Download Introduction to Chemical Reactions and Equations

ENTRY QUIZ
1.What is chemical reaction?
2.Give an example?
3.What is at the left?
4.What is at the right?
5.What the arrow means?
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th
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ELEMENT
SILICON
Si
Atomic number 14
Atomic number 28.09
Metalloid, widely used in computers
Atomic structure 14 p, 14 e, 14n
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Chemical
Reactions
and
Equations
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Phase Notation
• A chemical equation can also show the physical
state of the materials in the reaction. This is done
with phase notations:
• (s) solid (l) liquid
(g) gas
• (aq) aqueous or dissolved in water
• (ppt) or ( ) means “a precipitate forms”
• ( ) means “a gas bubbles off”
•
means “is heated”
• Phase notations are written to the lower right of
each formula. Clues to phase notations are often
given in equations written in words.
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An example
• When pellets of sodium hydroxide are dropped into
an aqueous solution of iron(III) chloride, iron(III)
hydroxide precipitates and sodium chloride stays
dissolved.
• 3NaOH(s) + FeCl3(aq)
Fe(OH)3 (ppt) + 3NaCl(aq)
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Word Equations: show the names of
the reactants and the products
Reactants

Products
Methane + Oxygen  Carbon Dioxide +
Water
• The arrow (yield sign) and indicates that a
reaction took place.
Skeleton Chemical
Equations
The formulas of the reactants and
products are written without indicating
their relative amounts.
Example:
CH4+ O2 CO2 +H2O
Balanced Chemical
Equations
• The chemical formulas and the relative amounts
of the reactants and products are written.
Example:
CH4 + 2O2 CO2 + 2H2O
• The coefficients indicate the relative amounts of
each substance.
• Equal number of atoms of each element must
appear on both sides of the balanced equation.
Why does a chemical reaction have to
be balanced?
• In any chemical or physical
change, mass is neither created
or destroyed
• Mass is CONSTANT
• Law of Conservation of Mass
(Antoine Lavoisier)
Combustion of Methane
CH4 + 2O2
CO2 +
Atoms are rearranged!
2H2O
Example: Decomposition of
hydrogen peroxide in the presence of
a catalyst.
• Word equation:
hydrogen peroxide  water +
oxygen
Example: Decomposition of
hydrogen peroxide in the presence of
a catalyst.
• Skeleton equation:
H2O2

H 2O
+
O2
Example: Decomposition of
hydrogen peroxide in the presence of
a catalyst.
• Balanced Equation:
2H2O2

2H2O
+
O2
Example: Decomposition of
hydrogen peroxide in the presence of
a catalyst.
Balanced equation showing
• the catalyst (MnO2)
• the state of the reactants and products:
MnO2
2H2O2(l)

2H2O (l)
+
O2 (g)
Activity 4. Calculate
balance at both sides
Balance each decomposition equation first!
1. H2O --- H2 + O2
2. HCl -- H2 + Cl2
3. MgO -- Mg + O2
4. NH3 -- N2 + H2
5. Fe2O3 --- Fe + O2
6. Al2O3 -- Al
+ O2
7. CO2
--- C
+ O2
8. CO
---- C
+ O2
9. NaCl
---- Na + Cl2
10. KCl
---- K
+ Cl2
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EXIT QUIZ
1.What is chemical reaction?
2.What are signs of chemical reaction?
3.How do we describe chemical reaction?
4.Do we have to balance chemical equations?
5.What if we don’t balance equations?
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