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Unit 4 ~ Chemical Reactions (Chapters 6, 7, 18) And you 4-1 Evidence for Chemical Reactions (Section 6.1) Color change gas Energy change Precipitate 4-2 Writing and Balancing Chemical Equations (Sections 6.2, 6.3) • Chemical equations express in symbols the qualitative experiences in real life and allow scientists to calculate quantitative relationships between the chemical species. • Like How??? • Word Expression: • Sodium solid is added to liquid water, producing aqueous sodium hydroxide and hydrogen gas. • Chemical Equation: • Na(s) + H2O(l) → NaOH(aq) + H2(g) • But notice that the reaction is as unbalanced as Mr. Wozniak Na(s) + H2O(l) → NaOH(aq) + H2(g) • • • • Balanced” Chemical Equation: 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) Phase symbols: (s) = solid (l) = liquid (g) = gas (aq) = aqueous or dissolved in water • Other symbols: • → means “yields” or “produces” • ∆ means “heat added” Note: • chemicals on the left side of the → are the reactants • while the right side are the products Balancing Equations: • Equations must be balanced to satisfy the Law of Conservation of Matter. In other words, there must be equal numbers of atoms of all elements on both sides of the arrow which, in turn, will necessitate equal mass on both sides of the →. • To “balance” an equation, you may only change the coefficients in front of the chemicals, NEVER the subscript (this would change the substance!). Example: KCl → K + Cl2 • Acceptable: 2KCl → 2K + Cl2 • Unacceptable: KCl2 → K + Cl2 Useful HINTS for balancing include: • 1) Use the concept of least common multiple. • 2) Start with elements that appear only once on both sides of the →. • 3) even numbers can not be changed into odd numbers with a whole number coefficient but odd numbers can be changed into even numbers!!! Practice for you!!! • • • • ____KClO 2 2 + ____O 3 2 3 → ____KCl Change odd to even with those oxygen atoms! Balance those oxygen atoms Now balance those K and Cl atoms! Try the next one please 3 • ____C3H8 + ____O 5 4 2 → ____CO2 + ____H 2O And another please • ____C 2 5H10 + ____O 15 2 → ____CO 10 10 2 + ____H2O • When balancing a hydrocarbon reaction, it can be beneficial to start with a coefficient 2 in front of the hydrocarbon – then reduce at the end if needed.