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Alevel Chem > 18 Kinetics + Kp > Flashcards

18 Kinetics + Kp Flashcards

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1
Q

Define reaction rate

A

change in concentration of reactant or product per unit time

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2
Q

What is zero order?

A

the concentration has no effect on the rate

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3
Q

What is first order?

A

the concentration is directly proportional to the rate
eg. if the conc doubles, the rate also doubles

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4
Q

What is second order?

A

the concentration squared is proportional to the rate
eg. if the conc doubles, the rate is x4

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5
Q

Define the overall order

A

the sum of orders of each reactant

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6
Q

Rearrange the rate equation to find k:
rate = k [A][B]

A

k = rate/ [A][B]

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7
Q

What are the units for rate?

A

mol dm-3 s-1

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8
Q

What happens to the rate constant (k) if you increase the temperature?

A

k also increases - more frequent collisions, increasing the energy of each collision. The conc and orders remain the same, so k must increase for the rate equation to balance.

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9
Q

Why do small changes in temperature produce large changes in reaction rates?

A

As temp increases, there are more successful collision, and a greater proportion of molecules have enough energy to react. Particles only react if collisions have sufficient energy to start bond breaking.

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10
Q

What is the rate determining step?

A

The slowest step in a multi-step reaction. If a reactant doesn’t appear in the rate equation, it won’t be involved in the rate determining step.

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11
Q

What does the Arrhenius Equation show?

A

Shows how rate constant varies with temperature and activation energy.

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12
Q

What is the Arrhenius Equation?

A

k = Ae^-Ea/RT

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13
Q

What is the simplified form of the Arrhenius Equation?

A

lnk = lnA - Ea/RT
(natural log = opposite of exponential)

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14
Q

What happens to k if you increase Ea?

A

Decreases - less particles with sufficient energy for a collision to occur.

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15
Q

What happens to k if you increase T?

A

Increases - particles have more kinetic energy, so higher chance of successful collisions.

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16
Q

Rearrange the Arrhenius Equation to make Ea the subject.

A

Ea = RT x ( LnA - Lnk)

17
Q

Rearrange the Arrhenius equation to make T the subject.

A

T = Ea/R x (LnA - Lnk)

18
Q

In the Arrhenius equation, what do k, Ea, T and R represent? (with units)

A

k = rate constant
Ea = activation energy (J/mol)
T = temperature (K)
R = gas constant (8.31 JK-1mol-1)

19
Q

What is the equation for the gradient?

A

gradient = change in y
/ change in x

20
Q

Describe how you would find Ea from a graph of Ln k against 1/T.

A
  1. Find the gradient by drawing a tangent
  2. Multiply the gradient by the gas constant 8.31.
21
Q

Describe how you would find ln A from a graph of ln k against 1/T.

A

Substitute the gradient and coordinates of any point on the line into ln k = -Ea/RT + ln A, and rearrange.

22
Q

Define the total pressure of a gas.

A

The sun of all partial pressures of the individual gases.

23
Q

What is a mole fraction?

A

The proportion of a gas mixture that is made up of a particular gas.

24
Q

Equation to calculate mole fraction.

A

moles of substance A / total moles of all substances
*sum of all mole fractions = 1

25
Q

Equation to calculate partial pressures.

A

mole fraction of A x total pressure
*sum of partial pressures = total pressure

26
Q

What is Kp?

A

The equilibrium constant for a reversible reaction where all reactants and products are gases.

27
Q

Write a Kp expression for the equilibrium:
aA + bB <===> dD + eE

A

Kp = (pD)d(pE)e / (pA)a(pB)b

28
Q

What happens to the equilibrium if you increase / decrease temperature?

A

Increase: shifts in the endothermic direction (positive H)
Decrease: shifts in the exothermic direction (negative H)

29
Q

Is Kp affected by temperature?

A

Yes - changing temp changes how much product is formed at equilib. This changes the mole fractions of gases present, which changes their partial pressures.

30
Q

Is Kp affected by pressure?

A

No - the equilibrium will shift to keep it the same.

31
Q

Is Kp affected by a catalyst?

A

No - it just gets the system to equilibrium more quickly.

Alevel Chem (2 decks)

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