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A level chemistry > module 3.2 physical chemistry > Flashcards

module 3.2 physical chemistry Flashcards

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1
Q

define enthalpy change (delta H)

A

enthalpy change (delta H) is the heat energy transferred in a reaction at constant pressure. units are KJ mol-1

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2
Q

what are the standard conditions

A

pressure: 100 Kpa
temperature: 298K (25c)

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3
Q

what is delta H when reaction is exothermic

A

exothermic reactions give out energy. delta H is negative

temperature often rises

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4
Q

what is delta H when reaction is endothermic

A

endothermic reactions absorb energy. delta H is positive

temperature often decreases

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5
Q

what is the activation energy Ea

A

is the minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction

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6
Q

standard enthalpy changes

A

enthalpy changes under standard comditions

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7
Q

what is enthalpy affected by

A

temperature and pressure

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8
Q

define: standard enthalpy change of reaction deta(r)H

A

delta(r)H is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions

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9
Q

define: standard enthalpy change of formation

delta(f)H

A

is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

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10
Q

define: standard enthalpy change of combustion

delta(c)H

A

is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions

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11
Q

define: standard enthalpy change of neutralisation

delta(neut)H

A

is the enthalpy change when an acid and an alkali react together, under standard conditions to form 1 mole of water

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12
Q

bond breaking is…

A

endothermic (delta H is positive)

as you need energy to break bonds

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13
Q

forming bonds is…

A

exothermic (delta H is negative)

energy is released

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14
Q

enthalpy change for a reaction is the

A

overall effect of the bond breaking and bond forming

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15
Q

if you need more energy to break bonds than is released when bonds are made

A

delta H is positive.

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16
Q

+ve and -ve ions are attracted to each other in what

A

ionic bonding

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17
Q

the +ve nuclei are attracted to the -ve charge of shared electrons in

A

covalent bonds

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18
Q

the amount of energy you need to break a bond is called what?

A

bond dissociation enthalpy. always in gaseous compounds

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19
Q

average bond enthalpies are not what

A

exact

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20
Q

define: average bond enthalpy

A

the energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

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21
Q

to measure enthalpy change for a reaction what two things must you know

A
  • the number of moles of the stuff thats reacting

- the change in temperature

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22
Q

what is the equation used to calculate enthalpy change

A

q= mc(delta)T
q= heat lost or gained. (in J) same as the enthalpy change is pressure remains constant
m= mass of water in the calorimeter, or solution in the insulated container (in g)
c= specific heat capacity of water 4.18 J g-1 K-1
delta T = the change in temp of water or solution (in K)

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23
Q

state hess’s law

A

the total enthalpy change of a reaction is always the same, no matter which route is taken

24
Q

enthalpy change of reaction =

A

total energy absorbed - total energy released

25
Q

a reaction won’t take place between two particles unless (2)

A
  • they collide in the right direction / orientation. they need to be facing each other the right way
  • the collide with a least a certain minimum amount of kinetic energy
26
Q

the theory of how things react is called

A

collision theory

27
Q

the minimum amount of kinetic energy particles need to react is called the

A

activation energy

28
Q

what does the boltzmann distribution look like

A
  • plots number of molecules agaist kinetic energy to see what proportion of the molecules in a substance have the activation energy of above to react.
29
Q

what happens to the boltzmann distribution curve if you increase the temperature

A
  • a greater proportion of molecules will have at least the activation energy and be able to react. this pushes the curve to the right to have a lower curve
30
Q

how does increasing the temperature change the rate of reaction

A
  • molecules have more energy move faster and collide more often increasing the reaction rate.
31
Q

how does increasing concentration change the reaction rate

A

if you increases conc. of reactants in solution, the particles will be closer together. so they will collide more frequently. if there are more collisions, they have more chance to react. increasing reaction rate

32
Q

how does increasing pressure change the rate of reaction

A

if your reactants are gases, increasing pressure pushes particles closer together, increasing the chance of successful collisions

33
Q

how does the use of catalysts change the rate of reaction

A

catalysts lower activation energy by providing a different pathway. if activation energy is lower more molecules have energy to react and so rate of reaction increases

34
Q

define catalysts

A

a catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. the catalysts is chemically unchanged at the end of the reaction.

35
Q

define a heterogenous catalysts

A

a catalyst that is in a different phase from the reactants. i.e different physical state

36
Q

explain how the Haber process is a example of a heterogeneous catalysts

A

gasses are passed over a solid ion catalyst

reaction happen on surface of the catalyst. so increasing the surface area of the catalyst increases rate of reaction

37
Q

what is the equation for the haber process

A

N2 + 3H2 2NH3

Fe

38
Q

the haber processes is used to make what

A

ammonia NH3

39
Q

define a homogeneous catalysts

A

catalysts that are in the same physical state as the reactants
- usual is an aqueous catalyst for a reaction between two aqueous solution.

40
Q

how do homogeneous catalysts work:

A

work by forming an intermediate species.
the reactants combine with the catalyst to make an intermediate species, which then react to form the products and reform the catalysts

41
Q

why is the ion catalyst so important in the haber process

A

if it wasn’t for the catalyst the temperature would have to be much higher for reaction to happen quick enough
this is expensive and reduce the amount of ammonia produced

42
Q

the use of catalysts is good for the environment because(3):

A
  • lower temps saving energy less CO2 emissions and fossil fuel reserves persevered
  • reduce waste by allowing different reaction to be used with a better atom economy
  • catalytic converters reduce pollution
43
Q

what is the formula for reaction rate

A

rate of reaction = amount of reactant used or product formed / time

44
Q

name the two ways to investigate reaction rates

A
  • change in mass

- volume of gas given off

45
Q

define dynamic equilibrium

A

dynamic equilibrium is where a reversible reaction reaches the point in a closed system where the concentrations of the products and reactions remain constant

46
Q

define le chateliers principle

A

if there is a change in concentration, pressure or temperature the equilibrium will move to help counteract the change

47
Q

what happens if you increase concentration in a dynamic equilibrium

A
  • if you increase the concentration of the reacts eqm wull move to the right to produce more product to keep concentration the same.
  • if you increase conc. of products eqm will move left to produce more reactants
48
Q

what happens when you increase pressure in a dynamic equilibrium

A

only affects gases

- if you increase pressure eqm shifts to the side with fewer gas molecules. thus reducing the pressure

49
Q

what happens if you increases the temperature in a dynamic equilibrium

A
  • if you increase the temperature eqm will shift in the endothermic direction and absorb this heat. reducing the temperature
50
Q

what effect do catalysts have on dynamic equilibrium

A

catalysts have no effect on the position of eqm. they speed up the forward and reverse reactions by the same amount.
this can’t increase yield but they do mean eqm is reached faster

51
Q

the equilibrium constant Kc can be worked out when

A

you have a homogeneous reaction that’s reached dynamic equilibrium

52
Q

Kc tells you what?

A

Kc tells you how far left or right the equilibrium is.

53
Q

what is the expression for equilibrium constant Kc

A

aA + bB dD eE

kc = [D]d [E]e / [A]a [B]b

54
Q

the larger the value of Kc the more

A

products there are at equilibrium so eqm lies to the right

55
Q

the smaller the value of Kc the more

A

reactants there are at eqm. so eqm lies to the left.

56
Q

what is the one thing that can change Kc

A

Kc varies with temperature

A level chemistry (5 decks)

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