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Chem 177 Ch.3 Ouline

Chem 177 Ch.3 Ouline - Chapter 3 Stoichiometry Calculations...

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Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations 1 of 6 3.1 Chemical Reaction Equations: Chemical reaction equations are just the symbolic representation of chemical reactions. Reactants Products Example combustion reaction (See section 3.2) – burning often reaction with O 2 . Combustion of C, H, and O containing compounds always lead to the formation of CO 2 and H 2 O as shown below: CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (l) Sometimes, rxn conditions are shown on the arrow: pressure heat - temperature catalyst – a species that increases rxn rate (but not consumed during the reaction) Chemical equations are quantitative. Remember the law of conservation of mass. 1. Conservation of mass: mass products = mass reactants 2. Conservation of charge: total charge on products = total charge on reactants 3. # of atoms of each type on left side = # of atoms of each type on right side Thus equations NEED to be balanced - See: “Guidelines for Balancing Chemical Equations.pdf” For example, CH 4 (g) + O 2 (g) CO 2 (g) + H 2 O (l) Count number of atoms of each type on each side. 1 C, 4 H, 2 O 1 C, 2 H, 3 O Thus, C atoms are balanced. Need to choose next atom to balance. O exists in 2 compounds on the right side of equation, so skip for now. Balance H, by adding a 2 in front of H 2 O. CH 4 (g) + O 2 (g) CO 2 (g) + 2 H 2 O (l) C and H now in balance, check O. 2 on left side; 4 on right. Don’t want to change the coefficient in front of CH 4 , CO 2 , or H 2 O (otherwise would unbalance C and H). So want to add a coefficient to O 2 . To get 4 O on left side, put 2 in front of O 2 : CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) Equation is now balanced and all coefficients integral. Double check: Left side: 1 C, 4 H, 4 O Right side: 1 C, 4 H, 4 O

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Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations 2 of 6 Think in terms of particles. Reaction equation refers to individual molecules, atoms, ions, units. See Sample Exercise 3.1 (page 80). 3.2 Pattern of reactivity (Types of reactions): i) Combination rxns –usually metal and nonmetals are involved –2 or more substances combine to form a single product. Consider the corrosion reaction which involves iron and oxygen Fe + O 2 What product, assume Fe reacts to form Fe 3+ . Fe + O 2 Fe 2 O 3 Balance: 2 Fe (s) + O 2 (g) Fe 2 O 3 (s) Multiply by 2 on every component in the reaction, we get… 4 Fe (s) + 3 O 2 (g) 2 Fe 2 O 3 (s) ii) Decomposition reactions – single reactant reacts to form multiple products. 2 NaN 3 (s) 2 Na (s) + 3 N 2 (g) NaN 3 : sodium azide, unstable compound.
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