Chapter 3Stoichiometry: Calculations with Chemical Formulas and Equations CHEMISTRY The Central Science 9th Edition
Chemical Changes • Chemical properties describes the reaction a substance undergo to form new substances. • The study of chemical changes is at the heart of chemistry. • Some chemical changes or simple and some or complex. • For example, changes that occur in your brain and eyes allow you to see and think. • In this chapter, chemical changes will be used to explore the quantity of substances consumed and produced in a chemical reaction. • Chemical Equations are used to describe chemical reactions.
Defining Chemistry • Lavoisier: mass is conserved in a chemical reaction. • His careful measurements turned chemistry into a science (Father of Chemistry) • Investigate the reaction of hydrogen with oxygen to produce water (2H2 + O2 2H2O).
Chemical Equations • The chemical equation for the formation of water can be visualized as two hydrogen molecules reacting with one oxygen molecule to form two water molecules: • 2H2 + O2 2H2O Products Reactants
Reading Chemical Equations • The plus sign (+) means “react” and the arrow points towards the substance produce in the reaction. • The chemical formulas on the right side of the equation are called reactants and after the arrow are called product. • The numbers in front of the formulas are called stoichiometric coefficients. • 2Na + 2H2O 2NaOH + H2 • Stoichiometric coefficients: numbers in front of the chemical formulas; give ratio of reactants and products. Coefficient Reactants Products
Understanding Chemical Equations Coefficients and subscripts included in the chemical formula have different effects on the composition.
Balancing Chemical Equations • Law of conservation of mass: matter cannot be lost in any chemical reactions. O
Class Practice Problem • Balance the following equations: • (a) Na(s) + H2O(l) NaOH(aq) + H2(g) • (b) Al(s) + HCl(aq) AlCl3(aq) + H2(g) • (c) C2H4(g) + O2(g) CO2(g) + H2O(l)
In Combination reactions two or more substances react to form products: 2Mg(s) + O2(g) 2MgO(s) The Mg has combined with O2 to form MgO (ionic compounds). Decomposition reactions is when one substance undergoes a reaction to produce two or more substances: 2NaN3(s) 2Na(s) + 3N2(g) (the reaction that occurs in an air bag) The NaN3 has decomposed into Na and N2 gas. Combination and Decomposition Reactions
Patterns in Chemical Reactivity • The periodic table can be used to predict how elements will react in a combination reaction: • 2K(s) + 2H2O(l) 2KOH(aq) + H2(g) • All alkali metals will react with water to form the hydroxide compound and hydrogen. • Thus, if let M represent the alkali metal, we able to write: • 2M(s) + 2H2O(l) 2MOH(aq) + H2(g) • Alkali metal + water Metal hydroxide + hydrogen
Formula and Molecular Weights Formula weights (FW) is the sum of the atomic weights of each atom in the chemical formula. FW (H2SO4) = 2AW(H) + AW(S) + 4AW(O) = 2(1.0 amu) + (32.0 amu) + 4(16.0) = 98.0 amu If the chemical formula is also its molecular formula then the weight is called the molecular weight (MW). MW(C6H12O6) = 6(12.0 amu) + 12(1.0 amu) + 6(16.0 amu) Formula Weights
Formula Weights • Percentage Composition from Formulas • Percent composition is the atomic weight for each element divided by the formula weight of the compound multiplied by 100:
Class Practice Problem • Calculate the FW of C12H22O11. • Calculate the percent composition of H2O. • 12C = 12 x 12.01 = 144.12 • 22H = 22 x 1.01 = 22.22 • 11O = 11 x 16.00 = 176.0 • C12H22O11 = 342.34 amu • % H = 11.21% • % O = 88.79%
Molar Mass • Molar mass: mass in grams of 1 mole of substance (units g/mol, g.mol-1). • Experimentally, 1 mole of 12C = 12 g, which can be written as 12g/mol.
The Mole • The unit we use to express the quantity of atoms, ions, and molecules that an object contains is called mole. • Mole: convenient measure chemical quantities. • The actual number of atoms, ions, or molecules in 1 mole of something = 6.0221367 1023(Advogadro’s number) of that thing. • Thus, • 1 mole of 12C atoms = 6.02 x 102312C atoms • 1 mole of H2O molecules = 6.02 x 1023 molecules • 1 mole of NO3- ions = 6.02 x 1023 ions
Visualizing The Mole Concept Different Units
Class Practice Problem • How many C atoms are in 0.350 mol of C6H12O6? • C atoms = [0.350 mol C6H12O6 (6.02 x 1023 molecules/1 mole C6H12O6)(6 C atoms/1 molecule)] = 1.26 x 1024 C atoms
Class Practice Problem • Converting moles to mass • Calculate the number of moles of glucose C6H12O6 in 5.380 g of C6H12O6. • Moles of C6H12O6 = [5.380 g C6H12O6 (1 mole C6H12O6/ 180 g C6H12O6)] = 0.02989 mol C6H12O6
Class Practice Problem • Converting mass to particles • Calculate the number of atoms of Cuin 3 g of Cu? • Atoms of Cu = [3 g Cu (1 mole Cu/180 g Cu)(6.02 x 1023 atoms Cu/1 mole Cu)] = 3 x 1023Cu atoms
Empirical Formulas from Analyses • Start with mass % of elements (i.e. empirical data) and calculate a formula, or • Start with the formula and calculate the mass % elements. • For example: • Ascorbic acid contains 40.92 percent C, 4.58 percent H, and 54.50 percent O by mass. What is the empirical Formula? The experimentally determined molecular weight is 176 amu.
Empirical Formulas from Analyses • Molecular Formula from Empirical Formula • Once we know the empirical formula, we need the MW to find the molecular formula. • Example 3.14, page 98 • Subscripts in the molecular formula are always whole-number multiples of subscripts in the empirical formula
Quantitative Information from Balanced Equations • Balanced chemical equation gives number of molecules that react to form products. • Interpretation: ratio of number of moles of reactant required to give the ratio of number of moles of product. • These ratios are called stoichiometric ratios. • NB: Stoichiometric ratios are ideal proportions • Real ratios of reactants and products in the laboratory need to be measured (in grams and converted to moles).
Limiting Reactants • If the reactants are not present in stoichiometric amounts, at end of reaction some reactants are still present (in excess). • Limiting Reactant: one reactant that is consumed
Limiting Reactants • Theoretical Yields • The amount of product predicted from stoichiometry taking into account limiting reagents is called the theoretical yield. • The percent yield relates the actual yield (amount of material recovered in the laboratory) to the theoretical yield:
The End of Chapter 3The test will cover Chapters 1-3, Scheduled for September 28, 2005Chemistry Feud will be held on Monday, September 26, 2005Homework: 3.9, 3.11, 3.15, 3.17, 3.19, 3.21, 3.25, 3.27, 3.31, 3.33, 3.43, 3.47