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Georges chem > enthalpy chp 9 > Flashcards

enthalpy chp 9 Flashcards

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1
Q

what is enthalpy

A

enthalpy (H) is a measure of the heat energy in a chemical system.

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2
Q

what is meant by a chemical system

A
  • the chemical system refers to the atoms, molecules, or ions making up the chemicals of the reactants and products
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3
Q

can enthalpy be measured

A

enthalpy cannot be measured but enthalpy changes can

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4
Q

what does a negative enthalpy value tell us

A
  • a negative enthalpy change value means the reaction is exothermic because the energy that was in the chemical system has been transferred to the surroundings in the form of heat this causes the surroundings to heat up.
  • energy has been lost giving a negative value.
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5
Q

what does a positive enthalpy change value tell us

A
  • energy from surroundings drawn into chemical system
  • make surroundings colder.
  • more energy in system than before so gives positive value
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6
Q

what is the law of conservation of energy

A

the law of conservation of energy states that energy cannot be created or destroyed.

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7
Q

what are the features of exothermic reactions

A
  • heat out of the system
  • the energy transferred from the system = energy transferred to the surroundings
  • enthalpy change is negative
  • the temperature of the surroundings increase
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8
Q

what does a exothermic enthalpy profile diagram look like

A
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9
Q

What are the features of endothermic reactions

A
  • heat into the system
  • energy transferred from the surrounding = energy transferred to the system
  • enthalpy change is positive
  • the temperature of the surroundings decreases as it lose energy
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10
Q

what does a endothermic enthalpy profile diagram look like

A
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11
Q

what is activation energy

A
  • the energy input required to break bonds in reactants
  • activation energy is the minimum energy required for a reaction to take place
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12
Q

would reactions with lower or higher activation energy occur quicker

A
  • reactions with lower activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings,
  • very large activation energies may take place extremely slowly or not at all.
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13
Q

what is meant by standard enthalpy changes

A

the enthalpy change that occurs during a reaction under standard conditions

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14
Q

what are the units and values for standard conditions

A
  • standard pressure is 100 kPa ( 1 atmosphere is 101 kPa)
  • standard temperature is 298k (25 °C)
  • standard concentration is 1 mol dm-3 (only relevant for solutions)
  • standard state is the physical state of a substance under standard conditions
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15
Q

what is the enthalpy change of reaction

A
  • the enthalpy change that accompanies a reaction in the molar quantities shown in a stated chemical equation under standard conditions
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16
Q

what is the enthalpy change of formation

A

The standard enthalpy change of formation is the enthalpy change that takes place when 1 mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

17
Q

what is the enthalpy change of combustion

A
  • the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions
18
Q

what is the enthalpy change of neutralisation

A
  • the energy change that accompanies the reaction of an acid by a base to from 1 mole of H20 under standard conditions

the value of enthalpy change of neutralisation is the same for all neutralisation reactions
- 57 Kj mol^-1

19
Q

What does a 1k rise in temperature relate to in °C

How do you convert from degrees to kelvin

A
  • A 1 k rise in temperature is the same as a 1°C rise in temperature
  • °C + 273 = K
  • K - 273 = °C
20
Q

what is the formula for calculating energy change

A

q = energy change
m = mass of substance that is being heated
c = specific heat capacity (4.18 Jg^-1K^-1)
T = temperature

21
Q

what is meant by specific heat capacity

A

the specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1 k

22
Q

what things could make a enthalpy change of combustion value less accurate

A
  • heat loss to the surroundings other than the water
  • incomplete combustion of fuel (lower heat given off than complete combustions)
  • evaporation of fuel from the spirit burner wick (effect calculation made at the end)
  • non standard conditions, the conditions for the experiment are unlikely to be identical to standard conditions

all but the last of these reasons would result in a less exothermic value than expected

23
Q

what could be done to reduce the effect of the errors on the enthalpy change of combustion value

A

use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion

24
Q

how would you correct for cooling on a cooling curve

A

extrapolate the cooling curve section of the graph back to the time when the substance was added

25
Q

what is average bond enthalpy

what are some important points to remeber about average bond enthapy

A

average bond enthalpy is the energy required to break 1 mole of a specified type of bond is a gaseous molecule

-energy is always required to break bonds
-bond enthalpies are always endothermic
-bond enthalpies always have a positive enthalpy value

26
Q

what are the limitations of average bond enthalpies

A

The actual bond enthalpy can vary depending on the chemical environment of the bond

27
Q

is bond breaking endo or exothermic

A

-energy is required to break bonds and so bond breaking is endothermic
-△H is positive

28
Q

is bond forming endo or exothermic

A

-energy is released when bonds form making bond forming exothermic
-△H is negative

29
Q

what determines if a reaction is endo or exothermic

A

The difference between the energy required for bond breaking and the energy released by bond making determines wether a reaction is exothermic or endothermic

30
Q

how can average bond enthalpy be used to calculate enthalpy change of reaction

A
31
Q

what does hess’s law state

A

hess’s law states that, if a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.

comes from the idea of conservation of energy

32
Q

what are the formulas for calculating the enthalpy change of reaction from enthalpy change of formation and reaction

A

Georges chem (29 decks)

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