Chapter 5 Chemical Reactions

5.1 The Nature of Chemical Reactions Chemical reactions happen not only in the laboratory. They happen all around us & in us everyday, all day. When things grow, ripen, decay or burn a chemical reaction is taking place.

Chemical reactions change the substance Chemical reactions take place when substances undergo chemical changes & form new substances. See fig. 5.1 on p. 148 Production of gas & a change in color indicate a chemical change. Sugar, water, yeast + flour make bread. CO 2 is produced causing the bread to rise. Old bonds break & new bonds are formed.

Chemical reactions rearrange atoms! When gasoline is burned in a car’s engine, chemical reactions are taking place: C 8 H 18 (isooctane) and O 2 are the reactants. CO 2 and H 2 O are the products. Reactants are the substances that undergo a chemical change. – [isooctane and oxygen] Products are the new substances which result from that chemical change. – [carbon dioxide and water]

Chemical Reactions The products contain the same kinds and numbers of atoms as the reactants. These same atoms are merely rearranged as a result of chemical bonds which have occurred.

Energy & reactions Energy is always needed to start a chemical reaction. This energy is needed to break the bonds that are holding the reactants together. This energy can come in the form of heat, a spark, electricity, sound, or light. A spark is needed to start the reaction to start a cars engine. (No gas station)

Forming bonds releases energy After energy initiates a chemical reaction & bonds are broken, energy is released when new bonds are formed. See fig. 5.4 on p. 150 Energy is always conserved in a chemical reaction.

Forming bonds releases energy The energy of an explosion was stored in the reactants as chemical energy. Chemical energy: the energy stored in atoms & molecules. This energy can be released when a substance reacts.

Released energy/absorbed energy Exothermic reaction: is a reaction that transfers energy from the reactants to the surroundings, usually as heat. (Heat is released.) The reaction releases energy to the surrounding. Endothermic reaction: a reaction in which energy is transferred to the reactants usually as heat from the surroundings. (Heat is absorbed.) The reaction takes energy from the surroundings.

Endothermic reactions When an endothermic reaction occurs, there is normally a drop in temperature. Endothermic reactions are used in cold packs for strain and sprain injuries. See fig. 5.5 on p. 152

5.2 Types of Reactions

Synthesis Reactions Synthesis: a reaction of at least 2 substances that form one new, more complex substance. A + B => AB 2Na + Cl 2 => 2NaCl i.e. sodium and chlorine gas combine to produce table salt

Decomposition reactions Decomposition: a reaction in which one compound decomposes into at least two new products. AB => A + B 2H 2 0 => 2H 2 + O 2 i.e. water decomposes into hydrogen gas and oxygen gas.

Combustion reactions Combustion: a reaction in which a compound and oxygen burn. If there is not sufficient oxygen when a carbon containing fuel is burned, carbon monoxide gas, CO (poisonous gas), will be produced instead of carbon dioxide gas, CO 2. See p.156

Single replacement reaction Single replacement: a reaction in which atoms of one element replace atoms of another element in a compound. AB + C => AC + B 3 CuCl Al => 2 AlCl Cu See fig on p. 157 H 2 O + K => KOH + H 2 i.e. water and potassium react to produce potassium hydroxide and hydrogen gas

Double replacement reaction Double replacement: a reaction when two compounds appear to exchange ions. AB + CD >> AD + BC Pb(NO 3 ) 2 + K 2 CrO 4 >> PbCrO 4 + 2KNO 3 lead nitrate reacts with potassium chromate to produce lead chromate and potassium nitrate.

Oxidation/Reduction Reactions Redox Redox Reactions substances are reduced if they gain electron(s) substances are oxidized if they lose electron(s) If one substance is reduced, the other substance will be oxidize. Both occur simultaneously.

Redox reactions Some redox reactions do not involve ions. Oxidation is a gaining of oxygen or loss of hydrogen Reduction is a loss of oxygen or a gaining of hydrogen. Respiration & combustion are examples of redox reactions

Radicals have electrons available for bonding When a covalent bond is broken such that at least 1 unpaired electron is left on each fragment of the molecule, these fragments are called radicals. Hydrogen has 1 electron available for bonding so hydrogen is a radical. Radicals react quickly to form covalent bonds. Polymerization reactions can occur when radicals are present producing synthetic fibers, plastic bags & wraps. Radicals are also formed when coal & oil are produced.

5.3 Balancing Chemical Equations Chemical equation: an equation that uses chemical formulas & symbols to show the reactants & products in a chemical reaction. A chemical equation describes how atoms are rearranged to make new substances. Reactants (what is going to undergo a change) are found on the left side of the arrow. Products (what the atoms turn into) are found on the right of the arrow. The arrow means “yields” or “produces” or “gives” or “makes”

Balanced equations = conservation of mass When the # of atoms of each element on the right side equals the # atoms on the left side, the equation is balanced. The carbons on the left have to = the carbons on the right but do NOT have to = anything else in the equation. A balanced chemical equation follows the law of conservation of mass.

How to balance chemical equations Formulas must remain in tact. Subscripts can NOT be changed, moved or added. They must stay exactly as they are. Subscripts tell us how many atoms of the element that immediately precedes it. If parentheses, then the subscript multiplies everything inside the parentheses. Coefficients can be added & put in front of any formula. The coefficient multiplies all the atoms after it until you reach the arrow or a “+” sign.

Determining mole ratios Coefficients tell how many moles of each substance are involved in the reaction. 2 Mg + O 2 >> 2 MgO can be read as 2 moles of Mg gets together w/1 mole of O 2 ( the 1 is understood if no coefficient is present) to produce 2 moles of MgO. In a balanced chemical equation, the total mass of the reactants is = to the total mass of the products.

The law of definite proportions If you use twice as much magnesium, then twice as much oxygen must also be used. This demonstrates the law of conservation of mass as well as the law of definite proportions. Law of definite proportions: a compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed.

Mole ratios can be derived from balanced equations Mole ratio: the smallest relative number of moles of the substance involved in a chemical reaction.

5.4 Rates of Change Chemical reactions can occur at different speeds. Fast: explosion of nitroglycerin Slow: rust forming Reaction rates can be altered.

Factors affecting reaction rates A potato slice takes 5:00 to fry in 200C A potato slice takes 10:00 to cook in boiling water (100C) Sliced potatoes take 10:00 to cook in boiling water. A whole potato takes 30:00 to cook in boiling water.

Most reactions go faster at higher temperatures Particles move higher temperatures. The faster motion increases the energy & this increases the # of collisions. The more collisions, the greater chances for the particles to react. This speeds up the reaction.

Increased surface area speeds the reaction Surface area is the part of a substance that can be seen. If you cut up a potato, you can see more of the potato (you can see the inside of the potato) therefore you have increased the surface area. More area of the potato is exposed to the hot liquid. This speeds up the cooking. This is also true for most chemical reactants. Large surface area means that more particles come in contact w/the other reactants.

Concentrated solutions react faster The more particles that are present, the faster the reaction. More particles = greater concentration. Ex.: a little bleach in a washing machine vs a lot of bleach in a washing machine.

Reactions are higher pressures A gas at high pressure is more concentrated than a gas at low pressure. High pressure = high concentration = greater collisions = faster reaction.

Massive, bulky molecules react more slowly Massive molecules move more slowly at the same temperature than smaller molecules (Kinetic Theory) Slower movement = less collisions = slower chemical reaction.

Catalysts change rate of reactions Catalyst: a substance that changes the rate of a reaction but itself is not consumed by the reaction. Catalysts are not reactants or products. Most catalysts speed up a reaction. Catalysts that slow a reaction are called inhibitors. Catalysts can be expensive but still “worth it” because they can be re-used.

Enzymes are biological catalysts Enzymes: proteins that are catalysts for chemical reactions in living things. Enzymes are fragile; work only at certain temperatures.

Equilibrium Systems In most reactions, all or nearly all reactants are turned into products whether the reaction be fast (nitroglycerin) or slow (rust) Anything that might be leftover is insignificant.

Some changes are reversible Not all reactions go to completion, some are reversible. Burning wood is NOT reversible. When you see 2 arrows pointing in opposite directions, this indicates that the reaction is reversible. See p. 173

Equilibrium results when rates balance. Equilibrium: the state wherein a chemical reaction & its reverse happen at the same time & same rate. Ex.: CO 2 in a closed bottle of soda. CO 2 is mixed in the drink and in the air in the bottle. These CO 2 molecules are continually changing places. When the top is removed from the bottle, equilibrium no longer exists and the CO 2 escapes into the room.