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H2 CHEMISTRY SG > REACTION KINETICS > Flashcards

REACTION KINETICS Flashcards

1
Q

What is activation energy?

A

Ea is the minimum energy the reactants must possess before a reaction can occur.

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2
Q

What is the rate determining step?

A

It is the slowest step in the reaction mechanism and it determines the overall rate of reaction.

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3
Q

Define rate of reaction

A

It is the change in concentration of a reactant or product per unit time.

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4
Q

What is quenching?

A

Quenching stops the reaction or slows the reaction down considerably. This is done so that the composition of the reactants does not continue to change and can represent the concentration of the products and reactants at that specific time.

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5
Q

How to quench a chemical reaction?

A
  1. Cool the aliquot rapidly in ice bath (eg).
  2. Add large amounts of h20 (preferably cold) to dilute and cool the aliquot.
  3. Add an inhibitor to slow the reaction down (eg. add reactant to react catalyst away)
  4. Add an excess of reagent that can react away one of the reactants rapidly.
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6
Q

What is a rate equation?

A

The rate equation is an experimentally determined mathematical expression of reaction rate in terms of concentrations of reactants.

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7
Q

How to know the units of k the constant?

A
  1. Assume mol/dm/s is the units of rate.

2. Divide according to rate = k(a)(b)

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8
Q

What does a zero order reaction mean?

A

Rate is constant at all concentrations of reactants and is independent of the concentration of the reactants.

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9
Q

What does the concentration-time graph for concentration of reactants look like for zero order reaction? (Specify axes too).

Why?

A

For reactants,
Time / s takes the x- axis. X-axis is for independent variable, the changed variable.
Concentration of reactants / moldm^-3 will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Linear graph that is decreasing.
Concentration of reactants decreases with time (getting used up).
Linear as gradient is constant at all concentrations as it is independent of the concentration of the reactant.

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10
Q

What does the concentration-time graph for concentration of products look like for zero order reaction? (Specify axes too).

Why?

A

For products,
Time / s takes the x- axis. X-axis is for independent variable, the changed variable.
Concentration of products / moldm^-3 will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Linear graph that is increasing.
Concentration of products increases with time (being produced).
Linear as gradient is constant at all concentrations as formation of product = rate of consumption of reactant = constant.

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11
Q

What does the rate-concentration graph look like for zero order reaction? (Specify axes too).

Why?

A

Concentration of reactant / mol dm^-3 takes the x- axis. X-axis is for independent variable, the changed variable.
Concentration of products / moldm^-3s^-1 will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Horizontal graph of y=k.
Rate is constant.

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12
Q

What does a first order reaction mean?

A

When concentration of reactant is multiplied by two, rate is also multiplied by two. When concentration of reactant is multiplied by three, rate is also multiplied by 3. Rate of reaction is directly proportional to concentration of reactant.

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13
Q

What does the concentration-time graph for concentration of reactants look like for first order reaction? (Specify axes too).

Why?

A

Concentration of reactant / mol dm^-3 takes the y- axis.
Time / s will take the y-axis. X-axis is for independent variable, the changed variable. Y-axis is for dependent variable which is the measured variable.

Exponentially decreasing graph.

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14
Q

What does the concentration-time graph for concentration of products look like for first order reaction? (Specify axes too).

Why?

A

Concentration of products / mol dm^-3 takes the x- axis. X-axis is for independent variable, the changed variable.
Time / s will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Exponentially increasing graph.

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15
Q

What does the rate-concentration graph look like for first order reaction? (Specify axes too).

Why?

A

Concentration of reactant / mol dm^-3 takes the x- axis. X-axis is for independent variable, the changed variable.
Rate / moldm^-3s^-1 will take the y-axis. Y-axis is for dependent variable which is the measured variable.

A straight line passing through the origin should be obtained. y = mx + c, rate = k(A) + 0

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16
Q

What is an important concept that only applies to first order reactions?

A

They have constant half-lives which is independent of the concentration of the reactant. T1/2 = ln2/k
-try to recall graph, the x-axis area remains constant.

17
Q

How to find time taken for a percentage of a substance to decompose when given the half life of a certain duration?

A
  1. Assume the reaction starts with x mol of that substance.
  2. After n half lives, _ percentage of x mol of that substance remains.
  3. Amount of substance remained = 1/2^n x = percentage leftover x
  4. Find N
  5. N x time taken = time taken
18
Q

What does a second order reaction mean?

A

When concentration of reactant is multiplied by 2, the rate is multiplied by 2^2 times. When multiplied by 3, the rate is multipled by 3^2. Rate of reaction is directly proportional to (A)^2.

19
Q

What does the concentration-time graph for concentration of reactants look like for second order reaction? (Specify axes too).

Why?

A

Concentration of reactant / mol dm^-3 takes the x- axis. X-axis is for independent variable, the changed variable.
Time / s will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Exponentially decreasing graph. At a faster rate compared to 1st order reaction. Successive half lives are NOT constant.

20
Q

What does the rate-concentration graph look like for second order reaction? (Specify axes too).

Why?

A

Concentration of reactant / mol dm^-3 or [A]^2 (preferred) takes the x- axis. X-axis is for independent variable, the changed variable.
Rate / moldm^-3s^-1 will take the y-axis. Y-axis is for dependent variable which is the measured variable.

Preferred
Straight linear line passing through origin.
Not preferred
Exponentially increasing graph.

21
Q

What are the methods to determine orders of reaction?

A
  1. Initial Rates method

2. Isolation Method

22
Q

How does the Initial Rates method work?

A
  1. Tangent at t=0
    Can determine order of reaction of a reagent by varying the initial concentration of this particular reagent while keeping the temperature and initial concentrations of all other reagents constant.
  2. Compare using indices/comparison method.
  3. Pick 2 experiments wherein only 1 reactant shows varied concentration.
  4. Clock reactions.
23
Q

How does a Sulfur Clock reaction work?

A

Na2S2O3(aq) + 2HCl(aq) → S(s) + SO2(g) + 2NaCl(aq)
(Not following closely to notes)
1. Measure V1 V2 of Na2s2o3 and HCL respectively.
2. Pour V1 into conical flask first and then V2 and swirl.
3. Stop the stopwatch when printed pattern is obscured by the fixed amount of S produced.
4. Rate = 1/t
5. Repeat with different starting volumes of one of the reagents while keeping total volume of reagents constant (DI) to determine how rate depends on the concentration of the reagent.
6. The faster the formation of a small but constant amount of Sulfur, the shorter the time taken for the cross to disappear.

24
Q

How does the Iodine Clock reaction work?

A

Reaction 1: H202 + 2I- + 2H+ —> 2H20 + I2
Mix known concentrations and volumes of H202, KI and an acid in the presence of starch and a small fixed volume of NA2S2O3
Reaction 2: 2S2O32- + I2 —> 2H20 + I2

25
Q

How does the Isolation Method work?

A
  • More than one reactant
  • Leads to pseudo-zero/first/second order reaction
  • Keep the concentration of all reactants constant except one by adding large excess to the other reactants, able to determine the order of reaction with respect to that reactant.
26
Q

How to keep the concentration of a reactant constant?

A
  1. By adding the reactant in large excess
    If a reactant is in large excess, only a small fraction of it will be reacted, leaving its concentration relatively constant.
  2. By regenerating the reactant (Catalyst)
    e.g Iodine Clock Reaction
    Na2s2o3 will reduce I2 product back into I-. Concentration of I- in reaction mixture is kept relatively constant.
    Rate = k’ [H2O2] where k’= k[I-]
    Conc of catalyst is constant as it is regenerated.
27
Q

Define order of reaction.

A

Rate = k[A}^n

The order of reaction with respect to A is n.

28
Q

What is a catalyst?

A

A catalyst is a substance that increases the rate of reaction by providing an alternative reaction mechanism of lower activation energy without itself undergoing any permanent chemical change.

29
Q

What is autocatalysis?

A

A reaction is said to be autocatalysed if the intermediate or product can act as a catalyst for the reaction.

H2 CHEMISTRY SG (1 decks)

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