Question

11. Which of the following is a skeleton reaction?
A) CH2 +02 → CO2 +H,0
B) Fe,0, +2 A1 → 2Fe + Alo
c) AgNO, + NaCl → AgCl + NaNO,
D) Ca(OH)2 +CO,> CaCO, + H2O
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Answer 1

Key Takeaways

•    A chemical equation is a concise description of a chemical reaction.
•    Proper chemical equations are balanced.

Write and balance the chemical equation for each given chemical reaction.

   Hydrogen and chlorine react to make HCl.

   Ethane, C2H6, reacts with oxygen to make carbon dioxide and water.

Solution

   Let us start by simply writing a chemical equation in terms of the formulas of the substances, remembering that both elemental hydrogen and chlorine are diatomic:

   H2 + Cl2 → HCl

   There are two hydrogen atoms and two chlorine atoms in the reactants and one of each atom in the product. We can fix this by including the coefficient 2 on the product side:

   H2 + Cl2 → 2HCl

   Now there are two hydrogen atoms and two chlorine atoms on both sides of the chemical equation, so it is balanced.

   Start by writing the chemical equation in terms of the substances involved:

   C2H6 + O2 → CO2 + H2O

   We have two carbon atoms on the left, so we need two carbon dioxide molecules on the product side, so that each side has two carbon atoms; that element is balanced. We have six hydrogen atoms in the reactants, so we need six hydrogen atoms in the products. We can get this by having three water molecules:

   C2H6 + O2 → 2CO2 + 3H2O

   Now we have seven oxygen atoms in the products (four from the CO2 and three from the H2O). That means we need seven oxygen atoms in the reactants. However, because oxygen is a diatomic molecule, we can only get an even number of oxygen atoms at a time. We can achieve this by multiplying the other coefficients by 2:

   2C2H6 + O2 → 4CO2 + 6H2O

   By multiplying everything else by 2, we don’t unbalance the other elements, and we now get an even number of oxygen atoms in the product—14. We can get 14 oxygen atoms on the reactant side by having 7 oxygen molecules:

   2C2H6 + 7O2 → 4CO2 + 6H2O

   As a check, recount everything to determine that each side has the same number of atoms of each element. This chemical equation is now balanced.

Test Yourself

Write and balance the chemical equation that represents nitrogen and hydrogen reacting to produce ammonia, NH3.

Answer

N2 + 3H2 → 2NH3

Many chemical equations also include phase labels for the substances: (s) for solid, (ℓ) for liquid, (g) for gas, and (aq) for aqueous (i.e., dissolved in water). Special conditions, such as temperature, may also be listed above the arrow. For example,

2NaHCO3(s) −→−200°C Na2CO3(s)+CO2(g)+H2O(ℓ)