Chapter 5: Chemical Reactions

Symbols Used in Chemical Equations

--> = reaction arrow Δ = heat (written over the arrow) (s) = solid (l) = liquid (g) = gas (aq) = aqueous solution (reactant dissolved in water)

mole ratios

-1 mol N2/1 mol O2 = 2 reactants N2 and O2 -1 mol N2/2 mol NO = reactant-product N2 and NO -1 mol O2/2 mol NO = reactant-product O2 and NO

balanced chemical equation

also tells us the # of moles of each reactant that combine and the # of moles of each product formed.

Avogadro's Number

the definition of a mole is based on the # of atoms contained in exactly 12g of the carbon-12 isotope, the # is called Avogado's #, named after the Italian scientist Amadeo Avogadro, who first proposed the concept of a mole in the 19th century. -1 mole, abbreviated as mol, always contains an Avogadro's # of particles. -1 mole of C atoms = 6.02 x 10^23 C atoms -1 mole of H2O molecules = 6.02x10^23 H2O molecules -1 mole of vitamin C molecules = 6.02x10^23 vitamin C molecules conversion factor= 1 mol/6.02x10^23 atoms

energy diagram

the energy changes in a reaction are often illustrated on an energy diagram, which plots energy on the vertical axis, and the progress of the reaction on the horizontal axis. -the reactants are written on the left side and the products on the right side, and a smooth curve that illustrates how energy changes with time connects them. -the products are assumed to be lower in energy than the reactants. -http://image.wistatutor.com/content/feed/tvcs/chapter-2-7.png

coefficients

the number written in front of any formula; coefficients show the number of molecules of a given element or compound that react or are formed. -when no number precedes a formula, the coefficient is assumed to be "1". -when a formula contains a subscripts, multiply its coefficient by the subscript to give the total number of atoms of a given type in that formula. -ex. 2 O2 = 4 O atoms' 2 H2O = 4 H atoms + 2 O atoms -coefficients are used because all chemical reactions follow a fundamental principle of nature. -used to balance an equation, making the # of atoms of each elements the same on both sides of the equation. -the coefficients in a balanced chemical equation tell us the # of molecules of each compound that react or are formed in a given reaction. -are used to form mole ratios, which can serve as conversion factors. -in a chemical equations tell us the ratio of the # of molecules or moles that are involved in a chemical reaction.

reaction coordinate

the progress of the reaction.

conversion factors

to multiply 2 #s in scientific notation, multiply the coefficients together and add the exponents in the powers of 10. -(3.0 x 10^5) x (2.0 x 10^2) = 6.0 x 10^7 ∙add exponents (5+2); multiply coefficients (3.0x2.0) -to divide 2 #s in scientific notation, divide the coefficients and subtract the exponents in the powers of 10. -6.0 x 10^2/2.0 x 10^20 = 3.0 x 10^-18 ∙divide coefficients (6.0/2.0); subtract coefficients (2-20) -grams of reactant-(molar mass conversion factor)-->moles of reactant-(mole-mole conversion factor)-->moles of product-(molar mass conversion factor)-->grams of product.

mole-mole conversion factor

use the mole ratio from the coefficients in the balanced equation to convert the # of moles of 1 compound (A) into the # of moles of another compound (B).

equilibrium

when the rate of the forward reaction equals the rate of the reverse reaction, the net concentrations of all species do not change, and the system is at equilibrium. -the forward and reverse reactions do not stop once equilibrium has been reached. -the net concentrations of all reactants and products do not change.

transition state

when the reactants approach each other, their electron clouds feel some repulsion, causing an increase in energy until a maximum value is reached. -in this state, the bond between A and B is partially broken and the bond between B and C is partially formed.

normal body temperature for humans

37°C

chemical reactions

a chemical change-chemical reaction-converts one substance into another; heart of chemistry; involve breaking bonds in the starting materials, called reactants, and forming new bonds in the products. -ex. combustion of methane (CH4) in the presence of oxygen, forms CO2 and H2O. -may be accompanied by a visible change: 2 colorless reactants can form a colored product; a gas may be given off; 2 liquid reactants may yield a solid product, sometimes heat is produced, or a characteristic visible change. -when molecules come together and react, bonds are broken in the reactants and new bonds are formed in the products. -breaking a bond requires energy, while forming a bond releases energy.

oxidizing agent

a compound that gains electrons (is reduced) while causing another compound to be oxidized. -ex. Cu^2+=oxidizing agent, since it causes Zn to lose electrons and become oxidized.

reducing agent

a compound that loses electrons (is oxidized) while causing another compound to be reduced. -ex. Zn=reducing agent, since it causes CU^2+ to gain electrons and become reduced.

law of conservation of matter

atoms cannot be created or destroyed in a chemical reaction.

reversible reaction

can occur in either direction, from reactants to products or from products to reactants; reactants can come together and form products, and products can come together to re-form reactants. -forward and reverse reaction double arrows

mole

defines a quantity, much like a dozen items mean s 12, the only difference is that mole is much larger; quantity that contains 6.02 x 10^23 items, usually atoms, molecules, or ions. -1 mole, abbreviated as mol, always contains an Avogadro's # of particles.

Le Châtelier's Principle

general rule used to explain the effect of a change in reaction conditions on equilibrium; if a chemical system is at equilibrium is disturbed or stressed, the system will react in the direction that counteracts the disturbance or relieves the stress. -added reactant drives the equilibrium to the right. -added product drives the equilibrium to the left. -removing water from the reaction drives the equilibrium to the right; a decrease in concentration of 1 product results in more of the forward reaction to form more product. -adding reactant=equilibrium favors the products. -removing reactant=equilibrium favors the reactants. -adding product=equilibrium favors the reactants. -removing product=equilibrium favors the products.

endothermic

heat is absorbed; ΔH is positive; the products are higher in energy than the reactants. -when energy is absorbed, the reaction is said to be endothermic and ΔH is positive (+). -when ΔH is positive, more energy is needed to break bonds than is released in forming bonds. The reactants are lower in energy than the products. -ex. 6 CO2(g) + 6 H2O(l) --> C6H12O6(aq) + 6 O2(g); ΔH = +678 kcal/mol <--(heat is absorbed) -in this reaction energy is absorbed, ΔH is positive (+), and the reaction is endothermic. Since energy is absorbed, the products are higher in energy than the reactants.

exothermic reaction

heat is released; ΔH is negative; the products are lower in energy than the reactants. -when energy is released, the reaction is said to be exothermic and ΔH is negative (-). -when ΔH is negative, more energy is released in forming bonds than is needed to break bonds. The products are lower in energy than the reactants. -ex. CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(l); ΔH = -213 kcal/mol <--(heat is released) -in this reaction energy is released, ΔH is negative (-), and the reaction is exothermic. Since energy is released, the products are lower in energy than the reactants. -when the products are lower in energy than the reactants, ΔH is negative and the reaction is exothermic.

reaction rate

how fast the reaction occurs. -increasing the concentration of the reactants increases the # of collisions, so the reaction rate increases. -increasing the temperature increases the reaction rate; increasing the temperature increases the kinetic energy, which increases the # of collisions

half reactions

individual reactions written to emphasis which electrons are gained and lost. -ex. oxidation half reaction: Zn --> Zn^2+(+ 2e^-) [loss of electrons=oxidation] -reduction half reaction: Cu^2+(+ 2e^-) [gain of electrons=reduction] -->Cu

thermoregulation

involves the brain and the circulatory system. -an infection in the body is often accompanied by a fever; that is, the temperature in the body increases. A fever is part of the body's response to increase the rates of defensive reactions that kill bacteria. The respiratory rate and heart rate increase to supply more oxygen needed for faster reactions.

redox reaction

involves the transfer of electrons from 1 element to another. -always has 2 components: 1 that is oxidized and 1 that is reduced. -ex. Zn + Cu ^2+ --> Zn^2+ + Cu -Zn loses 2 electrons to form Zn^2+, so Zn is oxidized; Cu^2+ gains 2 electrons to form Cu metal, so Cu^2+ is reduced. -ex. 2 Li + I2 --> 2 LiI; I2 gains 2 e^-, forming 2 I^-; I2 is reduced; each Li atom loses 1 e^-, forming Li^+; Li metal is oxidized. -oxidation: 2 Li --> 2 Li^+ +2 e^- -reduction: I2 + 2 e^- --> 2 I^-

catalyst

is a substance that speeds up the rate of a reaction. A catalyst is recovered unchanged in a reaction, and it does not appear in the product. -accelerate a reaction by lowering the energy of activation. -have no effect on the energies of the reactants and products. Thus, the addition of a catalyst lowers Ea but does not affect ΔH. -http://duch.sd57.bc.ca/~rmcleod/Chemists_Corner/PE_Diagram_Multi_files/Catalyst-2.gif

chemical equation

is an expression that uses chemical formulas and other symbols to illustrate what reactants constitute the starting materials in a reaction and what products are formed. -written with reactants on the left and the products on the right, separated by a horizontal arrow-a reaction arrow-that points from the reactants to the products. -ex. CH4+2O2(reactants)-->CO2+2H2O(products)

atomic weight

is the average mass of an element, reported in atomic mass units (amu). -use amu to calculate the mass of a compound.

reduction

is the gain of electrons by an atom. -results in the loss of oxygen atoms or the gain of hydrogen atoms. -ex. Cl2 + 2e^- --> 2Cl^- -O2 + 4e^- --> 2O^2- -Cu^2+ + 2e^- --> Cu -Ag^+ + e^- --> Ag

oxidation reactions

is the loss of electrons from an atom. -results in the gain of oxygen atoms or the loss of hydrogen atoms. -ex. Na-->Na^+ + e^- -Mg-->Mg^2+ + 2e^- -2Cl^- --> Cl2 + 2e^- -2O^2- --> O2 + 4e^-

molar mass

is the mass of 1 mole of any substance, reported in g/mol. -used to determine how many atoms or molecules are contained in a given mass. -the value of the molar mass of an element in the periodic table (in g/mol) is the same as the value of its atomic weight (in amu). -ex. carbon's atomic weight is 12.01 amu; carbon's molar mass is 12.01 g/mol; 1 mole of carbon atoms weighs 12.01 g. -in the value of the molar mass of a compound in grams equals the value of its formula weight in amu. -a very useful quantity because it relates the # of moles to the # of grams of a substance. -ex. since the molar mass of H2O is 18.02 g/mol = 18.02 g H2O/1 mol

energy of activation (energy barrier)

is the minimum amount of energy needed for a reaction to occur; the difference in energy between the reactants and the transition state; symbolized by Ea. -fundamental characteristic of a reaction. -the energy of activation is often called the energy barrier that must be crossed. -the height of the energy barrier, the magnitude of the energy of activation, determines the reaction rate. -when the energy of activation is high, few molecules have enough energy to cross the energy barrier and the reaction is slow. -when the energy of activation is low, many molecules have enough energy to cross the energy barrier and the reaction is fast. -the difference between the reactants and the products is the ΔH.

formula weight

is the sum of the atomic weights of all the atoms in a compound, reported in amu. -used for both ionic and covalent compounds. -ex. formula weight of ionic NaCl, sodium chloride, is 58.44 amu, which is determined by adding up the atomic weight of Na (22.99 amu) and Cl (35.45 amu). -formula weight of NaCl = 58.44 amu: atomic weight of 1 Na = 22.99 amu, atomic weight of 1 Cl = 35.45 amu.

molecular weight

is used in place of formula weight for covalent compounds, since they are composed of molecules, not ions.

heat of reaction (enthalpy change)

measures the difference between the energy needed to break bonds in the reactants and the energy released from the bonds formed in the products. In other words, ΔH indicates the relative strength of bonds broken and formed in a reaction; the energy absorbed or released in any reaction; ΔH. -is given a positive (+) or negative (-) sign depending on whether energy is absorbed or released.

completion

reactants that are completely converted to products through reactions.

oxidation-reduction reactions

reactions that involves electron transfer. -a common type of chemical reaction involves the transfer of electrons from 1 element to another. -oxidation and reduction are opposite processes, and both occur together in a single reaction called an oxidation-reduction or redox reaction. -when considering neutral atoms, metals lose electrons and nonmetals gain electrons. -when considering ions, cations tend to gain electrons and anions tend to lose electrons.