Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

La Chimie H2 > Thermochemistry > Flashcards

Thermochemistry Flashcards

1
Q

Define enthalpy change

A

ΔH of a rxn is amt energy absorbed or released in chem rxn wrt no of mol of rxt or pdt specified

ΔH = sum of Hpdt - sum of Hrxt

where
Hpdt is enthalpies of pdt,
Hrxt is enthalpies of rxt

Unit: kJ mol -1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Explain endothermic and exothermic reactions

A
  1. Endo
    - rxn absorb energy fr surrounding
    - temp of surrounding decrease
    - sum Hpdt > sum Hrxt (ΔH>0)
    - pdt r thermally LESS stable than rxt
    - energy profile diagram shows increase
  2. Exo
    - rxn release energy to surrounding
    - temp of surrounding increase
    - sum Hpdt < sum Hrxt (ΔH<0)
    - pdt r thermally MORE stable than rxt
    - energy profile diagram show decrease
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What to take note about thermochemical equations?

A
  • MUST write state symbol (aq,s,l,g) for ALL eqn
  • MUST write sign (to know if exo or endo)
  • MUST write ΔH > or < 0
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are conditions affecting enthalpy change?

A
  • amt substance (eg if twice, twice enthalpy change)
  • state of rxt & pdt (change state can release/absorb energy)
  • temp & Pa (usually standard condit n; if changed, enthalpy change affected)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Define standard enthalpy change of reaction, ∆Hϴ

A

∆Hϴ or ∆Hrϴ is amt energy absorbed or released in chem rxn when molar qty stated in chemical eqn react under standard condit n 298K, 1 bar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What does standard conditions mean?

A

(denoted by super script ϴ)
- temp = 298K = 25 deg C
- Pa = 1 bar = 1.0e5 Pa
- sol n at conc of 1 mol dm-3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Define standard enthalpy change of combustion, ∆Hcϴ

A

∆Hϴ of substance is energy RELEASED when 1 mol of substance completely burnt in oxygen under standard condit n 298K, 1 bar

NOTE:
- always exo
- water in eqn is liquid, NOT gas at standard condt n, and CH4 is gas, C8H18 is liquid
- values of ∆Hcϴ aka energy value of fuel; more exo the ∆Hcϴ, the larger amt heat liberated upon complete combust n fr given amt fuel

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Define standard enthalpy change of neutralisation, ∆Hnϴ

A

∆Hϴ is energy RELEASED when acid & base react form ONE mol water under standard conditions 298K, 1 bar

NOTE:
∆Hnϴ always exo, same value for strong acid, strong base rxn

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why is enthalpy change of neutralisation involving weak acids or bases is lower than -57.3kJ mol-1

A
  • weak acid & bases oni slightly dissociated in aqueous sol n
  • some of energy evolved fr neutral n process is used to further dissociate weak acid/base completely
    => enthalpy change of neutral n involving weak acids, bases is less exo than that btw strong acid, strong base
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Define standard enthalpy change of formation, ∆Hfϴ

A

∆Hfϴ of substance is energy CHANGE when 1 mol of substance is formed fr its element under standard condit n 298K, 1 bar

NOTE:
- ∆Hfϴ of elements is ZERO
- Graphite is used as basis for carbon since it is the most stable form of C vs other form eg diamond
- ∆Hfϴ is measure of stability relative to element (the more exo value, greater stability relative to element - less energy content)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Define standard enthalpy change of atomisation of element, ∆Hatϴ

A

∆Hatϴ of element is energy ABSORBED when one mole gaseous atoms is formed fr element in its standard state under standard condit n 298K, 1 bar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Define standard enthalpy change of atomisation of compound, ∆Hatϴ

A

∆Hatϴ of compound is energy ABSORBED when one mole of compound in a given state is being broken down into its constituent gaseous atoms under standard condit n 298K, 1 bar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Define bond energy (BE)

A

of covalent bond is avg energy absorbed to break 1 mol of COVALENT bond in gas phase into constituent gaseous atom under standard condit n 298K, 1 bar

NOTE:
BE of homonuclear diatomic molecule (ie X-X) is twice of ΔHatθ of constituent element X2 (so, usually BE found in databooklet)
Eg
BE (Cl-Cl) = 2ΔHatθ(Cl)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Define First electron affinity (1st EA)

A

of element is energy released when 1 mol gaseous atom gain 1 mol e- form 1 mol singly charged negative gaseous ions

Note:
Always exo, unlike 2nd EA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What to take note about First and Second Ionisation Energy?

A
  • always endo
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Define Second electron affinity (2nd EA)

A

of element is energy ABSORBED when 1 mol singly charged negative gaseous ion gain 1 mol e- form 1 mol doubly charged negative gaseous ions

Note:
Always endo bcos e- added to -ve ion (energy needed overcome repuls n)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Define lattice energy ΔHlattθ

A

ΔHlattθ of ionic cpd is energy released when 1 mol of ionic cpd is formed fr constituent gaseous ion under standard condit n 298K, 1 bar

18
Q

Define Standard Enthalpy change of hydration ΔHhydtθ

A

ΔHhydθ of an ion is energy released when 1 mol gaseous ions is hydrated under standard condit n 298K, 1 bar

19
Q

Define standard enthalpy change of solution ΔHsolθ

A

ΔHsolθ of substance is energy CHANGE when 1 mol of substance is completely dissolved in a solvent form infinitely dilute sol n under standard condit n 298K, 1 bar

Note:
Can be exo or endo depending on chemical properties

20
Q

Define standard enthalpy change of fusion ΔHfusθ

A

ΔHfusθ of substance is energy absorbed when 1 mol of substance is converted fr solid to liquid state w/o change in temp at 1 bar

21
Q

Define standard enthalpy change of vaporisation ΔHvapθ

A

ΔHvapθ of substance is energy absorbed when 1 mol of substance is converted fr liquid to gaseous state w/o change temp at 1 bar

Note:
Energy required to overcome intermolecular attract n in liquid state to vaporise => endo

22
Q

Define standard enthalpy change of sublimation ΔHsubθ

A

ΔHsubθ of substance is energy absorbed when 1 mol substance is converted fr solid to gaseous state w/o change temp at 1 bar

23
Q

Give formula for temperature change ΔT

A

ΔT = Highest/lowest temp - initial temp

ΔT>0 indicate temp increase

ΔT<0 indicate temp decrease

24
Q

How to measure temperature change?

A
  • directly recording initial and highest/lowest temp reached
  • monitor temp over time, plot temp-time graph & extrapolate to find max/min temp (for exo & endo rxn respectively)
25
Q

Give formula for heat absorbed/released by solution Qsol

A

Qsol = mcΔT = VcΔT (Only magnitude)

where
m is mass of sol n in cup (g)/V is vol of sol n (cm3) (when density assumed 1g cm-3),
c is specific heat capacity of sol n (when assumed, 4.18) (J g-1 K-1),
ΔT is temp change (K)

26
Q

Give formula for heat absorbed/released by reaction Qrxn

A
  • Qrxn = Qsol (assume no heat loss; 100% efficiency)
  • Qrxn = (100/x)Qsol
    where
    x% is efficiency of heat transfer
27
Q

Give formula for enthalpy change ΔH

A

ΔH = +/- (Qrxn/n) x N

where
Qrxn is heat released/absorbed,
n is amount of limiting reagent,
N is stoichiometric coeff of limiting reagent in eqn (usually 1)

*MUST HAVE SIGN

28
Q

What to take note about calculating enthalpy change of reaction ΔH

A
  • all enthalpy changes of rxn MUST hv correct sign
  • reversing chem eqn oso reverse sign of ΔHθ
  • multiply chem eqn by factor oso multiply ΔH by same factor
  • ΔH can be calculated theoretically or any following methods (if can):
    1 Hess’ Law
    2. Standard enthalpy change of format n
    3. Standard enthalpy change of combust n
    4. Bond energy
    5. Algebraic method
29
Q

Define Hess’ law

A

Enthalpy change for chem rxn is same regardless of route by which chem change occur, provided initial state of rxt & final state of pdt r same

30
Q

How to calculate enthalpy changes with Hess’ Law?

A

By Hess’ Law,

dH1 = dH2 + dH3 + dH4

(sum of dH in clockwise direct n = sum of dH in anti-clockwise direct n)

*enthalpy change is path-independent (regardless of route)

31
Q

What are the general steps for using Hess’ Law

A
  1. construct balanced chem eqn for rxn to find unknown (aim eqn)
  2. identify thermochem eqn for all given ΔH info in qn
  3. form energy cycle (include state symbol, species along arrow, balance species, include appropriate ΔH notat n, values)
  4. apply Hess’ Law solve for unknown ΔH
32
Q

Give formula for standard enthalpy change of formation. When can this formula be used?

A

ΔHr = sum ΔHθf (pdt) - sum ΔHθf (rxt)

*use oni when given values of enthalpy change of form n of ALL rxt, pdt

33
Q

When using formation or combustion formulae, what to take note?

A
  • ΔHr is not always the unknown to find
34
Q

Give formula for standard enthalpy change of combustion. When can this formula be used?

A

ΔHr = sum ΔHθc (rxt) - sum ΔHθc (pdt)

*use oni when given enthalpy change of combust n of ALL rxt, pdt
NOTE:
- sum of RXT - PDT, not other way ard like for form n
- same eqn may sometimes represent more than 1 type of enthalpy change (but same process)
eg
C(s) + O2(g) –> CO2(g)
ΔHθf (CO2) = ΔHθc (C)

35
Q

Give formula for bond energy method

A

ΔHr = sum BE(rxt) - Sum BE (pdt)

NOTE:
- energy absorbed in rxt => (+ve sign)
- energy released in bond form n in pdt => (-ve sign)

36
Q

Enthalpy change of reaction that is calculated differs slightly from actual value. Suggest why

A

BE calculat n is approximat n method as BE values given in data book r avg value, not specific to cpd in rxn

37
Q

What are the steps to find enthalpy change using algebraic method

A
  1. construct balanced chem eqn (aim eqn)
  2. write thermochem eqn for all given enthalpy change in qn (optional if u clear of eqn involved)
  3. manipulate given eqn so rxt in aim eqn appear on left side, pdt appear on right side (ensure balanced eqn)
  4. add eqn so same substance on both side cancel out
  5. enthalpy change of rxn = sum given enthalpy changes of rxn added
38
Q

How does energy diagram show endothermic or exothermic?

A
  • Upwards arrow mean energy absorbed (endo)
  • downward arrow mean energy released (exo)
39
Q

What to take note about Born-Haber Cycle

A
  • apply ONI to ionic cpd
  • use upward arrow for endo rxn (+), downward arrow for exo rxn (-)
  • follow this sequence in making cycle:
    F orm n (exo)
    A tom n of metal then non-metal (endo)
    I on fr IE (metal) then EA (non-metal) (2nd EA is endo, larger mag than 1st EA, which is exo)
    L attice energy (exo)
  • use ruler, ensure arrows are rather proportional in mag
40
Q

Why is theoretical lattice energy for eg FeO different from value obtained using Born-haber cycle?

A

Fe2+ cation hv high charge density, so polarise O2- anion e- cloud, induce partial covalent character. Theoretical value no consider partial covalent character of FeO

NOTE:
- calculat n using born-haber cycle involve expt values, presense of partial cov character in ionic cpd is accounted for
- for theoretical values (fr use of | ΔHlatt | ∝ (q+)(q-)/(r+ + r-) , oni pure ionic lattice structure considered, not partial cov character usually

41
Q

Give formula for standard enthalpy change of solution. What can be deduced?

A

fr Hess’ Law,

ΔHθsol = sum ΔHθhyd - ΔHθlatt

  • more exo, more soluble salt (unless data suggest otherwise)

Deduct n:
- salt likely soluble if ΔHθsol < 0, bcos enough hydrat n energy released to compensate for lattice energy required to break down ionic cpd
- salt NOT likely soluble if ΔHθsol > 0, bcos insuff hydrat n energy released compensate for lattice energy needed break down ionic cpd

NOTE: some salts still soluble w endo ΔHθsol of small mag (due to entropy)

La Chimie H2 (25 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions