CHAPTER 7 Chemical Reactions: An Introduction as expected, you will use this stuff ‘til the day you leave the class!

7.1 Evidence for a Chemical Reaction matter can combine or break apart to produce new kinds of matter w/ different properties = chemical reaction how can you recognize one??? look for clues!

Evidence of Chem Reaction Bubbles form (gas formation) – Alka-Seltzer in water, baking powder/soda in dough; some metals in acid all give off gas = evidence of chm rxn!

Color changes leaves, peeled apple, burned toast; all evidence of chemical rxn

Appearance of a new substance : precipitate forms (e. g Appearance of a new substance : precipitate forms (e.g. solid from two liquids)

Changes in heat and/or light produced/lost– lit candle, burning charcoal, fuel oil on fire; all give off heat/light = chm rxn may take in heat or light (e.g. photosynthesis, baking soda in water)

7.2 Chemical Equations hydrocarbons are a group of cmpds which release E in chm rxns; e.g.… propane + oxygen  carbon dioxide + water + energy reactants  products above is too long, so... C3H8 + O2  CO2 + H2O + energy this is a chemical equation

Reversible rxns! rarely is a rxn one way; often both ways; e.g. H2 + O2  H2O + energy but also… H2O + energy  H2 + O2 so… H2 + O2  H2O + energy

What is it now!? What is it now? law of conservation of mass = in a chemical rxn matter (atoms) are neither created nor destroyed discovered by Antoine Laurent Lavoisier w/ experiments in a closed system

all atoms are all accounted for in typical chm rxn better way of stating it: all atoms are all accounted for in typical chm rxn correct chm eq will obey LawConsMass! e.g. C + O2  CO2

make reactant and product masses equal H2 + O2  H2O what is wrong? To balance equations, use coefficients (front numbers) (never change subscripts) 2H2 + O2  2H2O

coefficients (the numbers in front) will show the proportion of the diff cmpds, e.g.: 2H2 + O2  2H2O 2 hydrogens : 1 oxygen (or) 2 zillion hydrogens : 1 zillion oxygens (or) 2 mol hydrogen : 1 mol oxygen

7.3 Balancing Chemical Reactions when balancing, you may not mess w/ cmpd identities (formulae)! play only w/ coefficients! make sure you have correct chem formula first, THEN balance using coefficients! when counting, multiply atoms in cmpd by coeff coefficients are whole numbers – make sure you use whole numbers!! make sure it is the simplest ratio!! so there!

examples C2H4 burns in oxygen to form carbon dioxide and water C2H4 + O2  CO2 + H2O C2H4 + 3O2  2CO2 + 2H2O sodium and water react to form sodium hydroxide and hydrogen gas Na + H2O  NaOH + H2 2Na + 2H2O  2NaOH + H2

examples magnesium nitride is formed from its elements first, what is magnesium nitride? Mg3N2 Mg + N2  Mg3N2 3Mg + N2  Mg3N2

examples nitrogen monoxide gas reacts with hydrogen gas to form nitrogen gas and water NO + H2  N2 + H2O 2NO + 2H2  N2 + 2H2O

examples lithium metal reacts with chlorine gas to form solid lithium chloride Li + Cl2  LiCl 2Li + Cl2  2LiCl

examples diboron trioxide reacts with water to form boric acid, B(OH)3 B2O3 + H2O  B(OH)3 B2O3 + 3H2O  2B(OH)3

examples solid iron(III) oxide reacts with aqueous nitric acid to form aqueous iron(III) nitrate and water Fe2O3 + HNO3  Fe(NO3)3 + H2O Fe2O3 + 6HNO3  2Fe(NO3)3 + 3H2O

examples solid ammonium nitrite is heated to produce nitrogen gas and water vapor NH4NO2  N2 + H2O NH4NO2  N2 + 2H2O