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Chem 2 Chap 13

QuestionAnswer
Kinetics is the study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds.
There are 4 factors that influence the speed of a reaction nature of the reactants, temperature, catalysts, concentration.
Defining rate is how much a quantity changes in a given period of time.
The speed you drive your car is an example of rate – the distance your car travels (miles) in a given period of time (1 hour). So the rate of your car has units of mi/hr
What is the equation for speed? the change in distance(d)/the change in time(t).
The rate of a chemical reaction is generally measured in terms of how much the concentration of a reactant decreases in a given period of time or product concentration increases. For reactants, a NEGATIVE SIGN is placed in front of the definition.
As time goes on, the rate of a reaction generally slows down . Because the concentration of the reactants decreases. At some time the reaction stops, either because the reactants run out or because the system has reached equilibrium.
H2 (g) + I2 (g) ® 2 HI(g) For these reactions, the change in the number of molecules of one substance is a multiple of the change in the number of molecules of another. For the above reaction, for every 1 mole of H2 used, 1 mole of I2 will also be used and 2 moles of HI made. THEREFORE, the rate of change will be DIFFERENT!
The average rate is the change in measured concentrations in any particular time period. Linear approximation of a curve. The larger the time interval, the more the average rate deviates from the instantaneous rate.
The instantaneous rate is the change in ________ at any one particular time slope Is the CHANGE IN CONCENTRATION at any one particular time. slope at one point of a curve.
How is the instantaneous rate determined? Determined by taking the slope of a line tangent to the curve at that particular point.
In order to measure the reaction rate you need to be able to measure the concentration of at least one component in the mixture at many points in time. There are two ways of approaching this problem (1) for reactions that are complete in less than 1 hour, it is best to use CONTINUOUS MONITOURING of the concentration, or (2) for reactions that happen over a very long time, sampling of the mixture at various times can be used.
Continuous monitoring, polarimetry – measuring the change in the degree of rotation of plane-polarized light caused by one of the components over time.
Continuous monitoring, spectrophotometry – measuring the amount of light of a particular wavelength absorbed by one component over time. The component absorbs its complimentary color.
Continuous monitoring, Total pressure – the total pressure of a gas mixture is stoichiometrically related to partial pressures of the gases in the reaction.
Sampling, gas chromatography can measure the concentrations of various components in a mixture. For samples that have volatile components. Separates mixture by adherence to a surface.
Sampling draws off periodic aliquots from the mixture and doing _____ analysis? quantitative analysis. Titration for one of the components. Gravimetric analysis.
Nature of the reactants means what? what kind of reactant molecules and what physical condition they are in.
What reaction takes place faster, in smaller or larger molecules? Small molecules tend to react faster than large molecules;
How do gasses tend to react differently than liquids? gases tend to react faster than liquids which react faster than solids.
Powdered solids are more reactive than what? “blocks” more surface area for contact with other reactants.
ions react faster than what? molecules because no bonds need to be broken.
Increasing temperature means what to the reaction rate? increases reaction rate
Chemist’s rule of thumb – for each ____ rise in temperature, the speed of reaction ______. for each 10°C rise in temperature, the speed of the reaction doubles for many reactions.
What are catalysts? catalysts are substances which affect the speed of a reaction without being consumed.
most catalysts are used to do what to a reaction? What is this called? catalysts are used to speed up a reaction, these are called POSTIVE CATALYSTS.
Catalysts used to slow a reaction are called negative catalysts.
homogeneous Catalyst= present in same phase.
Heterogeneous catalyst = present in different phase.
Generally, the larger the concentration of reactant molecules, the faster the reaction
Concentration of gases depends on the partial pressure of the gas. The higher the pressure = the higher the concentration
Concentration of solutions depends on what ratio? the solute to solution ratio (molarity).
What is the definition of the Rate Law? The Rate Law of a reaction is the mathematical relationship between the rate of the reaction and the concentrations of the reactants.
The rate of a reaction is directly proportional to the concentration of each reactant raised to a power.
For the reaction aA + bB ---> products, the rate law would have the form given below n and m are called the orders for each reactant. k is called the rate constant.
What is the definition of the order of the reaction? The sum of the exponents on the reactants is called the order of the reaction.
What order is sublimation in? it is normally a zero order because only molecules at the surface can sublime and their concentration does not change when the amount of subliming substance decreases.
The exponent on each reactant in the rate law is called the ORDER, with respect to that reactant. So if the exponent of [A]^2, that would mean that it is in Order 2.
The OVERALL ORDER is the sum of the exponents (m+n).
What is integrated rate law? is a relationship between the concentrations of the reactants and time. It is a single reactant decomposing into products. A---> products. DEPENDS ON THE ORDER OF THE REACTION.
First-0rder integrated rate law the RATE is DIRECTLY PROPORTIONAL to the CONCENTRATION. Equation is- Rate=k[A]. Since rate- -[A]/change in time. WE CAN WRITE= -change[A]/change in time= k[A].
Definition of half-life (tsub1/2), what does it depend on? Of a reaction is the LENGTH OF TIME it takes for the CONCENTRATION of the REACTANTS to fall ½ its INITIAL VALUE. concentration of the reactants to fall to ½ its initial value. The half-life of the reaction depends on the order of the reaction.
First-Order Reaction Half-Life equation on pg 579. Look at pg 581.
Zero order reaction half-life equation on pg 580. Look at pg 581.
Rate law can only be found ______. can only be determined experimentally. Graphing.
Determining the rate law graphically, rate = slope of curve [A] vs. time.
Determining the rate law graphically, if graph [A] vs time is straight line, then exponent on A in rate law is, and the rate constaqnt= 0, rate constant = -slope
Determining the rate law graphically, if graph ln[A] vs time is straight line, then exponent on A in rate law is 1, rate constant = -slope
Determining the rate law graphically, if graph 1/[A] vs time is straight line, exponent on A in rate law is 2, rate constant = slope.
Arrhenius equation shows the relationship between the _____ and the _____. shows the relationship between the RATE CONSTANT (k) and the TEMPERATURE in Kelvins (T). pg 581.
In the Arrhenius equation, R is is the gas constant (8.314 J/mol*K).
In the Arrhenius equation, A is is a constant called the FREQUENCY FACTOR.
In the Arrhenius equation, E(sub)a is the ACTIVATION ENERGY or ACTIVATION BARRIER.
Summarizing Basic Kinetic relationships (see Table 13.2), the reaction order and rate law must be determined how? experimentally.
Summarizing Basic Kinetic relationships (see Table 13.2), the rate law relates the _____ of the reaction to the ______ of the reactant. the rate law relates the RATE of the reaction to the CONCENTRATION of the reactant.
Summarizing Basic Kinetic relationships (see Table 13.2), the integrated rate law relates the concentration of the reactant to ____? to time.
Summarizing Basic Kinetic relationships (see Table 13.2), the half-life is the time it takes for the concentration of a reactant to fall to one=half its initial value.
The half-lives of zero-order and second-order reactions depend on the initial concentration.
The Activation Energy shows the ______ of the molecule as the reaction proceeds. shows the ENERGY of the molecule as the reaction proceeds.
To get from the reactant to the product (in activation energy), the molecule must go through a high-energy intermediate state called the activated complex or transition state. Kind of like to get from A+B to product! A+B---> AB. Look at graph on pg 582.
The energy required to reach the activated complex is the activation energy.
What is the relation between the activation and the reaction rate? the higher the activation energy, the slower the reaction rate (at a given temperature).
The Frequency Factor represents the number of approaches to the activation barrier per unit time.
The exponential factor in the Arrhenius equation is a number between 0 and 1.
The exponential factor in the Arrhenius equation represents the fraction of reactant molecules with sufficient energy so they can make it over the energy barrier.
The higher the energy barrier (larger activation energy), the fewer molecules… …that have sufficient energy to overcome it. That extra energy comes from converting the kinetic energy of motion to potential energy in the molecule when the molecules collide.
What does temperature and the average kinetic energy of molecules have to do with one another? What does this mean for the energy of molecules being able to overcome the energy barrier? Increasing the temperature increases the average kinetic energy of the molecules. Therefore, increasing the temperature will increase the number of molecules with sufficient energy to overcome the energy barrier.
Therefore increasing the temperature will increase what? the reaction rate.
Summarizing Temperature and Reaction Rate, the frequency factor is the number of times that the reactants approach the activation barrier per unit time.
Summarizing Temperature and Reaction Rate, the exponential factor is the fraction of approaches that are successful in surmounting the activation barrier and forming products.
Summarizing Temperature and Reaction Rate, the exponential factor increases with increasing what? increasing temperature, but decreases with increasing activation energy.
In the collision model, a chemical reaction occurs after a sufficiently energetic collision between two reactant molecules.
For collision theory, for most reactions, in order for a reaction to take place, the reacting molecules must collide into each other.
Once molecules collide they may react together or they may not, depending on two factors 1) whether the collision has enough energy to "break the bonds holding reactant molecules together". 2) whether the reacting molecules collide in the proper orientation for new bonds to form.
Collisions in which these two conditions are met (and therefore lead to reaction) are called effective collisions.
The higher the frequency of effective collisions, the faster the reaction rate.
When two molecules have an effective collision, a temporary, high energy (unstable) chemical species is formed - called an activated complex or transition state.
For a collision to lead to overcoming the energy barrier, the reacting molecules must have sufficient kinetic energy so that when they collide it can form the activated complex.
In the collision model, the frequency factor can be separated into two separate parts, what are they? orientation factor (p) and the collision frequency (z).
Collision frequency (z) is the number of collisions that occur per unit time.
The proper orientation results when the atoms are aligned in such a way that the old bonds can ____ and the new bonds can_____ the old bonds can break and the new bonds can form
The more complex the reactant molecules, the less frequently they will collide with the proper orientation.
Reactions between atoms generally have, p=? p = 1
Reactions where symmetry results in multiple orientations leading to reaction have p slightly less than 1.
For most reactions, the orientation factor is less than 1. For many, p << 1
There are some reactions that have p > 1 in which an electron is transferred without what? without direct collision.
Most reactions occur in a series of small reactions involving how many molecules? 1, 2, or at most 3 molecules.
A reaction mechanism is a series of individual chemical steps by which an overall chemical reaction occurs.
Each step in a reaction mechanism is an elementary step.
Elementary steps cannot be broken down into simpler steps, they occur as they are written, what do they represent? (THEY REPRESENT THE EXACT SPECIES THAT ARE COLLIDING IN THE REACTION!)
What does an elementary step represent? It represents an interaction between the reactant molecules in the step.
A reaction intermediate form in one elementary step and is consumed in another. (they are crossed out before they get to the overall reaction equation!). Example on page 588.
An overall reaction equation shows only the staring substances and the ending substances, not the path between them.
Describing the series of steps that occur to produce the overall observed reaction is called a reaction mechanism.
Knowing the rate law of the reaction helps us understand the sequence of steps in the mechanism.
H2(g) + 2 ICl(g) --> 2 HCl(g) + I2(g) 1.) H2(g) + ICl(g) --> HCl(g) + HI(g) 2.) HI(g) + ICl(g) --> HCl(g) + I2(g) notice that the HI is a product in Step 1, but then a reactant in Step 2. Since HI is made but then consumed, HI does not show up in the overall reaction.
Materials that are products in an early step, but then a reactant in a later step are called intermediates.
Elementary steps are characterized by their ____? molecularity.
The molecularity is the number of reactant particles involved in a step.
What are the most common molecularities? There are two. Unimolecular. And bimolecular.
A unimolecular step involves ____ reactant particle 1 reactant particle
A bimolecular step involves ____ reactant particles. Are they different or the same particles? 2 reactant particles, though they may be the same kind of particle.
A termolecular step involves ____ reactant particles. Are these common? 3 reactant particles
Each step in the mechanism is like its own little reaction – with its own _______ energy and its own ________. activation energy and own rate law.
The rate law for an overall reaction must be determined how? experimentally. But the rate law of an elementary step can be deduced from the equation of the step.
Rate Determining Step, in most mechanisms, one step occurs _____ than the other steps. occurs slower than the other steps.
The step determines the rate of the _____? The step determines the rate of the overall reaction. The result is that product production cannot occur any faster than the slowest step – the step determines the rate of the overall reaction.
we call the slowest step in the mechanism the rate determining step
the slowest step has the largest activation energy.
the rate law of the rate determining step determines the _______ of the overall reaction. rate law of the overall reaction.
For a reaction mechanism to be valid, mechanisms can only be validated, not proven- two conditions must be met 1) the elementary steps in the mechanism must sum to the overall reaction. 2) the rate law predicted by the mechanism must be consistent with the experimentally observed rate law.
Reaction rates can be increased by using a catalyst.
Catalyst is a substance that increases the rate of a chemical reaction but is not consumed by the reaction.
A catalyst works by providing an alternative mechanism for the reaction.
Catalysts can be categorized into two types homogeneous and heterogeneous.
Homogenous catalysis the catalyst exists in the same phase or state as the reactants.
heterogeneous catalysis the catalyst exists in a different phase than the reactants.
A second example of heterogeneous catalysis involves the hydration of double bonds within alkenes. The catalysis occurs by the four-step process adsorption, diffusion, reaction, desportion.
The heterogeneous catalysis occurs by the four step process, adsorption the reactants are adsorbed onto the metal surface.
The heterogeneous catalysis occurs by the four step process, diffusion the reactants diffuse on the surface until they approach each other.
The heterogeneous catalysis occurs by the four step process, reaction the reactants react to form the products.
The heterogeneous catalysis occurs by the four step process, desorption the products desorb from the surface into the gas phase.
Because many of the molecules are large and complex, most biological reactions require a catalyst to proceed at a reasonable rate. Protein molecules that catalyze biological reactions are called enzymes.
Enzymes work by adsorbing the substrate reactant onto an active site that orients it for reaction
Within the enzyme there are 2 things active site, substrate.
Active site this is a specific area in the enzyme.
The properties and shape of the active site are just right to bind the reactant molecule, usually called the substrate.
 

 



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