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AS Chemistry > Chapter 6: Enthalpy Changes > Flashcards

Chapter 6: Enthalpy Changes Flashcards

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1
Q

Heat of reaction:

A

Amount of heat evolved or absorbed in a reaction, quantities of reactants and products being sane as represented by chemical equation

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2
Q

Energy change during reaction is mainly due to

A

Chnage in bond energy ie potential energy resulting from breaking and making of new bonds in reactants and products

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3
Q

Distinguish between system and surrounding

A

System: area of interest
Surrounding: in theory, everything else in the universe
Energy lost by system is gained by surroundings and vice versa

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4
Q

Distinguish between open and closed systems

A

Open: can exchange energy and matter with surroundings
Closed: only exchange energy but not matter

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5
Q

Heat content of a system at _ is _

A

Constant pressure

It’s enthalpy (heat inside)

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6
Q

System is reservoir of

A

Heat

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7
Q

According to law of conservation of energy total energy doesn’t chnage during process but

A

Can be transferred between system and surrounding

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8
Q

ΔH is

A

Used to denote enthalpy change

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9
Q

ΔH is +0 when

A

Heat is added to system

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10
Q

Chemical system ie _ consists of _ and _ of it’s particles. These add up to _

A

Substance
Kinetic(motion) and potential energy(position)
Internal energy

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11
Q

Internal energy of substance depends on

A

Physical state

Structure

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12
Q

Mole of a substance has _ heat content

A

Characteristic

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13
Q

If reaction happens at constant pressure, _ and _ are the same

A

Heat change

Enthalpy change

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14
Q

Enthalpy is heat content. We want to know _ ssincewe can’t really know the enthalpy as such

A

Enthalpy change

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15
Q

In exothermic reaction…

A
  • heat is given out to surrounding
  • heat content of reactants> products.
  • excess given out in forms of heat
  • then falls to room temp as heat is lost
  • net decrease in potential energy/heat content of system
  • ΔH is -ve
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16
Q

Endothermic reaction…

A
  • heat gained from surrounding
  • total heat content of reactants less than products
  • products of energy falls below room at first then rises to room temp becahse heat lost is gained from surrounding
  • ΔH postive as heat gained
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17
Q

From an energy profile diagram how to identify Ea and ΔH?

A

Ea: from reactants line to threshold peak

ΔH: reactants line to products line (+/-)

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18
Q

Differentiate between energy level and profile diagrams

A

Profile has threshold for Ea along with the 2 lines

Level only has 2 lines to get ΔH

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19
Q

Standard conditions for measuring enthalpy changes. And why?

A

Temperature : 298K (25°C)
Pressure: 100kPa
Physical state: All solutions conc 1mol/dm3

To compare enthalpy changes, conditions must be fixes

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20
Q

Standard state is

A

Most stable physical share of substance at 100kPa at given temp ie 298K

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21
Q

Standard enthalpy change denoted by

A

ΔHø

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22
Q

Kelvin scale:

A

Emphasizes relationship between average KE and temperate

Absolute temp in Ke directly proportional to KE/movement

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23
Q

Absolute zero

A

-273°C

Lowest possible temp where all movement stops

24
Q

Increase in temp depends on :

A
  • mass of object (smaller will have greater temp increase, as sane energy shared amongst less particles)
  • heat added
  • nature of substance
25
Q

Specific heat capacity:

A

Amount of heat needed to increase unit mass by 1K

26
Q

Specific heat capacity depends on

A

No of particles present in unit mass, which in turn depends on mass of individual particles

27
Q

Heat/energy change(^Hr)=

A

-mcΔt

28
Q

Standard molar enthalpy change ΔHrø=

A

ΔH÷n

29
Q

Amount of energy released depends on

A

Amount of fuel burnt

30
Q

Heat released by exo reaction is absorbed by _ and the temp of it _. Assuming all of it is absorbed by it

A

Water

Increases

31
Q

As water gains heat as enthalpy chnage of reaction is _ when temp of water increases

A

Negative

32
Q

Heat absorbed by reaction in an _ reaction from _, so temp of _. Enthalpy chnage of reaction is _

A

Endo
Water
Water decreases
Postive

33
Q

Why can we not accurately say water has indeed gotten all the heat content released from a combustion reaction?

A
  • some heat needed for copper calorimeter and some has gone to surrounding
  • combustion of fuel likely to be incomplete owing to limited oxygen
  • experiment maybe not performed under standard conditions
34
Q

Finding enthalpy change of reaction in solution using cooling correction:

A

Plot a cooling curve to improve result of experiment.

Keep recording temp of a volume of solution, in intervals. Add excess reagent and keep recording for temp rise and approx linear fall.

Thermometer takes time to respond to temp chnage of solution, even if reaction is instantaneous.
During that tine heat is lost.
To estimate amount lost and work out max temp, temp is recorded at series of times and a temp time graph is plotted.
Line of best fit through falling temp drawn and extrapolated. Another line drawn from time at which reagent was added.
Meeting point is actual max temp rise

35
Q

Largest error in experiment conducted in polystyrene cup

A

Heat lost to environment soon as temp rises above room temp
Max temp recorded is lower than true value obtained in perfectly insulated system

(We make some allowance by extrapolating cooling section of graph to start tike of reaction)

36
Q

Assumptions to proceed with calculation of enthalpy change in solution:

A

1) no heat loss from system
2) all heat goes from reaction to water
3) solution is dilute V(s)= V(H2O)
4) water has density of 1g/cm3 hence mass of sol=mass of water

37
Q

Define standard enthalpy change of reaction

A

Enthalpy change when the amounts of reactants shown in equation react to give products under standard conditions. Reactants and products must be in their standard states.

38
Q

Define standard enthalpy change of formation

A

Enthalpy change when one mole of a compound is formed from it’s elements under standard conditions. Reactants and products must be in standard states

39
Q

Define standard enthalpy change of conbustion

A

Enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. Reactants and products must be in their standard states

40
Q

Define standard enthalpy change of neutralisation

A

ΔHnø is enthalpy change when one mole of water is formed by reaction of an acid with an alkali under standard conditions

41
Q

Define standard enthalpy change of solution

A

ΔHsolø is enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions

42
Q

Define standard enthalpy change of atomisation

A

^Hatø is enthalpy change when one mole of gaseous atoms is formed from it’s element under standard conditions

43
Q

Define standard enthalpy change of hydration of an anhydrous salt

A

Enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions

44
Q

Hess’s law:

A

Enthalpy change for any chemical change is independent of route provided the starting conditions and final conditions, and reactants and products are same

(because is 1 is large and exo, and 2 is small and exo, P->Q could give a lot of energy, and take reverse route (2, small and endo) only taking in small amount of energy. This could repeat and be perpetual source of energy. This doesn’t happen)

Hence energy can neither be created nor destroyed (first law of thermodynamics)

45
Q

Hess’s law equation

A

Q=Q1+Q2

46
Q

Why are standard enthalpy measurements important

A
  • give a measure of stability of a substance relative to it’s elements
  • used to calculate enthalpy changes of all reactions especially ones we can’t measure in laboratory (hypothetical/real reactions)
47
Q

Use of enthalpy of formation

A

Used to calculate standard enthalpy change on any reaction

48
Q

General expression for ΔH reaction of any reaction

A

ΔHr=totalΔHproducts - totalΔHreactants

bcos reactants+reaction=products, from a formation cycle ie elements–>reactants or products, and reactants–>products

49
Q

ΔHrø measured in _ and written _

A

kJ/mol

At end of reaction

50
Q

Enthalpy of substance is equal to it’s

A

Stranded enthalpy of formation

51
Q

Enthalpy of all elements in their normal states under standard conditions is

A

Zero

52
Q

Bond energy:

A

Energy needed to break one mole of particular kind of covakent bond under standard conditions.
Aka bond dissociation energy

53
Q

For diatomic molecules in _, dissociation energy…

A

Gaseous state

Twice enthalpy of atomization

54
Q

Bond breaking is -

Bond making is -

A

Endo

Exo

55
Q

Another way to express ΔHrø in terms of bond energy

A

^Hrø= sum of bond enthalpy of bonds broken - sum on bond enthalpy of bonds formed

AS Chemistry (11 decks)

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