Chemical Reactions and Stoichiometry Ms. Grobsky
Wrapping Up the Types of Chemical Reactions We just got done investigating the different types of chemical reactions We can now answer the two questions that are a central focus in chemistry: When two substances are mixed, will a chemical reaction occur? Yes, if at least one of the four driving forces are present: Transfer of electrons Formation of a gas Formation of a precipitate (solid) Formation of water If a chemical reaction occurs, what will the products be? Products can be predicted based on the type of chemical reaction present: SR, synthesis, decomposition, combustion, double replacement precipitation, double replacement - neutralization
Making the Connection In this unit, you have also learned that chemical reactions follow the Law of Conservation of Matter All atoms present going in MUST COME OUT! This was done by balancing the chemical equation for the reaction using coefficients Coefficients represent number of moles Remember, chemical equations tell us what amounts of reactants to mix and what amounts of products to expect for a particular reaction Kind of like a chemist s recipe Therefore, it is necessary to be able to quantitatively calculate the quantity of reactants needed to make the products in a reaction
So, What is Stoichiometry? When you know the quantity of one substance in a reaction, you can calculate the quantity of any other substance consumed or created in the reaction Quantity meaning the amount of a substance in grams, liters, molecules, or moles This process the calculation of quantities in chemical reactions by looking at a chemical equation - is called stoichiometry Greek for measuring elements
Example N 2 (g) + 3H 2 (g) 2NH 3 (g) This equation can be read as: 1 mole of N 2 reacts with 3 moles of H 2 to produce 2 moles of NH 3 N 2 and H 2 always react in this mole ratio So, if you start with 10 moles of nitrogen gas and 30 moles of hydrogen gas, you will form 20 moles of ammonia
In General Use the coefficients from the balanced chemical equation to make mole ratios Ratios of balanced coefficients = mole ratios Makes connections between reactants and products Using this information, you can calculate the amounts of the reactants involved and the amount of product you might expect Keep in mind that any calculation done with this process is a theoretical value The real world isn t always perfect
Converting From Mole to Mole Practice! The following reaction shows the synthesis of aluminum oxide: 3O 2 (g) + 4Al(s) 2Al 2 O 3 (s) If you wanted to produce 24 moles of product, how many moles of each reactant would you need? Given: 24 moles of Al 2 O 3 Unknown: moles of Al moles of O 2
Need 2 separate equations to solve for the unknowns: 3O 2 (g) + 4Al(s) 2Al 2 O 3 (s) 4 mol Al 24 mol Al 2 O 3 2 mol Al 2 O 3 = 48 mol Al 24 mol Al 2 O 3 2 mol Al 2 O 3 3 mol O 2 = 36 mol O 2
Practice for You 1. How many moles of hydrogen will be produced if 0.44 mol of CaH 2 reacts according to the following equation? CaH 2 + 2H 2 O Ca(OH) 2 + 2H 2 (.89 mol H 2 ) 2. Iron will react with oxygen to produce iron (III) oxide. How many moles of iron (III) oxide will be produced if 0.18 mol of iron reacts? (.090 mol Fe 2 O 3 )
USING STOICHIOMETRY TO CONVERT FROM MASS TO MASS OR OTHER UNITS
A LOOK AT OTHER STOICHIOMETRY CALCULATIONS No lab balance measures moles directly Generally, mass is the unit of choice So, it is more convenient to expand our stoichiometric calculations to include any unit of measure that is related to the mole The given quantity can be expressed in numbers of particles or units of mass! All you need is a balanced chemical equation! Recall, the coefficients in a balanced reaction equation indicates the relative numbers of moles of reactants and products
General Steps to Solving ALL Stoich Problems For any typical stoichiometry problems, follow the steps below: 1. Write and balance the chemical equation 2. Identify the GIVEN and UNKNOWN in the balanced equation 3. Write the GIVEN 4. Convert given quantity to moles 5. Then, use the mole ratio from the balanced equation to convert from the moles of given to the number of moles of the unknown 6. The moles of the unknown are converted to the units that the problem requests
THE STOICHIOMETRY FRAMEWORK STEP 1 Write and balance the chemical equation STEP 2 Identify the GIVEN and UNKNOWN in the balanced equation STEP 3 Write the GIVEN STEP 4 Change the GIVEN to moles STEP 5 Use the MOLE RATIO (coefficient ratio): STEP 6 Change moles of UNKNOWN to grams X X X = ANSWER with units and chemical formula