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Module 5 (18-24) > Chapter 18 > Flashcards

Chapter 18 Flashcards

1
Q

What are the units for rate of reaction?

A

mol dm-3 s-1

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2
Q

How can you measure rate of reaction?

A

Measuring the decrease in the concentration of a reactant over time
or Measuring the increase in the concentration of a product over time

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3
Q

How is rate of reaction written?

A

rate =k [A] [B]

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4
Q

What does the order of a reactant show?

A

shows how the concentration of a chemical, typically a reactant, affects the rate of reaction

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5
Q

What does it mean when the order is 0?

A

Changing the concentration of the chemical has no effect on the rate of the reaction

Therefore, it is not included in the rate equation

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6
Q

What does it mean when the order is 1?

A

The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction

The chemical is included in the rate equation

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7
Q

What does it mean when the order is 2?

A

The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four

The chemical is included in the rate equation (appearing as a squared term)

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8
Q

What is the overall order?

A

the sum of the powers of the reactants in a rate equation

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9
Q

How do you calculate orders from data?

A

Identify two experiments where the concentration of the reactant you are looking at first changes, but the concentrations of all other reactants remain constant

Repeat this for all of the reactants, one at a time, until you have determined the order with respect to all reactants

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10
Q

How can the rate constant (K) be calculated?

A

using the initial rates and the rate equation - You rearrange the rate equation so rate over the initial rates = k

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11
Q

How does a 0 order Concentration time graph look?

A

Straight diagonal line from top left to bottom right.

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12
Q

What does the gradient mean in a 0 order Concentration-Time graph?

A

The gradient = rate of reaction

since its 0 order, the reactant has no effect on the rate of the reaction, so the rate = the rate constant (K)

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13
Q

How does a 1 order Concentration time graph look?

A

The graph is a curve going downwards and eventually plateaus.

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14
Q

How does a 2 order Concentration time graph look?

A

The graph is a steeper curve going downwards

because the concentration of the reactant decreases more steeply with time.

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15
Q

How can the order of reaction be deduced from a reactions half life?
(explain how 0,1 and 2 would look)

A

For a zero-order reaction the successive half-lives decrease with time (less time for concentration of reactant to halve)

The half-life of a first-order reaction remains constant throughout the reaction (same time for concentration to halve)

For a second-order reaction, the half-life increases with time (takes more time for concentration to halve)

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16
Q

How does a 0 order rate-concentration graph look?

A

The rate doesn’t depend on the concentration of the reactant- The rate of the reaction, therefore, remains constant throughout the reaction.

The graph is a horizontal line

rate = K

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17
Q

How does a 1 order rate-concentration graph look? How do you determine the rate of reaction?

A

Rate of the reaction increases as the concentration of the reactant increases.

Straight diagonal line.

rate = k[A]

gradient = rate of reaction

18
Q

How does a 2 order rate-concentration graph look?

A

The rate is directly proportional to the square of concentration of a reactant.

Curved line going up.

rate = k[A]2

19
Q

What does initial rate determine?

A

The rate at the very start of a reaction when t=0

20
Q

How do you find initial rate?

A

Drawing a concentration-time graph
Adding a tangent at t = 0
Calculating the gradient of the tangent

21
Q

What are clock reactions used for? How do they work?

A

Measuring the initial rate of reaction using a single measurement.

The time taken, t, for a specific visual change in the reaction to occur is measured

These changes could be a colour change or formation of a precipitate

22
Q

What is the assumption that a clock reaction depends on?

A

There is no significant change in the rate of reaction between the start of the reaction and the time when the measurement is taken.

23
Q

What is initial rate proportional to?

A

1/t

24
Q

How do you calculate rate constant from the half life?

A

k = ln2 / t1/2

rate constant = ln(2) over t half life.

25
Q

How does the iodine clock reaction work?

A

In each experiment, the solution is colourless at the start and the time t
is measured for the blue-black colour of the starch-iodine to appear. (starch is added to make the blue/black iodine is normally orange-brown)

The initial rate is proportional to 1/t
A graph of 1/t against concentration is then plotted.

Separate experiments are carried out using different concentrations of one of
the reactants and all other concentrations are kept constant. The colour change
is delayed by including a small amount of another chemical which actually removes iodine as it forms.
As soon as this chemical is all used up, the blue-black colour appears.

26
Q

What is the accuracy of a clock reaction dependant on?

A

The actual time measured, the longer it takes the less accurate the average rate of change will be. The rate will have changed over that period (so it changes less over a shorter period.)

27
Q

How does continuous monitoring work? (to investigate reaction rates)

A

collecting experimental data throughout the course of a reaction to plot a concentration-time graph .

Measuring the volume / amount of gas evolved over time

Measuring the mass of reactants lost over time

28
Q

How is colorimetry used to investigate reaction rates?

A

It measures colour absorbance which is proportional to the concentration of the coloured species.

Firstly you need to measure the absorbance of a set of standard solutions of iodine and obtain a calibration curve.

By calibrating the colorimeter with solutions of known concentration we can use the readings from this device to measure the concentration at a particular time. (draw tangents and work out the rate of reaction).

29
Q

What is the rate determining step?

A

The slowest step of a reaction.

(A chemical reaction can only go as fast as the slowest part of the reaction)

30
Q

Do the reactants of the slow or fast step appear in the rate equation?

A

If a reactant appears in the rate-determining step (slow step), then the concentration of that reactant will also appear in the rate equation

31
Q

What happens to the rate of reaction if temperature increases?

A

Increasing the temperature of a reaction increases the rate of a chemical reaction.

31
Q

What happens to the rate of reaction if temperature increases?

A

Increasing the temperature of a reaction increases the rate of a chemical reaction.

32
Q

What happens to the rate constant as the rate of reaction increases?

A

As the rate of reaction increases the rate constant will increase.

33
Q

What happens to K as temperature increases?

A

K increases as long as the concentration of the reactants remains unchanged.

34
Q

How many degrees increases does it take for rate of reaction to double?

A

approx 10 degrees. (The number of degrees needed to double the rate also changes gradually as temperature increases)

35
Q

What causes the increase of temperature to result in a higher rate/ rate constant.

A

Increasing temperature causes the particles to move around faster resulting in more frequent collisions

Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction.

36
Q

Does k change when the concentration of the reactants is changed?

A

The rate constant, k only remains constant if the concentration of the reactants is the only factor which is changed

37
Q

What equation shows the relationship between k, temperature and activation energy?

A

the Arrhenius equation

38
Q

What is the Arrhenius equation?

A

K= Ae ^ -Ea/RT

K= rate constant. A= Arrhenius Constant
R = Gas constant (8.31 JK-1mol-1)
T= Temperature (K)
e= mathematical constant

39
Q

What’s the easier way to use the Arrhenius equation?

A

Ln k = ln A - Ea/RT

40
Q

How can the Arrhenius equation be used in a graph?

A

Plot ln K against 1/t. You can use this to calculate Ea.

The gradient = -Ea/R (so if you times it by 8.31 you will get the activation energy)

The y intercept = ln A

if you write it in the form of y = mx + c
ln K = -Ea/R 1/t + ln A

Module 5 (18-24) (7 decks)

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