Introduction to Chemistry Chapter 21 Chemical Reaction Introduction to Chemistry Chapter 21

Chemical Changes Section 21-1

Chemical Reactions Atoms re-arrange to form new compounds. Seating chart analogy How do you know a reaction has occurred? Gas is produced. Change in temperature. Precipitate formed. Light given off. Reactants react to form products. Reactants Products.

Conservation of mass Law of Conservation of mass. Antoine Lavoisier Mass of the products will equal the mass of the reactants Discovered by heating Mercury (II) oxide to produce oxygen gas and mercury. Mercury (II) oxide  oxygen plus mercury. 10 g = 0.7g + 9.3g

Letters, Words & Sentences The atomic symbols are like the chemist alphabet. Those “letters” are combined to form words - the chemical formulas. Those words are combined into sentences called chemical equations. Like a sentence, they describe what happened.

Equation Symbols Symbol Meaning  Produces, yields, or forms. + Plus or and (s) solid (l) Liquid as in melted (g) gas (aq) Aqueous, dissolved in water

Coefficients Since mass is conserved, you sometimes need more than one molecule of a compound. To show the number needed, coefficients are placed in front of the compound. 2NaCl would mean two sodium chloride molecules are involved in the reaction.

A reaction . . . Lead (II) nitrate is added to potassium iodide. One molecule of lead (II) nitrate dissolved in water is added to two molecules of potassium iodide dissolved in water to produce solid lead (II) iodide and potassium nitrate dissolved in water. Pb(NO3)2 (aq) + 2KI (aq) PbI2 (s) + 2KNO3 (aq) Lead Nitrate and potassium Iodide

Reactions: Practice What is the solid that is formed? NiCl2 (aq) + 2NaOH (aq) Ni(OH)2 (s) + 2NaCl (aq) What is the solid that is formed? What product is dissolved in the solution? How many of each element are on the reactant side? How many of each element are on the product side?

Practice Answers NiCl2 (aq) + 2NaOH (aq) Ni(OH)2 (s) + 2NaCl (aq) What is the solid that is formed? Nickel (II) hydroxide. What product is dissolved in the solution? Sodium chloride How many of each element are on the reactant side? Ni: 1, Cl: 2, Na:2, O:2, H:2 (or OH:2) How many of each element are on the product side? Ni:1, Cl:2, Na:2, O:2, H:2 (or OH:2)

Chemical Equations Section 21-2

Balancing reactions Why balance? HgO  Hg + O2 2HgO  2Hg + O2 Matter can not be created or destroyed! HgO  Hg + O2 Where did the extra oxygen come from? 2HgO  2Hg + O2 Now there are 2 mercury atoms and 2 oxygen atoms on both sides. This is a balanced reaction. So, why is the oxygen O2 and not the mercury?

Great Question! The Oxygen is diatomic – two atoms. Hydrogen, Nitrogen, Oxygen, Fluorine, chlorine, and Bromine will pair up as gases. Hydrogen and the “upside down L” or “7”. Whenever you see these as gases the formula is X2. Cl2 (g), O2 (g), F2 (g), H2 (g)

Balancing Rules Write the reaction with all the proper formulas. Once you figure out the formula, do not touch the subscripts. Balancing is done with coefficients (numbers in front). Count the number of atoms of each element on both sides. Change one coefficient, recount, and check again. Continue changing coefficients and rechecking until it is balanced. Coefficients must be in the lowest ratio.

Balancing Practice Lithium metal is added to water to produce lithium hydroxide and hydrogen gas. Copper (II) nitrate is added to sodium hydroxide to produce copper (II) hydroxide and sodium nitrate. Do It!

Balancing: Tricks of the Trade Only change one coefficient at a time, and then recheck. If a polyatomic ion appears on both sides keep together. If an atom appears in more than one compound balance it last. If you have oxygen gas (O2) and need an odd number, double everything to need an even number of oxygen.

Classifying Chemical Reactions Section 21-3

Synthesis Two or more substances combine to form one new substance. More reactants than products. A + B  AB 2H2 + O2  2H2O

Decomposition One compound breaks apart in to many. Opposite of synthesis reaction. More products than reactants. AB  A + B H2O2  H2 + O2

Single Displacement Between a compound and an element. The element (usually a metal) replaces another in a compound. A + BC  B + AC Cu + 2AgNO3  Cu(NO3)2 + Ag

Activity Series Metals can be ranked by how reactive they are. List is called activity series. A metal on top of the series will replace a metal lower down. Metals on the bottom of the list are often found uncombined in nature. Gold, Silver and copper are on the bottom.

Making Predictions Series can be used to predict if a reaction will occur. Mg + Pb(NO3)2 ? Cu + FeCl3  ? Cu + AgNO3  ?

Double Displacement Between two compounds. Positive ions exchange places. AB + CD  AD + CB AgNO3 + NaCl  AgCl + NaNO3

Combustion Combining a substance with oxygen to produce carbon dioxide and water. Combustion of Methane: CH4 + O2  CO2 + H2O CH4 + 2O2  CO2 + 2H2O

Oxidation and Reduction In a reaction, there is a transfer of electrons from one compound to another. Reduction is the gain of electrons. Charge is reduced (becomes more negative) Oxidation is the loss of electrons. OIL RIG Oxidation Is Loss, Reduction Is Gain Oxidation and reduction occur together Redox reactions

Chemical Reactions and Energy Section 21-4

Activation Energy – The Hill A reaction is like a rock at the bottom of a hill. Push it to the top and it will roll down the other side on its own. Pushing it to the “top” of the hill is the activation energy. Some hills are larger than others. Bond breaking requires energy. Bond making releases energy. Ethanol cannon

Energy Out Bond making release more energy than the bond breaking requires. Making > Breaking. Called exergonic reactions Excess energy is released as heat or light. If heat is released, called exothermic reaction. Exo = Out Therm = Heat Feels hot to the touch.

Energy In Bond breaking requires more energy than bond making provides. Breaking > Making Called endergonic reactions Additional energy must be provided to the reaction. Light (photosynthesis), Electricity (hydrolysis of water) If the reaction absorbs heat, it is called an endothermic reaction. Endo = In Therm = Heat Feels cold to the touch. Barium hydroxide and ammonium chloride rxn.

Energy Diagram - Exothermic

Energy Diagram - Endothermic

Reaction Rates To react, molecules must collide with enough energy. Concentration If the molecules are tighter packed, greater chance of collision. Surface Area Reactions occur at surfaces – more surface, more reactions. Temperature The lower the temperature, the slower the molecules – less energy.

Catalysts and Inhibitors Catalysts speed up a reaction. Enzymes in body Inhibitors slow down the reaction. BHT and BHA prevent spoilage. Nitroglycerine and dynamite. A catalyst / inhibitor does not react. Demos: Hydrogen peroxide and MnO /