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Chapter 12 : THERMOCHEMISTRY

4.1 The Changes Of Energy (E) in chemical reaction

1. The Law of Conservation of Energy states that energy cannot be___________ or ____________. However, energy can be___________ from one form to another.For example, chemical energy heat energy

2. Exothermic reaction the reaction which ____________ theheat energy ____ surroundings

3. Endothermic reaction the reaction which ___________ theheat energy ____ surroundings

4. The table below shows the differences between exothermic &endothermic reaction:

Charateristics EXOTHERMIC ENDOTHERMIC

1.Heat energy

2.Thermometer

3.Changes ofenergy

Chemical energy Heat energy

4.Bonding

5.Energy LevelDiagram

6.Examples - dissovelsconcentrated acid/alkali in water

- combustion- rusting of iron- acid reacts with metal- neutralisation- Haber process

- Touch process

- dissovelsammonium salts inwater

- carbonate / nitratesalts decomposesby heat

- acid reacts withNaHCO3 & KHCO3

- photosynthesis

Exothemic Endothermic

5. Energy (E) - ability to do the works- measured in Joule (J)

- 1000J = _____ KJ

6. Heat changes - the changes of heat during the chemical( H) reaction

- unit : _________

- ve value for ______________

- +ve value for ______________

- Hproducts-Hreactants

7. Heat energy will be absorbed in _____________ bonds(-ve)

8. Heat energy will be released in _____________ bonds(+ve)

4.2 Calculation for the heat changes, H

1. H measured in ____________ unit

2. H = mc

m = the mass of reactant solution which immersed thethermometer

c = heat capasity of water = 4.18 J g-1 C -1

= temperature change , T

1

Reactant

particles

Solution in

beaker =

surroundings

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3. Energy level diagram

H1

H = -ve

H2

H2

H = +veH1

4. During the chemical reaction:Hx = heat change for reactionHy = Heat absorbed to break the bonds (+ve)Hz = Heat realeased when bonds formed (-ve)

Hx = Hy + Hz

If Hy < Hz, then Hxve (exo)If Hy > Hz, then Hx +ve (endo)

Example: C (p) + H2O (ce) CO (g) + H2 (g) ,Hx +ve

5. Calculation of heat change in experiment

4.3 Heat of precipitation

Diagram:

- Defination:

- AgNO3 (ak) + NaCl (ak) AgCl (p) + NaNO3 (ak)Ionic equation : Ag+ (ak) + Cl- (ak) AgCl (p)

-calculation:1.2.3.

4.4 Heat of displacement

Diagram:

- Defination:

- Mg (p) + FeCl2 (ak) Fe (p) + MgCl2 (ak)- ionic equation : Mg (p) + Fe2+(aq) Fe (p) + Mg2+ (aq)

2

E

E

Exothermic

Endothermic

H = H2 H1

E

C (s) + H2O (l)

HyCO (g) + H2 (g)

C (g) + 2H (g) + O (g)

Hz

Hx

Heat absorbed to

break the bonds

Heat realeasedwhen bonds

formed

Total heat

change

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Jisim bahan t/b & tidak diambil kira

bahan t/b yg berlainan memberi haba penyesaran berlainan

cth: haba t/b Zn dgn Fe2+ haba t/b Al dgn Fe2+

4.5 Haba Peneutralan

- maksud: haba yang dibebaskan a/p satu mol ion H+t/b dgnsatu mol ion OH- utk menghasilkan satu mol airpada

keadaan piawai

- Cth: HCl (ak) + NaOH (ak) H2O (ce) + NaCl (ak) H = -x KJ mol-1

H2SO4 (ak) + Mg (OH)2 (ak) 2H2O (ce) + MgSO4 (ak) H = -2x KJ mol-1

H+ (ak) + OH- (ak) H2O (ce)- kaedah penentuan Haba Peneutralan- campurkan larutan asid M1 sbyk V1 dgn larutan alkali M2sbyk

V2 & perubahan suhu maksimum disukat

Haba peneutralan antara asid kuat dgn alkali kuat adalah

tetap, iaitu 57.3 KJ mol-1

Kerana semua asid & alkali kuat mengion secara lengkap

Haba peneutralan antara asid lemah dgn alkali lemah

< drpd -57.3 KJ mol-1

Kerana semua asid & alkali lemah mengion secara

separa / tidak lengkap

Maka sebahagian haba yg terbebas semasa proses

peneutralan diseraputk memutuskan ikatan O-H dalammolekul asid lemah yg tidak mengion

4.6 Haba Pembakaran

- maksud: haba yang dibebaskan a/p satu mol bahan kimiaterbakar dgn lengkap dalam oksigen pada keadaanpiawai

- Cth: CH3OH (ce) + 3/2 O2 (g) CO2 (g) + 2H2O (ce)

- kaedah penentuan Haba Pembakaran

- membakar bahan api (alkohol) sbyk jisim m1 & haba ygdibebaskan diguna utk memanaskan air pada V1.Perubahan suhu maksimum disukat

haba pembakaran (KJmol-1)

Bilangan atom karbonper molekul alkohol

Kesimpulan: Haba pembakaran semakin meningkat dgnpertambahan bil. atom C per molekul alkohol

4.7 Nilai Bahan Api

1. Bahan api :

2. Bahan api berbeza mempunyai haba pembakaran yang

berlainan

3. Faktor pemilihan bahan api dalam kegunaan industri:

a) menghasilkan tenaga haba yg bykb) mempunyai harga / kos yang rendahc) mudah diperolehd) tidak mencemarkan alam sekitar

4. Nilai bahan api:

: utk membanding kos tenaga antara bahan api: unit KJg-1

5. Arang kok byk digunakan dalam industriKelebihan:

Keburukan:Sebab:- mengandungi bendasing seperti sulfur yang akanditukar menjadi sulfur dioksida yang mengakibatkan hujanasid. Hujan asid memusnahkan hutan rimba, hidupan akuatik

mati, struktur bangunan (logam / konkrit) terkakis.3

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Persamaan: S (p) + O2 (g) SO2 (g)

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