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Chemistry > Energetics, Equilibria and Kinetics > Flashcards

Energetics, Equilibria and Kinetics Flashcards

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1
Q

Exothermic

A

Reaction that releases more energy during bond making than it takes on for bond breaking

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2
Q

Endothermic

A

Reaction that requires more energy for bond breaking than it releases during bond making

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3
Q

Standard conditions

A

100kPa

298K

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4
Q

Specific Heat Capacity

A

Amount of heat needed to raise the temperature of 1g of a substance by 1K

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5
Q

Standard Enthalpy Change of Neutralisation

A

Enthalpy change when one mole of water is formed in a reaction between acid and alkali under standard conditions

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6
Q

Hess’ Law

A

Enthalpy change for a reaction is independent of the route taken

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7
Q

First Law of Thermodynamics

A

Energy can be neither made nor destroyed only converted from one form into another

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8
Q

Factors affecting bond dissociation enthalpy

A

Directly proportional to bond strength and inversely proportional to bond length

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9
Q

State why standard enthalpy of formation for oxygen is quoted as zero

A

All elements in their standard states have a standard enthalpy of formation of zero, as there is no change involved in their formation

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10
Q

Bond dissociation enthalpy

A

Enthalpy change required to break a given covalent bond in a particular molecule in the gaseous state

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11
Q

Mean bond enthalpy

A

Enthalpy change required to break a given covalent bond averaged over a range of different molecules all in the gaseous state

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12
Q

Describe steps to determine an accurate minimum/maximum temperature for an enthalpy change

A
  • start timer
  • record temperature every 30 seconds
  • add reactant
  • plot a graph of temperature against time
  • extrapolate minimum temperature to the point reactant is added
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13
Q

Explain why the value for enthalpy of formation obtained using mean bond enthalpies is different to the actual value

A

Mean bond enthalpies are not SAME as actual as they are averaged over a different molecules and bond dissociation enthalpy varies between molecules

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14
Q

Explain how enthalpy change will differ if referred to the formation of water in liquid state as oppose to gaseous

A

Value will be more exothermic since energy condensation is an exothermic process/energy is released to condense water vapour

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15
Q

Enthalpy of Formation

A

Enthalpy change when one mole of a compound is formed from its constituent elements with reactants in products in their standard states under standard conditions

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16
Q

Enthalpy of Combustion

A

Enthalpy change when one mole of a substance completely burns in oxygen under standard conditions

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17
Q

Suggest why calculated value for enthalpy change of combustion differs from the actual value

A
  • Heat loss to surroundings
  • Incomplete combustion
  • Heat capacity of apparatus
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18
Q

Equilibrium

A
  • Concentration of reactants and products does not change
  • Rate of forwards reaction = Rate of backward reaction
  • Closed system
  • Conditions do not change
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19
Q

Le Châtelier’s Principle

A

For any factor that changes a system at equilibrium the position of equilibrium will shift to oppose that change

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20
Q

Describe how increasing temperature affects the position of equilibrium

A

Position of equilibrium shifts to favour endothermic reaction to decrease temperature

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21
Q

Describe how increasing pressure affects the position of equilibrium

A

Position of equilibrium will shift to the side with fewer moles of gas to decrease pressure

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22
Q

Describe how catalysts affect position of equilibrium

A

Have no effect because they increase the rate of the forward and backward reaction by the same amount

23
Q

State why volume does not need to be known to calculate Kc for certain reactions

A

Volume cancels in Kc equation / same number of moles on either side of the equation

24
Q

Dynamic

A

Ongoing/Continuos

25
Q

Homogenous

A

All reactants are in the same phase

26
Q

Describe how to calculate Kc value for the reverse reaction if Kc for forward reaction is known

A

Reciprocal of Kc value for forward reaction

27
Q

Suggest how you can tell from equations that a substance is behaving as a catalyst

A
  • substance is unchanged

- offers alternative route (forms intermediate species)

28
Q

Activation energy

A

Minimum energy (1) required to break bonds for a reaction to occur (1)

29
Q

Suggest why at a constant temperature most molecules eventually react

A

Some particles always have energies higher than the activation energy (1) and molecules gain energy due to collisions (1)

30
Q

Catalyst

A

A substance that increases the rate of a chemical reaction (1) without being permanently changed or used up (1)

31
Q

Describe how a catalyst works

A

Catalyst lowers the activation energy (1) by providing an alternative route for the reaction (1) so that more particles have energies higher than the activation energy (1)

32
Q

Suggest a way collision frequency between particles can be increased without changing temperature

A

Increase concentration/ pressure

NOT catalyst because they ONLY cause more energetic collisions not a higher frequency

33
Q

Suggest why a small increase in temperature leads to a large increase in reaction rate

A

Many more particles have energies higher than the activation energy

34
Q

Explain why reactions occur faster at high temperatures

A

More molecules have energies higher than the activation energy (1) causing more energetic collisions (1) so there is a higher frequency of successful collisions (1)

35
Q

Give two requirements for a reaction to occur between gaseous molecules

A

Collision
Sufficient energy (higher than activation energy)
Correct orientation collision (last option)

36
Q

Disadvantages of high pressure/temperatures

A

Expensive to generate

High cost of plant (to resist high temperature /pressure, i.e. strong pressure vessel and equipment)

37
Q

Suggest why operating temperature increases without the use of a catalyst

A

Activation energy is higher than with catalyst (1) so temperature increased so more molecules have energies higher than activation energy (1)

38
Q

Suggest how yield can be increased without altering temperature or pressure

A

Recycle reactants

39
Q

Explain why rate of reactions decrease over time

A

Reactants are used up so lower frequency of collisions per unit time

40
Q

Transition state

A

some bonds are in the process of being made while others are being broken

41
Q

Suggest why Maxwell-Boltzmann distribution is asymptotic

A

No particles are zero energy and there is no maximum energy

42
Q

Suggest why catalysts are used in the form of a gauze

A

large surface area

cheaper (since gauze is thin wire mesh)

43
Q

Rate of reaction

A

Change in concentration over time

44
Q

Reasons percentage yield is lower than expected

A
  • reversible reaction
  • product lost in isolation process
  • product lost in other reactions
45
Q

Explain why reactions occur slow at lower pressures

A
  • molecules are spread further apart

- lower frequency of successful collisions between gas particles

46
Q

How to write Kc expression for heterogenous reaction

A

Do not include solids in Kc expression since its concentration remains constant

47
Q

How does changing conditions affect the value of Kc

A
  • changing concentration or addition of catalyst has no effect
  • temperature changes value of Kc
48
Q

Explain why increasing temperature affects the value of Kc but increasing concentration of reactants or using a catalyst does not

A
  • increasing temperature favours endothermic reaction (in order to decrease temperature) so concentration of products and reactants changes, hence value of Kc changes
  • increasing concentration of reactants means concentration of products must increase to oppose change so ratio of products to reactants is unchanged, hence value of Kc remains constant
  • catalyst increases rate of forward and backward reaction by same amount so concentration of products and reactants is unchanged, hence Kc remains constant
49
Q

Enthalpy Change

A
  • change in heat energy

- measured at constant pressure

50
Q

Suggest why you cannot measure enthalpy change of salt to form hydrated salt directly

A
  • difficult to measure temperature of solid
  • difficult to prevent solid from dissolving
  • impossible to add precise amount of water
51
Q

Explain what happens to the most probable energy and number of molecules that have the most probable energy when temperature increases or decreases

A

temperature increases

  • moves to right
  • fewer molecules have most probably energy

temperature decreases

  • move to left
  • more molecules have most probable energy
52
Q

Explain how to work out initial rate of reaction from measuring volume of product over regular intervals of time

A
  • plot graph of volume against time

- draw tangent and work out gradient at beginning

53
Q

Suggest why you cannot measure the enthalpy change of the following reaction directly C(s) + 1/2O2(g) -> CO(g)

A

difficult to prevent complete combustion

temperature of gases can be measured directly so ignore

Chemistry (10 decks)

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