Chapter 3: Stoichiometry: Calculations With Chemical Formulas and Equations

Chapter 3: Stoichiometry: Calculations With Chemical Formulas and Equations

• atoms are neither created nor destroyed during any chemical reaction
• stoichiometry – quantitative nature of chemical formulas and chemical reactions

3.1 Chemical Equations

• chemical equations – the way chemical reactions are represented
• reactants – starting substances
• products – substances produced from a reaction
• balanced equation – equation with equal atoms on both sides of the equation
• subscripts should never be changed in balancing an equation
• coefficients changes only the amount and not identity of the substance

3.2 Patterns of Chemical Reactivity

3.2.1 Using the Periodic Table

• periodic table can be used to determine reactivity of substances
• all alkali metals react with water to form their hydroxide compounds and hydrogen

3.2.2 Combustion in Air

• rapid reaction that produces a flame
• most combustion reactions in air involve oxygen
• hydrocarbons and related compounds produce CO2 and H2O during combustion

3.2.3 Combination and Decomposition Reactions

• combination reactions two or more substances react to form one product
• decomposition reaction one substance produces two or more substances

3.3 Atomic and Molecular Weights

3.3.1 The Atomic Mass Scale

• atomic mass unit (amu) – unit in measuring mass of atoms
• 1 amu = 1.66054*10-24g and 1 amu = 6.02214*1024amu

3.3.2 Average Atomic Masses

• atomic weight – average atomic mass

3.3.3 Formula and Molecular Weights

• formula weight – sum of the atomic weights of each atom in its chemical formula
• molecular weight – same as formula weight

3.3.4 Percentage composition from Formulas

• ((atoms of element)(AW)/(FW of compound) * 100

3.3.5 The Mole

• avogadro’s number – 6.02*1023 atoms
• molar mass – numerically equal to its formula weight
• grams <use molar mass> moles <use avogadro’s number> molecules

3.5 Empirical Formulas from Analyses

• empirical formula gives relative number of atoms in each element
• mass % elements >>> assume 100g sample >>> grams of each element >>> use atomic weights >>> moles of each element >>> calculate mole ratio >>> empirical formula
• "percent to mass, mass to mol, divide by small, multiply ‘til whole/"

3.5.1 Molecular Formula from Empirical Formula

• the subscripts in the molecular formula of a substance are always a whole-number multiple of the corresponding subscripts in its empirical formula

3.5.2 Combustion Analysis

3.6 Quantitative Information from Balanced Equations

• the coefficients in a balanced chemical equation can be interpreted both as the relative numbers of molecules involved in the reaction and as the relative numbers of moles
• stoichiometrically equivalent quantities
• grams reactant >> moles reactant >> moles product >> grams product
• grams of substance A >> use molar mass of A >> moles of substance A >> use coefficients of A and B from balanced equation >> moles of substance B >> use molar mass of B >> grams of substance B

3.7 Limiting Reactants

• limiting reactant – limits the amount of product formed

3.7.1 Theoretical Yields

• theoretical yield – the amount of product that is calculated to form
• actual yield – the amount of product actually formed