Unit 12: Chemical Kinetics
How fast a reaction runs, why that speed can't be guessed from the equation, and what a catalyst actually changes and what it leaves alone.
11 topics · 42 questions
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12.1 Reaction Rates and How They're Measured
- How it worksHow does tracking the disappearance of a reactant, or the appearance of a product, over time define a reaction's rate?
- Worked exampleFind the average rate of reaction for a reactant whose concentration drops from 0.100 M to 0.068 M over 25 seconds.
- Common mistakeWhy isn't a reaction's rate the same at the very start as it is once the reactants are mostly used up?
Your turn1 of 2
Find the average rate of reaction for a reactant whose concentration drops from 0.848 M to 0.277 M over 57 seconds.12.2 Rate Laws and Reaction Order
- How it worksHow does a rate law's exponent for each reactant have to be found experimentally instead of read off the balanced equation?
- Common mistakeWhy can't you predict a reaction's order just by looking at the coefficients in its balanced chemical equation?
- Going furtherWhat happens to a reaction's overall order once you add up the individual orders with respect to each reactant?
Your turn1 of 2
A reaction 2A + B → C has the coefficient 2 in front of A. Why can't you assume the reaction is automatically second order with respect to A just from that coefficient?12.2 Determining Rate Laws from Experimental Data
- How it worksHow does comparing rate changes between two trials, with only one concentration changed, isolate that reactant's order?
- Worked exampleFind the rate law for a reaction given three trials where doubling A doubles the rate and doubling B quadruples the rate.
- Common mistakeWhy doesn't a single experimental trial contain enough information to determine a full rate law?
- Going furtherWhat happens to the method of initial rates when a reaction turns out to be zero order in one of its reactants?
Your turn1 of 2
In three trials of a reaction, doubling the concentration of A alone doubles the rate, and doubling the concentration of B alone quadruples the rate. What is the rate law?12.3 The Integrated Rate Law for a Zero Order Reaction
- How it worksHow does a zero order reaction's concentration decrease in a straight line all the way down toward zero over time?
- Worked exampleFind the concentration remaining after 20 seconds for a zero order reaction starting at 0.50 M with a rate constant of 0.010 M per second.
- Common mistakeWhy doesn't a zero order rate law mean the reaction has no dependence on concentration at every possible concentration?
Your turn1 of 2
Find the concentration remaining after 19 seconds for a zero order reaction starting at 0.779 M with a rate constant of 0.0115 M per second.12.3 The Integrated Rate Law for a First Order Reaction
- How it worksHow does plotting the natural log of concentration against time turn a first order reaction's curve into a straight line?
- Worked exampleFind the concentration remaining after 60 seconds for a first order reaction starting at 0.80 M with a rate constant of 0.015 per second.
- Common mistakeWhy doesn't a first order reaction's half-life change as the reaction proceeds and its concentration drops?
- Going furtherWhat happens to a concentration-versus-time plot for a first order reaction once you graph it as ln(concentration) instead?
Your turn1 of 2
Find the concentration remaining after 75 seconds for a first order reaction starting at 0.787 M with a rate constant of 0.0212 per second.12.3 The Integrated Rate Law for a Second Order Reaction
- How it worksHow does plotting the inverse of concentration against time turn a second order reaction's curve into a straight line?
- Worked exampleFind the concentration remaining after 30 seconds for a second order reaction starting at 0.40 M with a rate constant of 0.050 per M per second.
- Common mistakeWhy can't you tell a reaction is second order just because its concentration-versus-time graph curves downward?
- Ties togetherHow does picking the correct integrated rate law by testing which plot comes out straight connect back to the point of collecting kinetics data?
Your turn1 of 2
Find the concentration remaining after 57 seconds for a second order reaction starting at 0.479 M with a rate constant of 0.0719 per M per second.12.3 Half-Life
- How it worksHow does half-life for a first order process stay constant while a zero order process's half-life keeps shrinking?
- Worked exampleFind the half-life of a first order reaction with a rate constant of 0.0231 per second.
- Worked exampleFind how much of a 100 mg sample remains after three half-lives have passed.
- Common mistakeWhy isn't a substance with a shorter half-life necessarily less dangerous than one with a longer half-life?
Your turn1 of 2
Find the half-life of a first order reaction with a rate constant of 0.141 per second.12.4 Collision Theory and Activation Energy
- How it worksHow do collision theory's requirements of proper orientation and enough energy together explain why most collisions fail to react?
- How it worksWalk me through an energy diagram showing reactants climbing over an activation energy barrier on their way to products.
- Common mistakeWhy isn't activation energy the same thing as a reaction's overall enthalpy change?
- Going furtherWhat happens to the fraction of collisions with enough energy to react once the temperature is raised?
Your turn1 of 2
Gas molecules collide billions of times per second, yet only a small fraction of those collisions actually produce a reaction. How do collision theory's two requirements together explain why most collisions fail?12.4 The Arrhenius Equation
- How it worksHow does the Arrhenius equation combine activation energy and temperature into a single predicted rate constant?
- Worked exampleFind the activation energy of a reaction given rate constants of 0.00512 per second at 300 K and 0.0752 per second at 350 K.
- Common mistakeWhy isn't the frequency factor A in the Arrhenius equation just a fudge factor with no physical meaning?
- Going furtherWhat happens to a plot of ln(k) versus 1/T for a reaction that obeys the Arrhenius equation?
Your turn1 of 2
Find the activation energy, in kJ/mol, given rate constants of 7 times 10−3 per second at 306 K and 85 times 10−3 per second at 344 K (use the two-point Arrhenius equation, R = 8.314 J per mol per K).12.6 Reaction Mechanisms and Elementary Steps
- How it worksHow does the slowest elementary step in a mechanism end up controlling the rate of the entire overall reaction?
- How it worksWalk me through identifying a reaction intermediate that appears in one elementary step and disappears in a later one.
- Worked exampleFind the rate law predicted by a mechanism whose slow step is A plus B forming C.
- Common mistakeWhy isn't a reaction intermediate the same thing as a catalyst, even though neither one shows up in the overall balanced equation?
Your turn1 of 2
A proposed mechanism's slow (rate-determining) step is A plus B combining directly to form C. What rate law does this mechanism predict?12.7 Catalysts and Catalysis
- How it worksHow does a catalyst lower a reaction's activation energy by opening up a completely different reaction pathway?
- How it worksWalk me through the difference between how a homogeneous catalyst and a heterogeneous catalyst speed up a reaction.
- Common mistakeWhy doesn't a catalyst change a reaction's equilibrium position, even though it speeds up how fast equilibrium is reached?
- Going furtherWhat happens to an enzyme's ability to catalyze a reaction once the surrounding temperature climbs too high and the enzyme denatures?
- Ties togetherHow does a catalyst lowering activation energy on both the forward and reverse reaction connect back to why it can't shift equilibrium?
Your turn1 of 2
Adding a catalyst to a reaction speeds it up dramatically without being consumed. How does a catalyst actually accomplish that?