burning in air means reacting with O

2

count the number of atoms on each side– count polyatomic groups as one “element” if on both sides– split count of element if in more than one compound on

one side

Mg(s) + O

(g) 2

MgO(s)

Mg

O