chemistry-class-11-unit-06-chapter-05-chemical-thermodynamics-l-5-8-gnshi4lv62c

Chemical Thermodynamics L-5

Recap

Essential concepts and definitions
Heat, work & energy internal energy
Calculation of work, heat in different process for an ideal gas
Enthalpy and heat capacity
Determination of $\Delta U$ and $\Delta H$
Change in enthalpy in different processes / reactions
Enthalpy of reaction
Enthalpy of formation
Enthalpy of phase transition
Enthalpy of other processes / reactions

Chemical Thermodynamics L-5

Enthalpy of reaction

$\Delta_r H_T^{\circ}=\sum_i a_i H _{m,T}^{\circ}-\sum b_i H _{m,T}^{\circ}$
Conventional temperature considered as $25^{\circ} {C}, 298.15 K$
$\Delta_r H^{\circ}=\sum_i a_i H_m^{\circ}-\sum b_i H_m^{\circ}$
Standard heat of formation / Standard enthalpy of formation
$\Delta_f H_T^{\circ}, \Delta_f H^{\circ} \quad T \Rightarrow 25^{\circ} \mathrm{C}$

Chemical Thermodynamics L-5

Enthalpy of reaction

standard molar enthalpy
$\Delta_r H_T^{\circ} \Rightarrow H _{m, T}^{\circ}$ $\Rightarrow$ ${H}_m^{\circ}$
Standard heat of formation
$\Delta_f H_T^{\circ}$ of a pure substance at a specified temp. T is $\Delta r H^{\circ}$ for the process/reaction in which one mole of substance in its standard state at T is formed from the corresponding separated elements at T, each being in its reference state/form/phase

Chemical Thermodynamics L-5

Enthalpy of reaction

Reference state/phase/form
Most stable state of the element at 1 bar pressure at the specified temperature T
- $T=25^{\circ} {C}$
- Reference states at $25^{\circ} {C}$
- Dihydrogen = $H_2(g)$
- Dioxygen = $O_2(g)$
- Carbon = C (graphite)
- Sulphur = S (rhombic)

Chemical Thermodynamics L-5

Enthalpy of formation

Standard heat of formation of water, $H_2O$ (l) -298K
$\Delta_rH^o$ at 298K
Product - $H_2O$ (1 mole) at standard state at 298K (1 bar)
Reactant-
- $H_2$ (g)- 1 bar , 298K
- $O_2$ (g)- 1 bar, 298K
Reaction:
- $H_2(g)+O_2(g) \rightarrow H_2O (l)$ 1 bar, 298K
- $\Delta_rH^o=\Delta_fH^o _{H_2O}$ 298K
- -286 KJ $mol^-1$

Chemical Thermodynamics L-5

Enthalpy of formation

$CH_4$
$C(graphite)+2H_2(g) \rightarrow CH_4(s)$
$\Delta_rH^o(298K)=-74.8 KJ mol^-1$
$\Delta_fH^o _{CH_4}(298K)=-74.8 KJ mol^-1$
$C_2H_5OH$
$2C(graphite)+3H_2(g)+ \frac{1}{2}O_2(l) \rightarrow C_2H_5OH(l)$
$\Delta_fH^o(C_2H_5OH)= -277 KJ mol^-1$
$HBr(g)$
At $25^oC$ , 1 bar, $Br_2(l)$
$\frac{1}{2}H_2(g)+\frac{1}{2}Br_2(l) \rightarrow HBr(g)$
$\Delta_fH^o(HBr(g))= -36.4 KJ mol^-1$

Chemical Thermodynamics L-5

Enthalpy of formation

$CaO(s)+CO_2(g) \rightarrow CaCO_3(s)$
$\Delta_rH^o \ne \Delta_fH^o(CaCO_3)(s)$
$\Delta_fH^o$ of an element in its reference state is taken as zero
$\Delta_fH^o(graphite)$
$C(graphite) \rightarrow C(graphite)$ 1 bar
$\Delta_rH^o = 0$
$\Delta_fH^{\circ}$ for elements at thier reference state = 0

Chemical Thermodynamics L-5

Enthalpy of reaction

$\begin{gathered} \Delta_r H^{\circ}=\sum a_i h_m^{\circ}-\sum b_i H_m^{\circ} \ a A+b B \rightarrow c C+d D \end{gathered}$
$Reactants(aA+bB) \xrightarrow I Products(cC+dD) \xrightarrow {III}$ constituent elements in their reference state at T
$Reactants(aA+bB) \xrightarrow {II}$ constituent elements in their reference state at T $\xrightarrow {III} Products(cC+dD)$
$\Delta_r H^{\circ}(I)=\Delta_r H^{\circ}(\text { II })+\Delta_r H^{\circ} \text { (III) }$

Chemical Thermodynamics L-5

Enthalpy of reaction

$-\Delta_r H^{\circ}(\mathbb{I})=a \Delta_f H_A^0+b \Delta_f H_B^0$
$\Delta_r H^{\circ}(\underline{Q})=C \Delta_f H_c^0+d \Delta_f H_D^0$
$\Delta_r H^{\circ}(I)=\Delta_r H^{\circ}(I)+\Delta_r H^0\left(\mathbb{I}^0\right)$
$c \Delta_f H_C^0+d \Delta_f H_D^0-\left(a \Delta_f H_A^0+b \Delta_f H_l^0\right)$
$\Rightarrow \Delta_r H^{\circ}=\sum a_i \Delta_f H_i^{\circ}-\sum b \Delta_f H_i^0 \quad (\text{at } \operatorname{temp} T)$
$\Rightarrow \mathrm{CH}_4(9)+2 \mathrm{O}_2(9) \rightarrow \mathrm{CO}_2(9)+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \text { (298K) }$
$\Delta_r H^0=\Delta_f H _{{CO} _2(g)}^0+2 \Delta_f H _{{H}_2 {O}(l)}^0-\Delta_f {H} _{{CH} _4(g)}^0-2 \Delta_f {H} _{{O} _2(g)}^0$
$-393.5 + 2(-285.8) - (-74.8)$
$-890.4 , \mathrm{kJ , mn}^{-1}$

Chemical Thermodynamics L-5

Enthalpy of reaction

Balanced equation along with $\Delta_rH^o$ value
Thermochemical equation (thermochemistry)
Physical states and allotropic state must be mentioned
$CH_4(s)+2O_2(g) \rightarrow CO_2(g)+2H_2O(l)$
$\Delta_rH^o(298K)= -890.4 KJ mol^{-1}$
stoichiometric coefficients = number of moles
$\Delta_rH^o \rightarrow$ extensive quantity
Reverse chemical reaction will have opposite sign but equal magnitude in $\Delta_rH^o$

Chemical Thermodynamics L-5

Enthalpy of reaction

$CaCO_3(s)\rightarrow CaO(s)+CO_2(g)$
$\Delta_rH^o=178.3 KJ mol^{-1}$
$2CaO(s)+2CO_2(g) \rightarrow 2CaCO_3(s)$
$\Delta_rH^o = -2\times 178.3 = -356.6 KJ mol^{-1}$

Chemical Thermodynamics L-5

Change in enthalpy

Enthalpy change during phase transition
Standard enthalpy of phase transition
$\Delta _{trs} H^{\circ}(T)$ often called latent heat
Phase change
Solid $\rightarrow$ liquid $\quad$ fusion $\Delta_fus H^{\circ}(T)$
Liquid $\rightarrow$ gas $\quad$ vapourisation $\Delta _{ vap } H^{\circ} (T)$
Solid $\rightarrow$ gas $\quad$ sublimation $\Delta _{sub}H^{\circ}(T)$
Phase transition - generally happens at different temperature and pressure

Chemical Thermodynamics L-5

Change in enthalpy

$H_2 O(l) \rightarrow H_2 O(g) \quad \Delta_r H^{\circ}=40.66\quad KJ mol^{-1}$
Pure $H_2O(l)-$ 1bar,373 k
Pure $H_2O (g)-$ 1bar,373 K
$\Delta_{rap}H^{\circ}_{H_2} o(l)\hspace{0.5mm}$ (at 373 k)
$=40.66 KJ mol^{-1}$
Standand enthalpy of vapourisation is the amount of heat required to vaporize one mole of a liquid at a constant temperature and under standard pressure (1 bar)
Standard enthalpy of fusion $\quad\Delta _{trs} H^{\circ}(T)$
Standard enthalpy of sublimation

Chemical Thermodynamics L-5

Change in enthalpy

$\Delta H$ - only depends on initial and final states
Sublimation: solid $\rightarrow$ gas
solid $\rightarrow$ liquid $\rightarrow$ gas
$H_2 O(s) \rightarrow H_2 O(g) \quad \Delta _{sub } H^{\circ}$
$\Delta _{sub } H^{\circ}=\Delta _{fus} H^{\circ}+\Delta _{vap } H^{\circ}$

Chemical Thermodynamics L-5

Hess's law

Hess's law of constant heat summation
$\Delta_f{H}^{\circ}$ (298k) of ethane gas
$2 C (graphite) + 3H_2 (s) \rightarrow C_2H_6$ (g) at 298k
- $\Delta_rH^{\circ}$
(I) $C_2 H_6(g)+\frac{7}{2} O_2(g) \rightarrow 2 CO_2(g)+3 H_2 O(l)$
- $\Delta_rH^{\circ}$ (298 k)=-1560
(II) $C (graphite)+O_2(g) \rightarrow CO_2(g)$
- $\Delta_rH^{\circ}=-393.5$
(III) $H_2(g)+\frac{1}{2} O_2(g) \rightarrow H_2 O(l)$
- $\Delta_r H^{\circ}=-286 KJ mol^{-1}$

Chemical Thermodynamics L-5

Hess's law

$(I) \times (-1)$
- $2 Co_ 2(g)+3 H_2O \rightarrow C_2 H_6 (g)+\frac{7}{2} O_2 (g)$
$\Delta r H^{\circ}=1560$
- $(II) \times 2$
$2 C(graphite )+2 o_2(g) \rightarrow 2 C o_ 2(g)$
- $\Delta r H^{\circ}=2 \times(-343.5)$
$(III) \times 3$
- $3H_2(s) + \frac{3}{2} O_2 (s) \rightarrow 3 H_2o (l)$
- $\Delta_rH^{\circ} = 3 \times (-286)$
$2 c (graphite) +3 H_2(g) \rightarrow C_2 H_6(g)-85 KJ mol^{-1}$
$\Delta_f H^{\circ}C_2H_6(g) \quad (298K)=-85 KJ mol^{-1}$

Chemical Thermodynamics L-5

Hess's law

$A \xrightarrow{\Delta r H^{\circ}} B$
$A \xrightarrow{\Delta_rH^{\circ} _{1}} C$
$C \xrightarrow{\Delta_rH^{\circ} _{2}} D$
$D \xrightarrow{\Delta_rH^{\circ} _{3}} B$
$\Delta_r H^0=\Delta r H_1^0+\Delta_r H_2^0+\Delta_r H_3^0$
$C (graphite)+ O_2 \rightarrow C$

Chemical Thermodynamics L-5

Hess's law

Given that the standard enthalpy of combustion of graphite is -393.51 kJ ${mol}^{-1}$ and that of diamond is -395.41 kJ ${mol}^{-1}$ , calculate the enthalpy of the graphite-to-diamond transition.
$C (graphite)+ O_2 \rightarrow CO_2 (g)\Delta _rH^{\circ} =-393 . 5kJ {mol}^{-1}$
$C (diamond) +O_2 \rightarrow CO_2 (g)\Delta _rH^{\circ} =-395 . 4 {mol}^{-1}$
$C (graphite) \rightarrow C (diamond)$
$\Delta_rH^{\circ} =-393 . 5 + 395.4$
$= 1.90 \quad KJ mol^{-1}$

Chemical Thermodynamics L-5

THERMODYNAMICS

Chemical Thermodynamics L-5

Recap

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of formation

Chemical Thermodynamics L-5

Enthalpy of formation

Chemical Thermodynamics L-5

Enthalpy of formation

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Enthalpy of reaction

Chemical Thermodynamics L-5

Change in enthalpy

Chemical Thermodynamics L-5

Change in enthalpy

Chemical Thermodynamics L-5

Change in enthalpy

Chemical Thermodynamics L-5

Hess's law

Chemical Thermodynamics L-5

Hess's law

Chemical Thermodynamics L-5

Hess's law

Chemical Thermodynamics L-5

Hess's law

Chemical Thermodynamics L-5

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