Chapter 7Chemical Reactions

The Nature of Chemical Reactions

• Chemical reactions occur when substances go through chemical changes to form new substances

• Reactant is a substance that participates in a chemical reaction

• Product is a substance that forms in a chemical reaction

The Nature of Chemical Reactions

• Chemical reactions always involve changes in energy– Energy must be added to break bonds– Forming bonds releases energy

• Energy is conserved in chemical reactions– Energy in a chemical reaction can change

form, energy is never created or destroyed

The Nature of Chemical Reactions

• Reactions that release energy are exothermic– Products have less energy than the reactants

• Reactions that absorb energy are endothermic– Products have more energy

Cold Pack

• Cold Packs 

A cold pack comes in a plastic bag made of tough white plastic. This bag is filled with a smaller bag and ammonium nitrate crystals. The smaller bag contains water, and is made of a thin weak plastic, so it is easy to break. When a cold pack is used, it must be "broken" by rupturing the inner bag. Breaking the bag releases the water, which dissolves the ammonium nitrate. The water and ammonium nitrate react completely, and within fifteen to twenty the cold pack will no longer feel cold. 

Cold Pack

• Cold packs make use of the heat transfer that occurs during chemical reactions, but in contrast to heat packs, utilize endothermic reactions. In the endothermic reaction between ammonium nitrate crystals and water, the heat required for the reaction to proceed from reactants to products is absorbed from the surrounding environment, resulting in a decrease in temperature of the pack noticeable to the touch. This reaction, equation (2) 

NH4NO3(s) + H2O(l) + heat → NH4+(aq) + NO3-(aq) (2)

Hand Warmers

• Air activated hand warmers contain cellulose, iron, water, activated carbon (evenly distributes heat), vermiculite (water reservoir) and salt (catalyst) and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours.

• Hand warmers work because of a rusting process. The rusting is a redox reaction and the equation is as follows: 4Fe(s) + 3O2(g) -> 2Fe2O3(s) .

Chemical Equations

• Uses symbols to represent a chemical reaction and shows the relationship between the reactants and products of a reaction

• Reactants yield Products

• CH4 + O2 CO2 + H2O

• Not Balanced?????

Chemical Equations

• Balanced chemical equations account for the conservation of mass

• CH4 + 2O2 CO2 + 2H2O

Chemical Equations

• A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction

• Equation: 2Mg + O2 2MgO• Moles: 2 + 1 2• Molar mass(g/mol): 24.3 + 32.0(16x2) 40.3• Mass calculations:• 24.3g/molx2mol + 32.0g/molx1mol • 40.3g/molx2mol • Mass(g): 48.6g + 32.0g 80.6g

Chemical Equations

• Mole ratios tell you the relative amounts of reactants and products

• Mole ratios can be converted to masses

Reaction Types

• Synthesis reactions combine substances

• A + B AB

• 2Na + Cl2 2NaCl

• Metal sodium reacts with chlorine gas to form sodium chloride (table salt)

Reaction Types

• Decomposition reactions break substances apart

• AB A + B

• 2H2O 2H2 + O2

• Electrolysis of water breaks down water into hydrogen gas and oxygen gas

Reaction Types

• Combustion reactions use oxygen as a reactant• CH4 + 2O2 CO2 + 2H2O

• When there is not enough oxygen during a combustion reaction, not all fuels are converted completely into carbon dioxide. In this case some carbon monoxide (CO) may form

• CO is a poisonous gas that lowers the ability of the blood to carry oxygen– No odor

Single Replacement

Reaction Types

• In single-displacement reactions, elements trade places

• AX + B BX + A

• 3CuCl2 + 2Al 2AlCl3 + 3Cu

Reaction Types

• Copper chloride dissolves in water to make a bright blue solution. If you add a piece of aluminum foil to the solution, the color goes away and clumps of reddish brown material form. The reddish brown clumps are copper metal. Aluminum replaces copper in the copper chloride to form aluminum chloride. Aluminum chloride does not make a colored solution, so the blue color goes away.

• Alkali metals undergo single-displacement reactions

Reaction Types

• In double-displacement reactions, ions appear to be exchanged between compounds

• AX + BY AY + BX

• Pb(NO3)2 + K2CrO4 PbCrO4 + 2KNO3

Reaction Types

• The yellow lines painted on roads are colored with lead chromate (PbCrO4). This compound can be formed by mixing solutions of lead nitrate, Pb(NO3)2, and potassium chromate, K2CrO4. In solution, these compounds form the ions Pb2+, NO3

-, K+, and CrO4-2. The lead ions and

chromate ions are more attracted to one another than they are to water molecules around them. Therefore, when the solutions are mixed, a yellow lead chromate compound forms, and settles to the bottom of the container. The lead and chromate ions form a compound. Potassium and nitrate ions are soluble together in water, so they do not form a compound. Instead, they stay in solution just as the lead and nitrate ions were before the reaction

Reaction Types

• Free radical is an atom or a group of atoms that has one unpaired electron– H+

– OH-

– SO42+

Reaction Types

• Electrons are transferred in redox reactions• Reactions in which one substance loses electrons

and another substance gains electrons are called oxidation-reduction reactions– Formation of rust:

• In the presence of water, atoms of iron metal, Fe, react with O2, to form rust, Fe2O3. Each iron atom loses three electrons to form Fe3+ ions, and each oxygen atom gains two electrons to form O-2 ions

Reaction Types

• Substances that accept electrons are said to be reduced (algebra)

• Substances that give (lose) up electrons are said to be oxidized (leo)

Reaction Types

• Some redox reactions do not involve ions

• Oxidation can be the gain of oxygen or the loss of hydrogen

• Reduction can be the loss of oxygen or the gain of hydrogen

Reaction Types

• Oxidation does not necessarily require oxygen. But they still use the name oxidation to describe any reaction in which an atom loses electrons

Reaction Rates and Equilibrium

• Anything that increases contact between particles will increase the rate of a reaction– Temperature– Surface area– Concentration– Pressure– Size and shape of reactant molecules

Reaction Rates and Equilibrium

• Catalysts speeds up or slows down a reaction but is not changed by the reaction– Substances that slow reactions are called

inhibitors– Enzymes are biological catalysts

Reaction Rates and Equilibrium

• Some processes may go in both directions which results in an equilibrium system

• Carbonated drink:• decrease• pressure• • CO2(gas above liquid) CO2(gas dissolved in liquid)• • increase• pressure

Reaction Rates and Equilibrium

• Equilibrium results when rates balance