Post on 14-Dec-2015
Chemical Equations
Honors Chemistry
Mr. Rivas
Chemical Equations
A chemical reaction is a process in which substances are changed into different substances.
The old substances are called reactants and the new substances are called products.
A chemical equation describes what happens in a chemical reaction.
Example of a chemical change
The reaction: When solid carbon reacts with oxygen gas, a
new substance, carbon dioxide gas is formed and energy is released
The equation:
C(s) + O2(g) CO2(g) Since this is an exothermic reaction we
can also write:
C(s) + O2(g) CO2(g) + heat
Chemical Equation
The chemical equation identifies: The original substances (reactants) and the
resulting substances (products). The phase of each
(s) Solid (l) Liquid (g) Gas (aq) Aqueous (dissolved in water)
The number of moles of each substance
Chemical Equation
The arrow reads yields or produces. It separates the reactants to the left from the products to the right.
Δ or ø means reactants must be heated.
N.R. means no reaction
indicates that the reaction is reversible.
Balancing Chemical Equations
In a chemical reaction the mass of the products must be the same as the mass of the reactants.
Steps: Write the unbalanced equation Balance the equation by making sure that the number
of atom of each element is the same in the products and the reactants.
Hint: Start by balancing an element that appears in only one reactant and one product
Example
Tin oxide is heated with hydrogen to form tin metal and water vapor.
Write the unbalanced equation SnO2(s) + H2(g) Δ Sn(s) + H2O(g)
Identify which elements are not balanced. Here we have two O atoms in the reactants and
only one in the products. We correct this by entering a coefficient of 2 for
water …
Example
This puts the hydrogen atoms out of balance.
SnO2(s) + H2(g) Δ Sn(s) + 2 H2O(g) To fix this we enter a coefficient of 2 for
hydrogen.
SnO2(s) + 2 H2(g) Δ Sn(s) + 2 H2O(g) Remember: coefficients are multipliers. When
we write 2H2O we mean 2x2 = 4 H atoms and 2x1 = 2 O atoms.
Remember…
When balancing a chemical equation you must never change the formulas of the reactants or products.
This means do not change the subscripts of the formulas.
The balancing must be achieved by writing the appropriate coefficient(s).
Balancing Equations Exercises
1) H2 + O2 H2O
2) NaOH + H2SO4 Na2SO4 + H2O
3) Na + O2 Na2O2
4) N2 + O2 N2O5
5) Cs + N2 Cs3N
6) NH3 + HCl NH4Cl
7) Li + H2O LiOH + H2
Answers to balancing equations
1) 2 H2 + O2 2 H2O
2) 2 NaOH + H2SO4 Na2SO4 + 2 H2O
3) 2 Na + O2 Na2O2
4) 2 N2 + 5 O2 2 N2O5
5) 6 Cs + N2 2 Cs3N
6) NH3 + HCl NH4Cl
7) 2 Li + 2 H2O 2 LiOH + H2
Worksheet
Complete Worksheet 9.1