Chapter 7 - The Chemical Reaction

1. Chemical Equations

• Chemical Equation - the representation of a chemical reaction using the symbols of elements and the formulas of compound.
• Reactants - the original reacting species, shown to the left of the arrow.
• Products - the species formed as a result of a reaction, shown to the right of the arrow.
• The reaction below is read as:

H₂ + O₂ ® H₂O

hydrogen reacts with oxygen to produce (yield) water

2. LAW OF CONSERVATION OF MASS - mass can be neither created nor destroyed.
   both sides of arrow must have the same mass - same # of atoms

• Balanced equation - has the same number and type of atoms on both sides of the equation.
• Coefficients - whole numbers in front of the symbols or formulas (one is omitted).

2H2 + 1O2 ® 2H2O

 

Remember you CANNOT change the molecules present (subscripts).

 

STEPS for Balancing

1. Count the number of atoms of each element on both sides of the equation
2. Balance all other atoms except hydrogen and oxygen
3. Generally, it is easiest to do hydrogen next followed by oxygen
4. Check to see if atoms are balanced and that coefficients are lowest possible whole number.

3. Types of Chemical Reactions

• Combustion Reactions - reaction of an element or compound with elemental oxygen (O2). ("burning")

Examples:

C(s) + O₂(g) ® CO₂(g)

4Al(s) + 3O₂(g) ® 2Al₂O₃(s)

If Just Carbon and Hydrogen present Produce CO₂ and H₂O.

• Combination Reactions - preparation of one compound from two or more elements or simple compounds.

2Mg(s) + O2(g) ® 2MgO(s)
2Na(s) + Cl2(g) ® 2NaCl(s)
CaO(s) + CO2(g) ® CaCO3(s)

• Decomposition Reactions - One compound forms two or more compounds or elements.

H2CO3(aq) ® H2O(l) + CO2(g)
2HgO(s) ® CaO(s) + CO2(g)

• Single-replacement reactions - one free element substitutes for another in a chemical compound.

Zn(s) + CuCl2(aq) ® ZnCl2(aq) + Cu(s)

• Double-replacement reactions - two ions exchange partners. Several possible outcomes:
  Set up a table of ions:

AgNO3(aq) + NaCl(aq) ® ?

1. Formation of a compound insoluble in water(ppt) - Precipitation reaction

AgNO₃(aq) + NaCl(aq) ® AgCl(s) + NaNO₃(aq)

2. Formation of water - neutralization reaction.
   An Acid Reacts with a base
   Acid - a compound that produces H+ ions in solution
   Base - a compound that produces OH- ions in solution

HCl(aq) + NaOH(aq) ® H₂O(l) + NaCl(aq)

3. Formation of a gas

Acid reacting with a salt.

HCl(aq) + NaCN(aq) ® HCN(g) + NaCl(aq)

Net Ionic Equations