- Chemical changes occur via chemical reactions.
- A combustion reaction is a particular type of chemical reaction in which a substance combines with oxygen to form one or more oxygen containing compounds. Combustion also emit heat.
- We represent a chemical reaction with a chemical equation. The substances on the left are reactants and the ones on the right are products.
- When a chemical equation is not balanced, it violates the law of conservation of mass because an atom is not formed out of nothing.
- To correct an unbalanced equation-or write an equation that closely represents what actually happens-we must balance it by changing the coefficients to ensure that the number of each type of atom on the left is equal to the right side. New atoms do not form during a reaction, nor do atoms vanish-matter must be conserved.
- What quantity or quantities must always be the same on both sides of a chemical equation?
- the number of atoms of each kind.
8.4 Mole to mole and mass to mass conversions
- The coefficients in a chemical equation specify the relative amounts in moles of each substance involved in the reactions.
- The numerical relationships between chemical amounts in a balanced chemical equation are called stoichiometry, which allows the prediction of the amount of products that will form in a chemical reaction based on the amount of reactants that react. It also allows is to determine the amount of reactants necessary to form a given amount of product.
- This is essential in knowing the quantity of chemical reactants to obtain products in the desired quantities.
- Think of it as a recipe. The amount of ingredients (cheese, crust, sauce) make a certain amount of pizzas (products).
2 C8H18 (l) + 25 O2 (g) ====> 16 CO2 (g) + 18 H2O (g)
- Making Molecules: Mole to Mole Conversions
- In a balance chemical equation, we have a "recipe" for how reactants combine to form products. From our balanced equation for the combustion of octane for example is the following stoichiometric ratio:
- 2 mol C8H18: 16 mol CO2
- We can use this ratio to determine how many moles of CO2 from when a given number of moles of C8H18 burns.
- Suppose we burn 22.0 mol C8H18, how many moles of CO2 form?
- 22.0 mol C8H18 X (16 mol CO2/2 mol C8H18)= 176 mol CO2
- The combustion of 22 moles of C8H18 adds 176 moles of CO2 to the atmosphere.
- Making Molecules: Mass to Mass Conversions
- This calculation is similar to the Mole to Mole conversions exept that we are now given mass of octane instead of the amount of octane in moles, Consequently we must first convert mass (in grams) to the amount (in moles). (pg. 280)
- Mass A ==> Amount A (in moles) ==> Amount B (in moles) ==> Mass B
- Practice Problems 8.4 and 8.5 (pg. 281 and 282)
Video Overview
Mole to Mole Conversions
Mass to Mass Conversions
Mass to Mass Conversions Part II
8.5 Limiting Reactant, Theoretical Yield, and Percent Yield
- The three most important concepts in reaction stoichiometry: limiting reactant, theoretical yield, and percent yield.
- Limiting Reactant (Reagent):The substance that limits the amount of products that can be made
- Excess Reactant: is any reactant that occurs in a quatity greater than is required to completely react with the limiting reactant.
- Theoretical Yield: The amount of product that can (theoretically) be made based on limiting reactants
- Actual Yield: The amount of product that is (actually) made. Always equal or less than theoretical yield.
- Percent Yield: Ratio percentage. (Actual Yield/Theoretical Yield) X 100
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