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Chemistry > 8.1 & 8.2 > Flashcards

8.1 & 8.2 Flashcards

1
Q

what is enthalpy

A

measure of the total energy of a system

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2
Q

what is the enthalpy change

A

the heat energy that is transferred between the system and the surroundings at constant pressure

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3
Q

define exothermic

A

where heat energy is transferred from the system to the surroundings

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4
Q

define endothermic

A

where heat energy is transferred from the surroundings to the system

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5
Q

give 3 examples of exothermic reactions

A

freezing water
combustion
dissolving NaOH in water

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6
Q

give 3 examples of endothermic reactions

A

melting ice
photosynthesis
dissolving ammonium nitrate in water

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7
Q

do exothermic reactions have a positive or negative enthalpy change

A

negative

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8
Q

do endothermic reactions have a positive or negative enthalpy change

A

positive

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9
Q

define standard enthalpy change

A

the enthalpy change measured at 100kPa and a stated temperature (298K) when the number of moles of substances in the equation as written react

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10
Q

define standard enthalpy change of combustion

A

the enthalpy change measured at 100kPa and a stated temperature (usually 298k) when one mole of a substance is completely burned in oxygen

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11
Q

summarise the experiment to determine the enthalpy change of combustion for a liquid

A

weigh a spirit burner with the liquid
add a known volume of water to a copper can
measure the initial temperature of the water
light the spirit burner
stir the water
once the temperature has risen by roughly 20 extinguish the burner and immediately weigh it
measure the final temperature

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12
Q

what is the equation to calculate heat energy (Q)

A

Q = mcT
m - mass of water
c - specific heat capacity (usually 4.18)
T - change in temp

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13
Q

what is the equation for the enthalpy change of combustion (#cH)

A

cH = Q/n

n - moles of ethanol burned

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14
Q

state some sources of error within the experiment for calculating total enthalpy of combustion

A
  • some heat energy produced dissipated to the surroundings not the water
  • incomplete combustion may have occurred
  • some heat energy produced may have been transferred to the copper not the water
  • conditions are not standard
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15
Q

define standard enthalpy change of neutralisation

A

the enthalpy change measured at 100kPa and a stated temperature (usually 298k) when one mole of water is produced by the neutralisation of an acid with an alkali

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16
Q

why do almost all strong acids and alkalis have the same standard enthalpy of neutralisation

A

because they fully ionise in aqueous solution so this reaction occurs,
H+(aq) + OH-(aq) -> H2O(l)
which is the same reaction for all strong acids and alkalis

17
Q

summarize the experiment to determine the enthalpy change of neutralization

A

use a pipette fitted with a safety filter to add 25cm^3 of 1mol/dm^3 of acid into an expanded polystyrene cup
measure temp of the acid
carry out the same technique to place 25cm^3 of <1mol/dm^3 of alkali in a beaker
measure temp of the alkali
add the alkali and the acid
stir and measure the maximum temperature reached

18
Q

state some sources of error within the experiment for calculating total enthalpy of neutralization

A

answers should be given to the same number of significant figures as T
uncertainty of measurements of pipette and thermometer
heat energy transferred to thermometer & polystyrene cup

19
Q

define standard enthalpy of formation

A

the enthalpy change measured at 100kPa and a specified temperature (298k) when one mole of a substance is formed from its elements in their standard states

20
Q

what is Hess’s law

A

states that the enthalpy change of a reaction is independent of the path taken in converting reactants into products, provided the initial and final conditions are the same

21
Q

define bond enthalpy

A

energy required to break one mole of the stated bond in gaseous state

22
Q

what is mean bond enthalpy

A

the enthalpy change when one mole of a bond averaged out over many different molecules is broken

23
Q

how do you calculate the enthalpy change of a reaction

A

total bond enthalpy for bonds broken - total bond enthalpy for bonds made

24
Q
A

Chemistry (46 decks)

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