Kinetics Chapter 6
The reaction between two substances A and B A + B --> C + D has the following rate expression: rate = k [B] Draw the graphical representation of (x against y): [A] against time [B] against time Rate against [A] Rate against [B]
([A] against time) - straight line with negative gradient; ([B] against time) - decreasing curve; (Rate against [A]) - any horizontal straight line; (Rate against [B]) - straight line through origin;
Deduce the order of reaction with respect to OH- and explain your answer. (IMAGE QUESTION)
0; because rate not affected by change in [OH-]
Excess 0.100 mol dm-3 nitric acid is added to a certain mass of powdered calcium carbonate at 20C. The rate of reaction is monitored by measuring the change in mass over time due to the loss of carbon dioxide. Determine the rate of formation of carbon dioxide when the nitric acid reacts at a rate of 2.00 x 10-3 mol cm-3 s-1.
1.00×103 mol cm3 s-1
Catalyst
A catalyst speeds up a reaction by providing an alternate pathway that has a lower activation energy. Catalysts are not consumed in the reaction. This means that at a constant temperature more particles have the energy needed to overcome the activation energy barrier
Transition states
Activated complex Have only a transient existence Occur at a PE minimum Exist part way through every step of a rxn
The reaction between gases C and D is slow at room temperature. Suggest two ways of increasing the rate of reaction between C and D other than increasing temperature.
Add Catalyst; increase pressure; increase [C] or [D]
Explain the significance of the Arrhenius constant.
Arrhenius constant refers to the geometric requirements of a reaction.
Relationship between temperature of the reactants and the reaction rate
As the temperature increases and the reaction time decreased, the rate of the reaction increased
Which of the following is (are) important in determining whether a reaction occurs? Energy of the molecules Orientation of the molecules Both
Both answers are correct
Which of the following is (are) important in determining whether a reaction occurs? Energy of the molecules Orientation of the molecules
Both are important
C4H9Br + OH- C4H9OH + Br- Give the equations for the mechanism of this reaction.
C4H9Br --> C4H9+ + Br-; C4H9+ + OH- --> C4H9OH;
Rate expression
Can be written to calculate the effect of initial reactant concentration on rate of a reaction Can only be written after experimental data has been collected and analyzed
Surface Area
Collisions can only occur at the surface of a solid since the reactants cannot collide with particles "inside". More surface area will increase the collisions frequency. Powders and granules have more surface area than large chunks. This factor is important when substances reacting are in immiscible phases Example: gas in liquid, solid in liquid or non-polar liquid in a polar liquid
Consider the reaction between solid CaCO3 and aqueous HCl. The reaction will be sped up by an increase in which of the following conditions? Concentration of the HCl Size of the CaCO3 particles Temperature
Concentration of the HCl and the temperature
What are the factors that affect reaction rates?
Concentration/ pressure (gases) Surface Area Catalyst Temperature
For many reactions, the rate approximately _________________________ for a _______________________ in temperature
Doubles Ten degree celcius
Unimolecular
Elementary step consists of a single species breaking up or undergoing rearrangement to form products. Radioactive decay, for example is uni molecular. If the rds is: A to products then rate [A] so the rate law is 1st order with respect to the reactant
Bimolecular
Elementary step involves the collision of two species that form a transition state that cannot be isolated
State the rule of thumb
Every ten degrees Celsius the rate doubles
Intermediates
Exist for a finite amount of time Occur at a PE minimum Formed in one step of a rxn and consumed in a subsequent step
Deduce the order of reaction with respect to both X and Y. Explain your reasoning. (IMAGE QUESTION) Write the rate expression for the reaction and calculate the rate constant, including its units. Calculate the initial rate of reaction when the initial concentrations of X and Y are 0.40 mol dm-3 and 0.60 mol dm-3 respectively.
Experiment 1-2 : [X] doubles and rate ×4; --> 2nd order for X; Experiment 2-3 : [Y] doubles and rate ×2; --> 1st order for Y; rate = k[X]2[Y] for experiment 1, 1.0×10-2 = k (0.25)2(0.25); --> k = 0.64 mol-2 dm6 s-1;; rate = 0.64 x [0.40]2[0.60]; = 0.061;
Deduce the order of the reaction with respect to A, giving a reason for your choice, and write the rate expression for the reaction.
First order; as half-life is constant; therefore rate = k[A];
Half Life Equation
Half life = ln2/ lamda
Sketch the above graph (Vol of CO2 produced against time when excess CaCO3 added to 2M HCl) on an answer sheet. On the same graph, draw the curves you would expect if: I. the same volume (x cm3) of 1.0 mol dm-3 HCl is used. II. double the volume (2x cm3) of 1.0 mol dm-3 HCl is used. Label the curves and explain your answer in each case.
I. Less CO2 because HCl is limiting reagent and so half the original volume of CO2 produced; II. Same amount of CO2 produced because moles of HCl is the same; but curve less steep because less frequent collisions
The rate expression can never prove absolutely that a particular mechanism is correct but it can provide evidence that other proposed mechanisms are wrong
Important
Collision Theory
In order for a chemical reaction to occur, the particles must: Collide physically In the right orientation, numerically assessed by a "steric factor" With sufficient energy (to overcome the activation energy barrier)
Rate determining step
It is the slowest step of the reaction and this step determines the rate of the reaction overall
How can you tell what the greatest initial reaction is on a graph?
It will start with the steepest slope
State and explain three ways in which the rate of this reaction could be increased.
Method 1: use powdered MgCO3; particles collide more frequently as surface area increased; Method 2: increase temperature; greater proportion of collisions are successful as more particles with E > Ea; Method 3: increase [acid]; increases frequency of collisions between reactants; Method 4: adding catalyst; lowers activation energy;
Suggest a possible mechanism that is consistent with the rate expression. Indicate which of the steps is the rate-determining step.
NO + O2 <--> NO3; (slow) NO3 + NO --> 2NO2; (fast)
The reaction between 2-bromo-2-methylpropane (CH3)3CBr, and warm aqueous 0.100 mol dm-3 sodium hydroxide solution to form 2-methylpropan-2-ol, is found to have the following rate expression. rate =k[(CH3)3CBr] Deduce what the effect on the rate of the reaction will be if the concentration of the sodium hydroxide solution is increased to 0.300 mol dm-3.
No effect as [OH-] doesn't appear in rate expression.
Carbon dioxide gas in the atmosphere reacts slightly with rainwater as shown below. CO2(g) + H2O(l) <--> H+(aq) + HCO3-(aq) Predict the effect, if any, of the presence of a catalyst on the acidity of rainwater. Give a reason for your answer. Use Le Chatelier's principle to predict the effect of the addition of a small quantity of an alkali on the acidity of rainwater. Explain what effect, if any, this would have on the equilibrium constant, Kc.
No effect; because catalyst speeds up both forward and reverse reactions equally; No effect on acidity; equilibrium shifts to the right; Kc would not change;
Explain the term nucleophilic substitution.
Nucleophilic Substitution: Replacement of one functional group by a species with a lone pair of electrons;
Define the term molecularity and deduce its value in the mechanism.
Number of reactant molecules in rate-determining step; 1 in this reaction;
Define the term order of reaction.
Power of a reactant's concentration in the rate equation.
The graph below shows the volume of carbon dioxide gas produced against time when excess calcium carbonate is added to x cm3 of 2.0 mol dm-3 hydrochloric acid. (IMAGE QUESTION) State and explain the change in the rate of reaction with time. Outline how you would determine the rate of the reaction at a particular time.
Rate decreases with time; as concentration of HCl decreases --> fewer (successful) collisions between reactants; draw tangent to the curve at time t; rate of reaction = m;
Rate of the reaction
Rate is defined as the change in concentration of reactants or products per unit time
Any property that differs greatly between the reactants and the products can be used to measure the rate of a reaction. Possibilities include:
Rate of gas production by collecting gas in a syringe Rate of gas production as measured by loss of mass of reactants (Massive gas preferred) Rate of change of concentration as measured by absorption of light (colored substance) Rate of increase in precipitate formation (cloudiness or its ability to block light) Conductivity change due to presence of ions, especially Hydrogen plus or hydroxide
State what is meant by the term rate of reaction.
Rate of reaction: Increase in [product] or decrease in [reactant] per unit time
Equation for Reaction Rate
Reaction rate is equal to the change in reactants divided by time or reaction rate is equal to the change in products divided by time
Molecularity
Refers to the number of species (particles) colliding in an elementary step. Often the molecularity of each elementary step is described
The rate of a reaction between two gases increases when the temperature is increased and a catalyst is added. What happens to the activation energy?
The activation energy does not change when the temperature increases, but the activation energy decreases when a catalyst is added
Consider the reaction between solid CaCO3 and aqueous HCl. The reaction will be sped up by an increase in which of the following conditions? Concentration of the HCL Size of the CaCO3 particles Temperature
The concentration of HCl and Temperature
Concentration/ Pressure (gases)
The concentration of a solution or the pressure of a gas indicates the number of particles per unit volume. A higher value means more particles packed closer together and will increase the frequency of collisions per unit time
Maxwell-Boltzmann Distribution Curve
The distribution of the molecular speeds of gas particles at a given temperature; as temperature increases, average speed increases and the distribution becomes wider and flatter.
For a given reaction, why does the rate of reaction increase when the concentrations of the reactants increased?
The frequency of the molecular collisions increases
Activation Energy
The minimum amount of energy required for a chemical reaction to occur
Which statement is correct for the reaction below 4P + Q --> 2R +2S The rate of formation of R is one half the rate of the disappearance of Q The rate of disappearance of Q is one quarter of the rate of disappearance of P The rates of formation of R and S are not equal The rate of formation of S is double the rate of disapperance of P
The rate of disappearance of Q is one quarter of the rate of disappearance of P
In the Haber process for the synthesis of ammonia, what effects does the catalyst have?
The rate of formation of NH3 would increase while the amount of NH3 would not change because the amount of moles tells the amount of NH3 being formed not the catalyst.
Excess magnesium was added to a beaker of aqueous hydrochloric acid on a balance. A graph of the mass of the beaker and contents was plotted against time. If the graph has two lines, both with negative slopes, and the second line is lower than the first line, what could have changed in experiment 2 to produce the second line? The same mass of magnesium but in smaller pieces The same volume of a more concentrated solution of hydrochloric acid A lower temperature
The same volume of a more concentrated solution of hydrochloric acid
Reaction Mechanism
The sequence of elementary steps that make up the overall reaction. A rxn mechanism will describe in detail the order in which bonds are broken and formed and changes in relative positions of the atoms
Which step in a multi-step reaction is the rate determining step? The first step The last step The step with the lowest activation energy The step with the highest activation energy
The step with the highest activation energy
What is the definition of half-life for a first order reaction? The time required for the quantity of a reactant to decrease by half Half the time required for a reactant to be completely used up Half the time required for a reaction to reach its maximum rate The time required for a reaction to reach half of its maximum rate
The time required for a reaction to reach half of its maximum rate
Half Life
The time taken for the reactant concentration to reach a value which is the average of its initial and final values
Transition State Theory
The top of the peak where the molecule is unstable which high energy Resembles neither products or reactants
State the units for the rate constant K
The units for k is the amount of volume over time times moles
Explain in terms of the collision theory what would happen to the rate if the reaction was conducted at 50C.
There are more particles with KE greater than or equal to Ea; --> so collisions more frequent, but most importantly, successful collisions more frequent; so rate increases;
Temperature
There are two parts to this explanation First, since temperature is a measure of kinetic energy of the particles, at higher temps, the particles have more kinetic energy and collision frequency will increase. The second part of the explanation is the less obvious, but even more important. When the sample is heated to a higher temperature, the average energy of the particles increases.
State and explain whether the total volume of carbon dioxide gas produced would increase, decrease or stay the same if more lumps of magnesium carbonate were used in the experiment.
This would make the reaction go slower, but the endpoint would be the same so the total volume of carbon dioxide gas would not change.
State and explain whether the total volume of CO2 gas produced would increase, decrease or stay the same if: more lumps of magnesium carbonate were used. the experiments were carried out at a higher temperature.
Volume of CO2 produced would be the same; as MgCO3 was already in excess; Volume of CO2 produced would be the same; as same quantities of reactants used;
Relationship between concentration and reaction rate
When the concentration decreased and time increased, the rate of the reaction occurred slower and slower
A suggested mechanism for this reaction is as follows. H2 + NO <-->X (fast step) X + NO → Y + H2O (slow step ) Y + H2 → N2 + H2O (fast step) State and explain whether this mechanism agrees with the experimental rate expression k = [NO]2 [H2].
agrees/yes; because slow step depends on X and NO; and X depends on H2 and NO; so NO involved twice and H2 once;
Describe how, using a graphical method, values of A and Ea can be obtained for a reaction.
k measured at different values of temperature; and a graph is plotted with ln k against 1/T; Y-intercept of graph is ln A; --> A = e^intercept; Gradient of graph = Ea ÷ R ; --> Ea = -R × gradient;
Based on the definition for rate of reaction, which units are used for a rate?
mol dm^-3 time^-1
Draw a graph to show the distribution of energies in a sample of gas molecules. Label the axes and label your curve T1. Using the same axes, draw a second curve to represent the distribution of energies at a higher temperature. Label this curve T2. State and explain, with reference to your graph, what happens to the rate of a reaction when the temperature is increased.
rate increases; as more molecules with ≥ Ea;
State the units for the rate constant, k (when order of reaction is 1)
s-1 min-1
Reaction Rate
the change in concentration of a reactant or product per unit time
