Reaction Kinetics Flashcards
What is rate of reaction?
Change in concentration of reactant or product per unit time
What is the gradient of concentration-time graph?
rate of reaction
If concentration-time graph is a 1/x² graph, what does this imply?
- gradient decreasing with time
- reaction is slowing down
- rate of reaction is decreasing with time
What is initial rate?
reaction rate at the start of the reaction where an extremely small amount of reactant has been used up
How to obtain initial rate?
determining the gradient of tangent drawn to the curve at time t=0
What is instantaneous rate?
rate of reaction at a particular instant in time
How to obtain instantaneous rate?
gradient of tangent drawn to the curve at that time
What is average rate of a reaction?
Calculated from final change in concentration of reactant or product over a particular period of time
How to obtain average rate of reaction?
Measuring gradient of line drawn between two points on the curve for the time interval
What happens if your calculate rate of reaction is negative?
- rate of reaction always positive
- add a negative sign to make it positive
What is rate equation or rate law?
Experimental relationship between the reaction rate and the concentration of reactants
What is the rate equation of aA + bB → cC + dD
rate of reaction = k[A]ᵐ[B]ⁿ
For rate of reaction = k[A]ᵐ[B]ⁿ, what is k, m, n and m+n?
- k is rate constant
- m is the order of reaction with respect to A
- n is the order of reaction with respect to B
- (m+n) is overall order of reaction
What is order of reaction?
It is the power of the reactant’s concentration term in the experimentally-determined rate equation is the order of reaction with respect to that reactant
What is rate constant, k?
proportionality constant in the experimentally determined rate equation
When is rate constant a constant for a given reaction?
At a given temperature
What affects rate constant, k?
- k increases when temperature increases
- k increases when Ea decreases (using a catalyst)
What is zero-order reaction?
rate of reaction is independent to [reactant]
What does the concentration time graph look like for zero-order reactions? How to interprete the graph?
- downward sloping linear line
- gradient = -k
- gradient is constant which means that rate is constant
What does the rate-concentration graph of zero order reaction look like?
horizontal straight line at k (vertical intercept)
What does the rate-time graph of zero order reaction look like?
horizontal straight line at k (vertical intercept)
What is a first-order reaction?
rate of reaction is directly proportional to [reactant]
What does the concentration time graph look like for first-order reactions?
curve where gradient is getting less steep over time
What is the rate-[concentration] graph of first-order reaction?
- upward sloping linear graph
- gradient = k
What is half-life?
time taken for the concentration of a reactant to be halved
What is unique to first-order reactions?
half-life is constant
What is the formula for half-life?
t₁/₂ = ln 2/k
How to calculate the fraction of reactant remaining for a first order reaction?
- using number of half-lives passed (n)
- fraction of reactant remaining = (1/2)ⁿ
What is second-order reaction?
rate of reaction is directly proportional to [reactant]²
What does the concentration time graph look like for second-order reactions? And what does it imply?
- curve where gradient is getting less steep over time
- half-life increases with time
- rate of reaction is slower than first order
What is the rate-concentration graph for second order reaction?
upward sloping curve with increasing gradient
What is the rate-concentration² graph for second order reaction?
- upward sloping linear graph
- gradient = k
What is pseudo-order reaction about?
Make a reactant A in large excess with respect to B such that [A] will not change significantly throughout the reaction and can be regarded as constant
How do you describe a pseudo reaction with A in excess?
rate = k[A]ᵐ[B]¹
- pseudo zero order with respect to A
- first order with respect to B
- overall pseudo first-order reaction
(pesudo is like appears like)
What is initial rates method?
It involves several experiments at which initial concentration of the reactant of interest is varied while keeping the concentration of other reactants constant
Why do you use initial rate?
- Initial rate is where an extremely small amount of reactant has been used up in an extremely small amount of time (differentiation)
- thus amount of reactants is close to the inital amount of reactants
- thus can just use initial amt. of reactants which is known (make things much easier)
- By varying concentration and seeing how rate changes, can find order of reaction and k
rate of reaction = k[A]ᵐ
(y=mx+c)
What experimental procedure can be done to determine rate of reaction?
A + B → C
- mix known concentration of A and B, where B is in excess
- determine concentration of reactant A or product C at various time of the reaction
- Plot concentration vs time graph
What does it mean if concentration vs time graph is a straight line?
- reaction is zero order
- rate of reaction = -gradient =k
What does it mean if concentration vs time graph is a curve?
- reaction can be first or second order
- determine half-life to find out
- if half life is approximately constant, reaction is first order
What is the significance of the slowest step?
- rate-determining step
- dictates the rate of the entire reaction
What does collision theory assume?
reactant particles must collide effectively before they can react
What does collision theory say about when a reaction will occur?
A reaction will occur only if the particles collid with:
* minimum amount of kinetic energy called activation energy and
* appropriate collision geometry/orientation
What is the area under the Maxwell-Boltzmann distribution curve?
Total number of particles
What is the transition state theory about?
It proposes that reacting particles with minimum activation energy will come together in appropriate orientation to form a transition state or activated complex, which either decomposes back into reactants or rearranges to form products
What is the highly unstable maximum energy state in the energy profile diagram?
transition state or activated complex
What is the activation energy on the energy profile diagram?
energy gap between the transition state and reactants
What happens in a transition state?
new bonds are being formed while old bonds are broken concurrently
Does transition state have a fix geometry?
- Expected to have
- Since reactant particles must be oriented correctly before arriving at the transition state
What do reactant particles need to get to the transition state?
It must possess minimum amount of energy to:
* weaken the bonds of reactant particles
* overcome steric hindrance between reactant particles so they can approach each other
* orientate the reactant particles to achieve correct geometry for reaction
What do reaction with low activation energies do?
- proceed quickly
- because a larger proportion of reactant particles will have enough energy to overcome the energy barrier
What do reactions with high activation energies do?
- proceed slowly
- rate of reaction is so low it may not occur even if it is energetically favourable (feasible)
What are the two types of stability?
- kinetic stability related to Ea
- thermodynamic stability related to ΔG
What happens to ΔG, ΔS and ΔH for enthalpy driven reaction?
ΔS ≈ 0, ΔG ≈ ΔH
What happens if ΔH < 0 or ΔG < 0?
- ΔH is negative product is more thermodynamically stable with respect to the reactant
- reaction is thermodynamically feasible
What happens if ΔH > 0 or ΔG > 0?
- ΔH is positive, product is thermodynamically unstable with respect to the reactant
- reaction not thermodynamically feasible
How is large Ea related to kinetic stability?
- If Ea is large, reactant is kinetically stable
- reaction is expected to be slow
How is small Ea related to kinetic stability?
- reactant is kinetically unstable
- reaction expected to be is fast
What does an intermediate look like on an energy profile diagram?
energy minimum but with an energy level higher than both reactants and products
What are the factors that affect the rate of reaction?
- conentration of reactant particles/pressure of gaseous reactants
- surface area of solid reactants
- temperature
- catalyst
How does the concentration of reactants affect rate of reaction?
- As concentration of reactant increases, number of reactant particles per unit volume increases
- increase in frequency of collisions between reactant particles
- frequency of effective collisions will increase
- rate of reaction increases
How does pressure of gaseous reactants affect rate of reaction?
- When pressure of reactant increases
- number of paricles per unit volume increases
- increase frequency of effective collisions
- rate of reaction increases
How does surface area of solid reactants affect rate of reaction?
- reactions take place faster when solids are in finely divided state
- the larger the surface area of contact between the reactant particles, higher frequency of effective collisions between the reactant particles
How does temperature affect rate of reaction?
- When temperature increases, average kinetic energy of reactant particles increases
- frequency of collisions between reactant particles increases, frequency of effective collisions increases, rate of reaction increases
- At a higher temperature, there is a larger number of reactants with energy greater than or equal to the activation energy
- increase in frequency of effective collisions between reactant particles
- increase rate of reaction
How does the Maxwell-Boltzmann distribution curve change when temperature increases?
- peak moves to the right
- curve broadens and flattens out
What is a catalyst?
Catalyst is a substance that increases the rate of chemical reaction but remains chemically unchanged at the end of the reaction
What are inhibitors and how do they work?
- Inhibitors decrease the rate of reaction
- they remove the reactive intermediates from reaction sequence
How does a catalyst work?
- Catalyst provides an alternative pathway of lower activation energy
- more reactant particles posses the energy required for an effective collision and frequency of effective collisions increases, rate of reaction increase
What happens to a catalyst at the end of the reaction?
- regenerated and chemically unchanged
- but physical state may change
What is homogeneous catalyst?
Catalyst and reactants are in same phase
What is the difference between phase and physical state?
- for two substance to be in same phase, physical state must be same
- however, two substance being in same state does not mean they are in same phase (eg. oil and water are in same physical state but they are in different phase because they are immiscible
What are some examples of homogeneous catalyst and their equations?
- NO₂ (g)
- 2SO₂ (g) + O₂ (g) → 2SO₃ (g)
- Fe²⁺ or Fe³⁺
- S₂O₈²⁻ (aq) + 2I⁻ (aq) → 2SO₄²⁻ (aq) + I₂ (aq)
- Electrophilic substitution of benzene
- C₆H₆ + X₂ → C₆H₅X + HX
What are the steps and equation for catalytic oxidation of SO₂ by NO₂?
- NO₂ (g) + SO₂ (g) → SO₃ (g) + NO (g)
- NO (g) + 1/2O₂ (g) → NO₂ (g)
Overall: SO₂ (g) + 1/2O₂ (g) → SO₃ (g)
Why is atmosphere SO₂ a pollutant?
- SO₂ can be oxidised to SO₃ but is very slow
- With NO₂ from car exhaust fumes, NO₂ acts as a catalyst
- SO₃ formed reacts with water to form sulfuric acid which forms acid rain
What is the equation of the oxidation of iodide ion by peroxodisulfate ion (S₂O₈²⁻)?
2I⁻ (aq) + S₂O₈²⁻ (aq) → I₂ (aq) + 2SO₄²⁻ (aq)
What is the reaction of the oxidation of iodide ion by peroxodisulfate ion (S₂O₈²⁻) slow?
- reaction is slow as it has high activation energy
- large amount of energy is required to overcome the repulsion between two negatively-charged reactant particles before they can collide
What is the mechanism of oxidation of iodide ion by peroxodisulfate ion (S₂O₈²⁻) using Fe²⁺?
- 2Fe²⁺ (aq) + S₂O₈²⁻ (aq) → 2Fe³⁺ (aq) + 2SO₄²⁻ (aq)
- 2Fe³⁺ (aq) + 2I⁻ (aq) → 2Fe²⁺ (aq) + I₂ (aq)
What is the mechanism of oxidation of iodide ion by peroxodisulfate ion (S₂O₈²⁻) using Fe³⁺?
- 2Fe³⁺ (aq) + 2I⁻ (aq) → 2Fe²⁺ (aq) + I₂ (aq)
- 2Fe²⁺ (aq) + S₂O₈²⁻ (aq) → 2Fe³⁺ (aq) + 2SO₄²⁻ (aq)
opposite order for Fe²⁺
What are heterogeneous catalyst?
catalyst and reactants are in different phases
What are some reactions catalysed by heterogenous catalysts?
- Haber Process (Fe(s) or Fe₂O₃ (s))
* N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) - Catalytic convertors (Pt(s) or Pd(s))
* CO (g) + 1/2O₂ (g) → CO₂ (g)
* C (s) + O₂ (g) → CO₂ (g)
* combustion of CxHy
* 2NO (g) + 2CO (g) → 2CO₂ (g) + N₂ (g) [Rh(s)]
What is the reaction, catalyst and nature of catalyst of contact process?
- 2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)
- V₂O₅ (s)
- heterogenous catalyst
What is the reaction, catalyst and nature of catalyst of hydrogenation of alkenes?
- C₂H₄ (g) + H₂ (g) → C₂H₆ (g)
- heteregoneous catalyst
- Pd(s) or Pt(s) or Ni(s)
What is the reaction, catalyst and nature of catalyst of decomposition of hydrogen peroxide?
- 2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)
- heterogenous catalyst
- MnO₂ (s)
What is the mode of action of solid heterogenous catalyst? (absorption theory)
- Absorption: Reactant particles form weak bonds to the active sides on the catalyst surface, this weakens or breaks the bond within the reactant molecules themselves, making them more reactive
- Reaction: Reactant particles are held on the catalytic surface in close proximity and in the correct orientation so they can readily react
- Desorption: Weak bonds between the product particles and the catalyst surface are broken, the products formed diffuse away from the surface of the catalyst and the active sites become available again
What is autocatalyst?
A type of catalysist where one of the products of the reaction catalyses the reaction
What is the concentration-time graph of autocatalysis reaction?
- initial rate of reaction is low (gentle gradient)
- as rate of reaction increases, [catalyst] increases, rate of reaction increases (steeper grad)
- rate of reaction decreases as [reactants] decreases despite the increase in [catalyst] (gentle grad)
Why are enzymes?
proteins that catalyst the chemical reactions in living systems
What are the characteristics of enzymes?
- dimensions of enzymes in colloidal range and cannot be classified as honogeneous or heterogeneous
- highly selective (only catalyse a specific reaction or type of reaction)
What are factors that affect rate of enzyme-catalysed reactions?
- substrate concentration
- pH of medium
- temperature
How does [substrate] affect rate of reaction?
- At low [S], reaction is first-order with respect to substrate becayse the rate of enzyme-substrate complex formation is directly proportional to [S]
- At high [S], adding more substrate cannot accelerate the reaction as all the active sites on the enzyme molecules are saturated/occupied, rate of reaction becomes independent of [S] and zero order with respect to substrate
What is substrate?
It is the substance that undergoes a chemical reaction under the influence of an enzyme
What happens to reaction-[E] graph when [S] is kept constant and in excess?
reaction rate is directly proportional to enzyme concentration
How does pH of medium affect rate of enzyme catalysed reaction?
- each enzyme has own optimum working pH
- rate of reaction maximum at optimum pH and decreases above or below the pH
How does temperature affect the rate of enzyme catalysed reaction?
- At optimum working temperature of enzyme, rate of reaction is highest
- very low temperatures deactivate enzymes
- very high temperatures denature enzymes
- beyond optimum working temperatures, enzyme loses its 3D conformation, and thus its active side, catalytic properties of enzyme will be lost
What are the two types of experimental method to determine rate of reaction?
- continuous method
- discontinuous method
What is continuous method?
- involves monitoring a reaction mixture from start to end of reaction
- any measurable physical property which is proportional to concentration can be monitored
What is volume measurement to determine rate of reaction suitable for and how can rate of reaction be determined?
- suitable for reactions that produce gaseous products
- Plot volume of CO₂ against time
- rate of reaction is gradient of tangent
What is mass measurement to determine rate of reaction suitable for and how can rate of reaction be determined?
- suitable for reactions that produce gas (higher density)
- plot mass of reaction mixture against time
- mass lost = mass of gas produced
- rate of reaction is gradient of tangent
What is gas pressure measurement to determine rate of reaction suitable for and how can rate of reaction be determined?
- suitable for reactions where stoichiometric amount of gaseous product is different of stoichiometric amount of gaseous reactant (change in amount of gas)
- pressure of gas is directly proportional to amount of gas at constant temperature and volume
- Plot total pressure against time
- rate of reaction is gradient of tangent
What is colorimetric method to determine rate of reaction suitable for and how can rate of reaction be determined?
- suitable for reactions with coloured reactants or products
- rate of reaction determined by measuring amount of light transmitted through the solution at regular time intervals
- colour intensity of chemical species directly proportional to concentration
What is conductometric method to determine rate of reaction suitable for and how can rate of reaction be determined?
- suitable for reactions where there is a change in the number of ions as the reaction proceeds
- electrical conductivity is directly proportional to concentration of ions present
- rate of reaction determined by measuring charge of electrical conductivity of solution with time
What can rotation of plane-polarised light be used to determine rate of reaction suitable for and how can rate of reaction be determined?
- if change of optical activity occurs during reaction, change in optical rotation can be measured
- rate of reaction is determined by change in optical rotation with time
What is sampling method?
requires a portion of reaction mixture to be extracted at regular time intervals and chemically tested to determine concentration at particular time
How can quenching be done?
- sudden cooling (by plunging reaction mixture in ice bath)
- dilution by adding excess solvent, eg. cold water
- addition of known excess of quenching agent
How to carry out sampling method?
- reactions are carried at constant temperature
- known concentration of reactants are mixed and stopwatch started
- sample of reaction mixture are drawn at suitable time intervals and quenched
- concentration determined by titration
How to determine rate of reaction using sampling method?
- Using concentration found, plot concentration time graph
- rate of reaction is determined by gradient of tangent
What is discontinuous method?
- It relies on measuring the time taken for an easily observable stage to be reached after the reactants are reached
- rate of reaction is inversely proportional to time taken
What is the rate of reaction obtained by discontinuous method?
- average rate
- a good approximation of initial rate if very small amount of reactants is consumed
What is the iodine-clock reaction?
- S₂O₈²⁻ (aq) + 2I⁻ (aq) → 2SO₄²⁻ (aq) + I₂ (aq)
- iodine produced is rapidly reduced back to iodide by S₂O₃²⁻
- S₂O₃²⁻ (aq) + I₂ (aq) → S₄O₆²⁻ (aq) + 2I⁻ (aq)
- I⁻ reacts with S₂O₈²⁻ to produce I₂ which reacts with S₂O₃²⁻
- This repeats until all S₂O₃²⁻ is fully reacted
- then I₂ will react with starch to form blue-black colouration
- experiment repeasted by varying [S₂O₈²⁻] or [I⁻]
- amount of S₂O₃²⁻ must be constant
