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chemistry > module 5 - physical chemistry and transition elements > Flashcards

module 5 - physical chemistry and transition elements Flashcards

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1
Q

what is short hand for concentration of reactant ‘A’

A

[A]

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2
Q

what is the correlation between rate and concentration

A

directly proportional

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3
Q

define zero order

A

when the concentration has no effect on rate of reaction

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4
Q

in a zero order reaction:

A
  • concentration of a reactant is to the power of 0
  • any number raised to the power of 0 =1
  • doesn’t appear in the rate equation
  • concentration does not influence the rate of reaction
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5
Q

in a first order reaction:

A

if the concentration of A is doubled (x2) the reaction rate is increased by a factor of 2^1
(directly proportional)

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6
Q

in a second order reaction:

A

if the concentration is of A is doubled, the reaction rate increases by a factor of 2^2 = x4

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7
Q

what is the rate equation

A

rate = K [A]^m [B]^n

k = rate constant
[ ] = concentration
m&n are orders

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8
Q

what is the trend in concentration-time graphs for:
zero order
first order
second order

A

zero: straight line with negative gradient
first order: downward curve with a decreasing gradient over time - half life is constant
second order: also a downward curve but steeper at the start and tails off more slowly

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9
Q

define exponential decay

A

first order reactions have a constant half life (time taken for half a reactant to be used up)

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10
Q

what are the trends in rate-concentration graphs for:
zero order
first order
second order

A

zero: straight horizontal line with zero gradient
first: straight line directly proportional
second: upward curve with increasing gradient

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11
Q

how can you find the rate constant (k) from rate-concentration graphs for:
zero order
first order
second order

A

zero - y intercept
first: gradient
second: cannot be determined

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12
Q

how do you work out the initial rate of reaction from a concentration-time graph

A

draw a tangent at T=0 and calculate the gradient

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13
Q

what does the rate equation only include?

A

reacting species involved in the rate-determining step.

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14
Q

what is the Arrhenius equation? And what do the letters stand for?

A

k = A e^(-Ea/RT)
k = rate constant
A = frequency factor
Ea = activation energy
R = gas constant (8.314)
T = temperature (Kelvin)

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15
Q

what is the logarithmic form of the Arrhenius equation

A

Ln k = - Ea/R (1/T) + Ln A

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16
Q

why is the logarithmic form of the Arrhenius equation useful?

A

so we can determine both Ea and A to be determined graphically

a plot of Ln k against 1/T gives a straight line graph in the form of y=mx+c
gradient m = -Ea/R

intercept c of Ln A on y axis

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17
Q

Bromine reacting with an excess of methanoic acid
Suggest how the concentration of the bromine could be monitored

A

Measure the reduction of colour of bromine (turns from brown-orange to colourless)

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18
Q

define closed system

A

a system in which reactants and products cannot be added or removed once the reaction begins

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19
Q

define dynamic equilibrium

A

the rate of the forward reaction is the same as the rate of the backwards reaction

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20
Q

define the haber process

A

the industrial process of making ammonia from nitrogen and hydrogen (N2g + 3H2 <–> 2NH3

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21
Q

list 5 reversible reactions

A
  1. haber process (N2 + 3H2 <–> 2NH3)
  2. disassociation of acids (HA <–> H+ + A-)
  3. combustion of glucose (C6H12O6 +6O2 <–> 6CO2 +6H2O)
  4. haemoglobin + oxygen (Hb + O2 <–> HbO2)
  5. ammonium chloride (NH4 + Cl <–> NH4Cl)
22
Q

when can dynamic equilibrium happen?

A

in a closed system in a reversible reaction

23
Q

how do catalysts alter equilibrium position

A

allows reaction to reach equilibrium faster, economically valuable

24
Q

what is Le Chatlier’s principle?

A

when a system at equilibrium is subject to change, the system moves to minimise the effects of change

25
Q

in terms of equilibrium how do reversible reactions react to changes in concentration

A

increased concentration: shifts equilibrium to the direction that produces less of that substance

26
Q

in terms of equilibrium how do reversible reactions react to changes in temperature

A

increased temperature, shifts equilibrium in the direction that takes in heat (endothermic)

27
Q

in terms of equilibrium how do reversible reactions react to changes in pressure (for gases)

A

increase in pressure shifts equilibrium in the direction that produces fewer moles of gas

28
Q
A
29
Q

What goes at the top of the equilibrium constant expression calculation?

A

The products

30
Q

What does a small Kc value indicate

A

The concentration of the reactants is larger than the products

31
Q

What substances should not be included in the expression for Kc

A

Solids

32
Q

What factor could permanently change the value for Kc and why?

A

Temperature because a temperature change will favour one direction increasing the concentration of either products or reactants

33
Q

What does a large Kc value indicate

A

There are more products than reactants

34
Q

Define heterogeneous in rate equations

A

Containing species in different states/ phases

35
Q

What does a Kc value of exactly 1 suggest

A

The equilibrium lies directly between products and reactants

36
Q

Why are aqueous species considered homogenous in liquid water

A

Because they are in a single uniform phase

37
Q

Define Kp

A

Equilibrium constant when all products and reactants are gases

38
Q

What are the 4 steps to calculate Kp

A
  1. Total moles of gas (products and reactants)
  2. Mole fraction of each gas
  3. Partial pressure of each gas
  4. Kp - (products/ reactants)
39
Q

What is different about the equilibrium equations for Kc and Kp

A

Kc contains square brackets to show concentration [ ]

Kp contains round brackets ( )

40
Q

define partial pressure and how do you work it out?

A

Partial pressure (p) A measure of a gas’s contribution to the total pressure exerted(P)

p(A) = x(A) x P

(Mole fraction x total pressure)

41
Q

Why is there a direct relationship between Kp and Kc

A

Because gas concentration and pressure are proportional to each other

42
Q

What does a value of Kp smaller than 1x10^ -2 suggest?

A

Equilibrium lies well within the reactants
Reactants are greater than the products

43
Q

What is the expression for determining mole fraction?

A

Mole fraction x(A) = moles of A/ total moles of gas

44
Q

What is meant by continuous monitoring

A

Data is collected throughout the reaction taking place

45
Q

What is used to monitor the change of a coloured solution during a reaction

A

Colorimeter

46
Q

How do you work out if half life is constant from a graph

A

Work out how long it takes for concentration to half
Then work out how long it takes for concentration to half again

47
Q

How do zero order and first order concentration-time graphs differ

A

Zero order are a straight line with negative correlation whereas first order are downward curves

48
Q

For a first order reaction what is the relationship between rate constant and half-life

A

k= ln2/ (t 1/2)

49
Q

How do zero and first order rate-concentration graphs differ?

A

Zero order are horizontal but first order are directly proportional

50
Q

Explain why, when hydrogen peroxide, acid and iodine are mixed together in a thiosulfate and starch solutions, the tubes go blue after a period of time rather than gradually from the start

A

Thiosulfate reacts with iodine until thiosulfate used up

Only then will the iodine react with starch and produce the blue-black colour

51
Q

A student says chemical reactions go faster at higher temperatures because the molecules have more frequent collisions
Evaluate this statement

A

Student is partially correct as higher temperatures increase the kinetic energy of the molecules resulting in more frequent collisions

However, collisions are not the only factor

chemistry (9 decks)

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