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A-level Chem > How fast? > Flashcards

How fast? Flashcards

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1
Q

Definition of rate of reaction

A

The change in concentration, of a reactant or product, in a given time.

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2
Q

What are the key factors that affect the rate of reaction?

A
  1. Concentration
  2. Pressure
  3. Catalyst
  4. Temperature
  5. Surface area
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3
Q

Write the general formula for a rate equation

A

Rate = k[X][Y][Z]

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4
Q

What is an overall equation?

A

This tells us the mole ratios of reactants and products.

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5
Q

What is the rate equation?

A

This tells us about the kinetics of the reaction, i.e. what controls the rate at which the reaction occurs and so how the rate can be increased.

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6
Q

Definition of the rate determining step

A

The slowest step in the reaction mechanism.

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7
Q

Explain how to determine if a reactant is zero order

A

If changing the concentration of a reactant (Z) does not change the rate, this means the reaction is zero order with respect to Z.

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8
Q

Explain how to determine if a reactant is first order

A

If changing the concentration of a reactant (X) changes the rate in the same way (proportionally) this means the reaction is first order with respect to X.

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9
Q

Explain how to determine if a reactant is second order

A

If changing the concentration of the reactant (Y), e.g X2 and the rate (X2)^2 i.e, X4, then this means the reaction is second order with respect to Y.

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10
Q

How to calculate the overall order of the reaction?

A

Total up all the orders of the species in the rate equation

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11
Q

How can a graph be used to calculate the rate at a given time?

A

Draw a tangent to the curve and calculate the gradient of the tangent.

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12
Q

Which equation is used to calculate the gradient of a line?

A

(Difference in Y) / (Difference in X)
(Change in concentration) / (change in time)

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13
Q

What happens to the rate as the reaction progresses?

A

Rtae decreases as the reaction progresses as there are fewer reactant molecules per unit volume so less frequent collisions so fewer successful collisions.

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14
Q

Definition of half-life

A

The time taken for the concentration to fall to half of its orginal value.

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15
Q

Explain the relationship between half-life and a first order reaction

A

the half-life of a first order reaction is constant.
The half-life of a first order reaction is independent of the concentration of the reactant.

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16
Q

How is the initial rate determined of a reaction from a graph?

A

A tangent is drawn at t=0 on a concentration-time graph. Then calculate the gradient of thr tangent.

17
Q

State and explain a method for measuring the initial rate of a reaction

A

Carry out a clock reaction and take a single measurement. Provided there is no significant change in this reaction, it can be assumed that the average rate of reaction over this time will be the same as the initial rate. The intial rate is then calculated as initial rate is proprotional to 1/t. The clock reaction is then repeated several times with different concentrations.

17
Q

State and explain a method for measuring the initial rate of a reaction

A

Carry out a clock reaction and take a single measurement. Provided there is no significant change in this reaction, it can be assumed that the average rate of reaction over this time will be the same as the initial rate. The intial rate is then calculated as initial rate is proprotional to 1/t. The clock reaction is then repeated several times with different concentrations.

18
Q

Describe the result of a common type of clock reaction

A

Iodine is formed- colours the solution brown. Solution will turn blue-black if starch is present. To delay the colour change, a small amount of sodium thiosulfate is used. This initially reacts with the iodine and removes it until all the sodium thiosulfate has reacted. After this, the blue-black colour appears if starch is present and brown if no starch is present.

19
Q

Write an ionic equ for the reaction between sodium thiosulfate and iodine

A

2S2O3^2-(aq) + I2(aq) …. S4O6^2-(aq) + 2I-(aq)

20
Q

What assumption is made for a clock reaction?

A

Assumed that the average rate of reaction over the time of the reaction will be the same as the initial rate. This approximation works providing less than 15% of the reaction has taken place.

21
Q

What equ is used to calculate the rate constant if the half-life is known?

A

k = ln2/0.5T
0.5T is the half life

22
Q

During a reaction, if temperature increases, what happens to the value of k?

A

Increases

23
Q

What is the Arrhenius equ?

A

K = Ae^(-Ea/RT)
Ea = activation energy
R = gas constant (8.314 JK^-1mol^-1)
Units of T = K (C into K: +273)
lnK = -Ea/RT + lnA (This is in the form of the equation of a straight line: y = mx+c and allows Ea and A to be determined graphically).
lnK = y-axis
1/T = x-axis
Ea = -m x R
e^c = A

24
Q

What is A?

A

The exponential factor. This takes into account the frequency of collisions and their orientation, so it varies little with temperature, so can be regarded as constant.

25
Q

What is c^-Ea/RT in the Arrhenius equ?

A

Referred to as the exponential factor. Gives the proportion of the molecules that have energy equal to or greater than the activation energy at a particular temperature. This demonstrates the number of molecules that will have successful collisions.

26
Q

What is the effect of e^(-Ea/RT) calculated at two different temps?

A

The proportion of molecules with energy greater than or equal to the activation energy varies as temperature changes. This will show the effect on the value of k and hence the rate.

27
Q

Draw a general rate-concentration graph for a zero order reaction

A

*Refer to how fast notes

28
Q

Draw a general concentration-time graph for a zero order reaction

A

*Refer to how fast notes

29
Q

Draw a general rate-concentration graph for a first order reaction

A

*Refer to how fast notes

30
Q

Draw a general concentration-time graph for a first order reaction

A

*Refer to how fast notes

31
Q

Draw a general rate-concentration graph for a second order reaction

A

*Refer to how fast notes

32
Q

Draw a general concentration-time graph for a second order reaction

A

*Refer to how fast notes

A-level Chem (30 decks)

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