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Chemistry: Energetics > Chemistry: Enthalpy > Flashcards

Chemistry: Enthalpy Flashcards

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1
Q

What is enthalpy?

A

measure of the heat content of a substance

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2
Q

What is enthalpy change?

A

change in heat content of a substance at a constant pressure

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3
Q

What are the standard conditions for measuring enthalpy change?

A

Pressure: 100kPa
Temperature: 298K

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4
Q

What is enthalpy change measured in?

A

KJmol-1

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5
Q

Define what is meant by the term standard enthalpy of formation

A

enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products in standard states

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6
Q

How is standard enthalpy of formation represented?

A

triangle-f-H

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7
Q

What is the equation for calculating standard enthalpy of formation?

A

=sum of enthalpy formation products - sum of enthalpy formation reactants

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8
Q

What is standard enthalpy of combustion?

A

enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions with all reactants and products in standard states

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9
Q

How is standard enthalpy of combustion represented?

A

triangle-c-H

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10
Q

What is the equation for calculating standard enthalpy of combustion?

A

=sum of enthalpy combustion reactants - sum of enthalpy combustion products

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11
Q

What is temperature related to?

A

average kinetic energy of particles within a system

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12
Q

What effect does increasing kinetic energy have on temperature?

A

increases, particles moving faster

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13
Q

What is heat a measure of?

A

total energy of all particles present in a given amount of substance

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14
Q

What three details are necessary to measure enthalpy change?

A

-mass of substance heated/cooled -temperature change -specific heat capacity of substance

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15
Q

What is the specific heat capacity of a substance?

A

amount of heat energy required to raise the temperature of 1g of substance by 1K

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16
Q

What are the units for specific heat capacity?

A

Jg-1K-1

17
Q

What is the specific heat capacity of water?

A

4.18 Jg-1K-1

18
Q

What equation is used to calculate heat energy given out?

A

q=m-c-change in t
q: heat energy given out (J)
m: mass of substance (g)
c: 4.18
change in t: change in temperature (K)

19
Q

What equation is used to find enthalpy change per mole?

A

q/number of moles reacting

20
Q

How do we calculate moles?

A

=mass/mr

21
Q

What apparatus is used to measure enthalpy of combustion?

A

flame calorimeter

22
Q

Using a flame calorimeter, explain how we would measure enthalpy change of combustion

A

-fixed volume of water (measured) in calorimeter -spirit burner weighed on 2d.p balance
-initial temperature of water measured and recorded -burner ignited, water stirred continuously -flame distinguished after 20C rise -burner re-weighed -final temperature of water measured and recorded

23
Q

What sources of error are there to the flame calorimeter experiment?

A

-heat loss to surroundings
-heat absorbed by calorimeter
-incomplete combustion

24
Q

What steps can we ensure to reduce heat loss?

A

-calorimeter not placed too far above flame -lid on calorimeter -shielding/draught excluder

25
Q

What is bond enthalpy?

A

enthalpy change to break one mole of covalent bonds in the gas phase

26
Q

What is the value for bond enthalpy?

A

an average taken from a range of molecules

27
Q

What equation is used to calculate bond enthalpy?

A

=sum of enthalpy bonds broken - sum of enthalpy bonds made

28
Q

How do we conduct an experiment for heat of neutralisation with sodium hydroxide and hydrochloric acid?

A

-50cm3 1moldm-3 HCl in one polystyrene cup, equal volume of sodium hydroxide in another
-record temperature of each for 4 minutes at every 30 seconds, confirm remain the same
-at 4 don’t record temperature and add solutions together then stir
-record temperature every 30 seconds for further 6 minutes
-draw cooling curve with temperature against time, extrapolate to 4 minutes
-use lines of best fit to find temperature difference

29
Q

What is Hess’s Law?

A

states that enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products

Chemistry: Energetics (2 decks)

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