CHM102 Lecture Notes - Lecture 7: Limiting Reagent, Magnesium, Boron Carbide
Balancing Chemical Reactions
- Any calculation involving a reaction MUST be based on a balanced reaction.
- Need to adjust the coefficients preceding molecular or atomic formulas to adjust
the number of atoms in reactants and products to be equal.
Helpful steps:
1. Write 'skeleton' equation of just reactants and products
2. Indicate state of compounds using: (g), (s), (l), or (aq)
3. Balance the equation, by trying coefficients to make balance (numbers in front of
formulas, Never subscripts within formulas)
4. Adjust coefficients to get lowest whole number ratios (usually)
Masses of Reactants and Products: Stoichiometry
- The calculation of amounts and masses of reactants and products in chemical
reactions is called stoichiometry.
- Stoichiometry is based upon the fact that matter is always conserved in chemical
reactions
- use to determine how much of reactants are used up, or products are produced
(or both!)
1) Must begin with balanced equation!!! Check to make sure all equations are balanced
before doing anything else.
2) get moles of given compound- must do calculations in terms of moles!!
3) relate moles of given compound to moles of desired compound
4) calculate grams if necessary
Ex: B12C3 (boron carbide - abrasive hardness 9.3, diamond=10) is prepared from the
reaction of B2O3 and carbon in an electric furnace. CO is a by product. What is the
minimum weight of carbon needed to prepare 150.g B12C3? (F.W. B12C3 =
165.72g/mol; B2O3= 69.6g/mol).
1) balance equation 6 B2O3 + 21 C → B12C3 + 18 CO
2) moles of B12C3 150.g (1mole/165.72g) = 0.905 moles B12C3
