Hess law and Rate of Reaction (copy)
6.4 Collision Theory and Rate of Reaction
Collision Theory
-chemical reactions occur only when enough energy (kinetic energy) is used to break old chemical bonds and to form new chemical bonds.
-particles are in motion and the motion increases as the temperature increase. Therefore their kinetic energy also increases.
-chemical reactions must involve the collision of reactant particles
-effective collisions have enough energy and have the correct orientation
-ineffective collisions result in the particles rebounding unchanged
-the rate of reaction depends on the frequency of collisions and the fraction of collisions that are effective.
rate = frequency of collision x fraction of collisions that are effective
From this theory we can see that:
-increasing the concentration or surface area of the reactants will result in an increased frequency of collisions
-adding a catalyst or changing the nature of reactants may decrease the energy requirements for a reaction, thus increasing the fraction of effective collisions
-while increasing the temperature will increase the frequency of collisions and increase the energy that the reactants have. (Temp. has the greatest effect on a reaction)
Representing the Progress of a Chemical Reaction
-potential energy diagrams can be used to illustrate the change of chemical reactions
starting with reactants on the left and products on the right.
Activation Energy (EA)
-the minimum potential energy that the reactants must have in order to form the product.
-is determined by analyzing the reaction rate at different temperatures
-ex. High reaction rate at room temperature→low EA
low reaction rate at room temperature→high EA
-this is true regardless of the reaction being exo- or endothermic
Activation Energy for Reversible Reactions
-many reactions are reversible. The activation energy of the forward reaction is different
from the reverse reactions as seen on a potential energy diagram. See p. 368
Activated Complex
-an unstable temporary chemical species that forms at the time of collision
-consists of atoms with partially broken and partially formed bonds. This causes the
instability since it can either revert back to reactants or move forward to form products.
Reaction Mechanisms
-one particle reactants may: - collide with something in order to decompose
-ex. another particle or the container walls
-absorb light energy to break bonds
-for reactions that involve 3 or more particles, statistically the chance of such collisions is low. Therefore scientist believe that reactions occur in a sequence of elementary steps. The overall sequence is called the reaction mechanism. In this reaction mechanism there may be one step that is slower than the rest of the steps and is called the rate determining step. Since here are a series of steps, there are some reaction intermediates produced, but get consumed as the reaction proceeds.
Since the reaction intermediates and activate complexes are very short lived, the reaction mechanism are only Abest guesses@. There are rules however for proposing reaction mechanisms:
-each elementary step must contain no more than 3 reactants (and usually only 2 )
-the slowest step must be consistent with the overall average rate equation
-ex. the coefficients of the rate determining step must be the exponents of the rate law.
-elementary steps must add up to the overall equation (using Hess’ Law)
6.1 Rate of Reaction
Some reactions such as the conversion of graphite to diamond take years or millions of years to complete. Others, such as the light reaction in your eyes take 10-15s. What accounts for these differences is studied. These studies may lead to improvements in medications, energy production, food processing and pollution controls.
Chemical Kinetics -is the study of ways to make chemical reactions go faster or slower.
-involved in the development of theories and models to explain and predict reaction rates.
Rate of Reaction -a change in an amount of substance per unit time
average reaction rate = change in concentration
elapsed time
r = Dc c = mol/L (c = cf –ci)
Dt t = seconds or minutes
The term average is used since over the course of reaction the rate slows as the reactants get used up.
Measuring Rate of Reaction
-the rate with respect to the concentration of reactants is negative since they get used up.
-the rate with respect to the concentration of products is positive since they are formed.
-the rate of using reactants and forming products are related through the coefficients of a balance equation.
***When concentration of a substance vs. time of reaction is plotted on a graph a curved line is obtained.***
Average Rate of Reaction
-the change in concentration of a substance over a period of time of reaction
Instantaneous Rate of Reaction
-the rate of a reaction at a particular point in time.
-on a graph this can be obtained by drawing a tangent at this point and finding the slope
-an alternative method (mid-point time): the average rate of reaction between two time
points is the instantaneous rate half way between those two points
Methods of Measuring Rate
There are many methods of measuring rate, however the method chosen depends on the reaction itself. An ideal method is direct and does not affect the reaction itself.
Reactions that Produce a Gas -measure the volume of the gas produced by: i. the downward displacement of water in a graduated cylinder over time. Or ii. Collection in an empty syringe. In both these methods the atmospheric pressure and the temperature (in K) must be known. (PV = nRT)
-alternatively the mass of the nongaseous compounds could be measured as the gas escapes
Reactions that Involve Ions -the electrical conductivity of a solution will change as the concentration of ions changes.
Reactions that Change Colour -use a spectrophotometer to measure the change in colour intensity.
-either a coloured reactant is consumed or a coloured product is formed. The amount of coloured light that passes through the sample will change.
Reactions that Involve Acids and Bases -measure changes in pH using a pH meter
-pH is a measurement of the conc. of H+ ions
UNIT D: ENERGY CHANGES AND RATES OF REACTION
Chpt. 5: Energy Changes
5.1 The Nature of Energy and Heat
Thermal Energy -the sum of kinetic energy that results from the motion of molecules
Chemical System -the substance undergoing the change (either physical or chemical change)
Surroundings -the systems environment which is anything that absorbs or loses thermal energy from or to the system.
Heat (Q) -energy transferred between substances
Exothermic -energy transferred to the surroundings
Endothermic -energy transferred from the surroundings
Temperature -a measure of the average kinetic energy of the molecules
Open System -energy and matter can flow out of a system and into the surroundings
ex. to the beaker, air, the surface the beaker sits on
Closed System -energy can move out but matter can not
Isolated System -neither energy nor matter can move out of a system
Measuring Energy Changes: Calorimetry
Calorimetry -a technique used to measure the amount of energy transferred.
-the amount of heat transferred can be calculated using the following equation:
Q = m c DT m -mass of object (g)
DT -temperature change of object (0C)
c - specific heat capacity of the object (J/g 0C)
specific heat capacity (c) -the quantity of heat required to raise the temperature of one gram of a substance by 1 0C. (Law of Conservation of Energy: quantity of heat given off when cooling.)
-units J
g 0C
Heat Transfer & Enthalpy Change
Kinetic Energy -moving electrons within atoms
-vibration of atoms connected by chemical bonds
-rotation and translation of molecules
Potential Energy -nuclear potential energy of protons and neutrons in the nucleus
-electric potential energy of atoms connected by chemical bonds.
It is hard to measure all the energies of a system but it is possible to measure the change in energy or enthalpy change (DH) of a change by measuring the energy change of the surroundings.
DHsystem = +/- |Q surroundings|
Law of Thermodynamics
First Law of Thermodynamics -Energy is neither created nor destroyed, only converted from
one form to another. Esystem = -Esurroundings. This also implies
that the amount of energy in the universe is constant, Euniverse = 0.
Second Law of Thermodynamics -when two objects are in thermal contact, heat is always
transferred from the higher temperature to the object of lower energy.
Comparing Categories of Enthalpy Changes
Any process is accompanied by the change in enthalpy of the system. The three processes we will consider are: Physical Changes, Chemical Changes and Nuclear Changes.
I. Physical Changes -changes in a substance that do not affect the chemical properties of
the substance. Ex. dissolving and change of state.
-the approximate range in change in enthalpy is 0.44 – 41 kJ/mol
Enthalpy of Solution ( Hsol) -energy changes associate with a solute dissolving in a
solvent. Energy is required to separate solute particles, and to
separate solvent particles. Energy is released when the solvent and
solute particles bond to each other.
Exothermic -when more energy is released by the solute -solvent bond
formation than what was required to separate the particles.
Hsol is negative
Endothermic -when more energy is required to separate the solute-solute
and solvent-solvent particles than what is released when solute
bonds to solvent particles.
Hsol is positive
Enthalpy of Phase Changes -energy is required to go from solid to liquid, Hmelt =+
-energy is required to go from a liquid to a gas, Hvap =+
-the reverse processes are exothermic (Hfre and Hcond)
II. Chemical Changes -all chemical reactions are associated with a change in enthalpy
-the approximate range in change in enthalpy is 200-900 kJ/mol of
reactant or product.
III. Nuclear Changes -some atoms spontaneously emit particles ( from their nuclei
and are transformed into other elements. There is a change in total mass, this change in
mass is converted to energy as seen by E = mc2. The range in energy is 1 x 108 – 2 x 1010 kJ/mol
Alpha Decay ( -a nucleus emits an alpha particle is a helium nucleus and energy
-the amount of energy released is 5.64 x 108 kJ/mol
Beta Decay ( -a neutron releases a β particle (electron) and becomes a proton.
-the amount of energy released is 1.13 kJ/mol
Two other classes of nuclear reactions are Nuclear Fission and Nuclear Fusion
Nuclear Fission -a nucleus can be split into smaller nuclei. The change in mass releases
large quantities of energy.
-the energy released is 2 x 1010 kJ/mol of uranium but also produces
nuclear waste that emits radiation for centuries.
Nuclear Fusion -where two smaller nuclei combine to form a larger nucleus.
-this the reaction that occurs on the sun
-the amount of energy released is 1.7 x 109 kJ/mol and has no nuclear
waste.
5.2 T 5hermochemical Equations and Calorimetry
In chemical reactions, energy is required to break bonds, and energy is released when bonds
form.
Molar Enthalpy -enthalpy change per mole of a substance
-DHx is the symbol, where x can represent any physical or chemical change
Thermochemical equations show the reaction of the system and the enthalpy change.
Endothermic Reactions
-energy is absorbed by the system from the surroundings
H2O (l) + 285.8 KJ ® H2(g) + O2(g)
Or
H2O (l) ® H2(g) + O2(g) DH = +285.8 KJ
Exothermic Reactions
-energy is released by the system to the surroundings:
H2(g) + O2(g) ® H2O (l) + 285.8 KJ
Or
H2(g) + O2(g) ® H2O (l) DH = - 285.8 KJ
Note: each reaction was written using two different methods
A third method of describing thermochemical equations is to use Molar Enthalpy of Reaction
-these reactions are indicated by DHx where x describes the type of change (phys. or chem.) that 1 mole of a substance undergoes. (See table 1 for examples)
-these reactions start and end with SATP conditions (25 0C and 100 kPa)
ex. Includes energy to reach reaction temperature and cool to 25 0C
Ex. Combustion of hydrogen
H2(g) + O2(g) ® H2O (l) DH0 com = - 285.8 KJ/mol H2
A fourth method of describing thermochemical changes is to use Potential Energy diagrams
-chemical potential energy is related to the bond strength. The breaking of bonds and the reforming of bonds and changing atom arrangement results in different potential energies.
-the diagrams visually show the change in potential energy
-the vertical axis has potential energy (Ep)
-since reactions are written left to right, the horizontal axis describes the reaction progress
Calorimetry
-measuring the energy change of a system
-a simple calorimeter is used (an insulated container)
-assumptions made: -DHsys = D |qsurroundings|
-no energy is transferred between the calorimeter and the outside
-negligible heat absorbed by the calorimeter
-dilute aqueous solutions have the same density and specific heat capacity as of pure water. ( Dw 1.00 g/mL and cw = 4.18 J/g0C)