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Chemistry Year 13 > Rate Equations > Flashcards

Rate Equations Flashcards

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1
Q

What is the definition of the rate of a reaction?

A

The change in molar concentration of a substance in a set time, usually one second

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2
Q

What are the units of rate of reaction if a reaction takes place in a solution or is a homogeneous gas reaction?

A

Mol dm-3 s-1

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3
Q

What is the equation for working out rate?

A

Rate = change in concentration/ change in time

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4
Q

What is the concentration of a substance represented by?

A

Writing the formula of the substance inside square brackets

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5
Q

How can we measure the rate of change of concentration of substances?

A

By measuring the concentration of substances at time intervals at a fixed temperature

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6
Q

What is the order of reaction?

A

The proportionality of the rate to the concentration of reactants where rate = k[A]’x (to the power of x) and x is the order

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7
Q

What is a first order reaction and how would you write it?

A

It is a reaction whose rate is proportional to the concentration of a reactant and the reaction is said to be first order with respect to reactant A

Rate = k[A]’

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8
Q

What is a reaction that is second order with respect to a reactant and how would you write it?

A

Where the rate of the reaction is proportional to the concentration of the reactant squared

Rate = k[B]’2

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9
Q

What is a reaction that is zero order with respect to a reactant and how is it written?

A

One where the rate of the reaction does not change when the concentration of rhe reactant changes

Rate = k[C]’0

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10
Q

How is the overall rate equation formed?

A

The individual effects of different reactants in rate are combined to form the overall rate equation

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11
Q

How do you find the overall order of a reaction?

A

By adding together the orders of the reaction with respect to each reactant

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12
Q

What is a general rate equation for two reactants?

A

Rate = k[A]’m[B]n

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13
Q

What is K in the rate equation?

A

The rate constant

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14
Q

Why would a reactant not appear in the rate equation?

A

In the rate is zero order with respect to that particular reactant

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15
Q

What is the rate constant affected by and how?

A

The temperature. As the temperature increases both the rate of reaction and rate constant increase

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16
Q

How do you calculate the units of the rate constant?

A

By using the rate equation. The units of k can be calculated by replacing each of the terms in the rate equation by their units and then cancelling out duplicated units

17
Q

What do the graphs look like for zero order, 1st order and 2nd order reactions with concentration against time?

A
  • Zero order = straight line down
  • first order = very curved line
  • second order = slightly less curved line

Check snap camera roll

18
Q

What happens as the reactant is used up in first, second and zero order reactions?

A
  • zero order the reaction continues at a constant rate as the reactant doesn’t affect the rate
  • first and second order the rate of reaction becomes slower and the gradient decreases
19
Q

How can the rate constant be determined from a zero order graph?

A

By the gradient of a graph of the concentration of that reactant against time

20
Q

How can you determine whether a reaction is first or second order?

A

The rate must be determined at different concentrations or reactants and and a graph of the rate must be plotted against concentration of one reactant. The shapes of these graphs are very distinctive

21
Q

What do the graphs look like for zero, first and second order reactions for rate against concentration?

A
  • zero order = straight line with a zero gradient
  • first order = diagonal line with a positive gradient
  • second order = curved line inwards

Check snap camera roll

22
Q

How do you work out the rate constant from graphs for first and second order reactions?

A

Use a rate against concentration graph and work out the gradient

23
Q

What are the two ways of plotting a graph of concentration against rate?

A
  • the continuous monitoring method

* the initial rate method

24
Q

What is the continuous monitoring method?

A
  • If the concentration of a reactant remaining in a solution can be measured at set time intervals then a single reaction can be monitored.
  • we plot a veaph of concentration against time and draw the line of best fit
  • then choose a number of points on the line of best fit and draw tangents at each of them. The gradient of each tangent is used to determine the rate ar that point
  • for each of our chosen points we now have the rate (tangent) and concentration
  • the rate and the concentration of the reactant at each point are then drawn in another graph which can be used to determine the order of reaction
25
Q

What is the initial rate method

A
  • Used when the concentration of a reactant remaining in a solution cannot be measured at set time intervals
  • the reaction is carried out multiple times in seperate experiments each with a different starting concentration of a reactant.
  • the rate of reaction is followed by measuring the amount or concentration of product (not reactant). A graph of the amount of product against time is plotted for each experiment
  • as we know the initial concentration of the reactant for each experiment we measure the rate for the first few seconds (the gradient of a tangent drawn to the line). This determines the initial rate of reaction for each experiment at the start when the concentration of the reactant is known
26
Q

How else can the order of a reaction be determined without a graph?

A

From sets of data involving the initial rate of a reaction

27
Q

What are the steps for working out the order of a reaction from sets of data involving the initial rate of reaction?

A

• step 1: identify a pair of experiments where the concentration of the reactant you are looking at changes
• step 2: work out what factor the concentration has increased or decreased by
• step 3: work 9ut what factor the rate has increased or decreased by
• step 4: compare the factors that tbe concentration and the rate have changed by:
- zero order reactions: the factor that the rate changes bt is zero
- first order reactions: both the concentrarion and the rate change by the same factor
- second order reactions: the favtor that the rate changes by is the factor that the concentration changes by squared

28
Q

How do you work out the order if there’s not suitable pairs of experiments in the data?

A

If you know the order of one reactant and the rate you can rearrange the rate equation to find it

E.g. y is first order

Rate = [y]1 [x]?

Rate/ [y]1 = [x]?

29
Q

What is the rate determining step?

A

The slowest step in a series of intermediate stages that determines the overall rate of the chemical reaction

30
Q

What are the rules with the intermediate steps and the species involved in the rate equation?

A
  • species that are only involved in steps after the rate determining step do not appear in the rate equation
  • species that appear in the steps up to and including the rate determining step are in the rate equation
  • a substance that acts as a catalyst will appear in the rate equation but not in the overall chemical equation for the reaction
31
Q

How do you find the rate equation and what can it be used to deduce?

A

We find the rate equation by experiment and then can use this to deduce possible mechanisms

32
Q

What is the arrhenius equation and what do the different symbols mean?

A

K = Ae ^ - EA/ RT

K= rate constant
A = arrhenius constant
e = mathematical constant with a value of approximately 2.718
EA = activation energy in J mol -1
T = absolute temperature in K
R = gas constant with a value of approximately 8.314 J K-1 mol -1
33
Q

What is the logarithmic form for the arrhenius equation?

A

ln k = lnA - EA/RT

34
Q

What does plotting a graph of ln k against 1/T do?

A

Gives a line of best fit where the gradient is equal to -EA/R and the intercept is equal to lnA

35
Q

What can values of J and T be used for (arrhenius equation)

A

To calculate the activation energy for a reaction

Chemistry Year 13 (16 decks)

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