Chapter 8: Chemical Reactions Focus: Use balanced chemical equations

to solve problems

Chemical equation: example (red velvet cake)

NaHCO3(aq) + HC2H3O2(aq) → NaC2H3O2(aq) + H2O(l) +CO2(g)

We read this as: Sodium bicarbonate reacts with acetic acid to produce (or yield or form) sodium acetate, water, and carbon dioxide.

Definitions: Reactant: any species to the left of the arrow (consumed) Product: any species to the right of the arrow (produced) Physical State: Gases, liquids, and solids are labeled with (g), (l), and (s), respectively. Chemical species that are dissolved in water are said to be aqueous and are labeled (aq). Balancing Chemical Equations: Dalton's atomic theory of matter: In a chemical reaction atoms are neither created nor destroyed; rather they are rearranged into different chemical compounds. • Balanced: same number and kind of atoms (elements) on each side of the equation • stoichiometric coefficients: numbers before chemicals

example: methane gas burns in oxygen to produce carbon dioxide and water CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)

1 C atom 1 C atom 4 H atoms → 4 H atoms 4 O atoms 4 O atoms • balanced by adding appropriate stoichiometric coefficients • requires a largely trial-and-error approach

Guidelines for balancing chemical reactions:

DO’s: 1. Change coefficients for compounds before changing coefficients for elements (never change subscripts!) 2. Balance elements other than H and O first 3. Treat polyatomic ions as units rather than individual elements (if they appear on both sides) 4. Use smallest whole-number coefficients DON’Ts: 1. do not change subscripts on atoms in chemical formulas 2. don’t add or subtract reactants or products examples: Balance:

NaOH + H3PO4 → Na3PO4 + H2O

P4O10 + H2O → H3PO4

SO2 + O2 → SO3

(NH4)3PO4 + NaOH → Na3PO4 + NH3 + H2O

C6H14 + O2 → CO2 + H2O

Calculations based on Balanced Chemical Equations: The balanced equation tells us the mole ratio in which chemicals react, and in which they produce products. example: 3 NaOH + H3PO4 → Na3PO4 + 3 H2O from this balanced equation we learn that 3 mol of NaOH reacts with 1 mol H3PO4 to produce 1 mol Na3PO4 and 3 mol water Question: How many moles of Na3PO4 can be made from 0.240 mol of NaOH? 3 NaOH + H3PO4 → Na3PO4 + 3 H2O

example where the input and output are masses: The thermite reaction can be used to create molten iron: (This reaction is commonly used to weld train tracks together) 2 Al (s) + Fe2O3 (s) → Al2O3 (s) + 2 Fe (l) How many grams of Fe2O3 are needed to produce 86.0 g Fe? Also determine how many grams of Al are needed.

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CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g) example: How many grams of water are formed from burning 1.00 kg of methane? Limiting Reactant Calculations: (new concept) Often one or more of the reactants is not completed used Excess reactant: not completely used up after a reaction is complete Limiting reactant: completed consumed during a reaction

(limits the amount of product produced)

example: Balanced reaction:

Limiting Reactant / Excess:

Which reactant is limiting and which is in excess?

example: 2 mol of N2 and 5 mol of H2 react to produce ammonia. N2(g) + 3 H2(g) → 2 NH3(g)

Determine which reactant is limiting and which is in excess?

example: PCl3 is made according to the following reaction:

P(s) + Cl2(g) → PCl3(l)

How much PCl3 is produced if 12.0 g of P and 35.0 g Cl2 are mixed?

Theoretical yield: the maximum amount of product predicted by the balanced equation

Actual yield: the amount produced in the lab The actual yield is usually less than the theoretical yield:

- sticks to glassware - gas evaporates - side reactions

Percent yield: ratio of actual to theoretical yield example: 2 mol of N2 and 5 mol of H2 react to produce 3 mol ammonia. N2(g) + 3 H2(g) → 2 NH3(g)

Determine the percent yield

% yield = actualtheoretical

× 100