Subiectul III

Cuprins

Subiectul F.
Subiectul G1. ( OBLIGATORIU PENTRU NIVELUL I)
Subiectul G2. ( OBLIGATORIU PENTRU NIVELUL II)

Subiectul F.

Acetilena, , este utilizată pentru aparatele de sudură autogenă a metalelor. Scrieţi ecuaţia termochimică a reacţiei de ardere a acetilenei, ştiind că rezultă dioxid de carbon şi vapori de apă.

Rezolvare:

$C_2H_2+\dfrac{5}{2}O_2=2CO_2+H_2O$ .

Determinaţi masa de acetilenă, exprimată în grame, care prin ardere degajă $\text{kJ}$ , cunoscând entalpia de combustie $\Delta _cH^0_{C_2H_2}=-801,3$ $\text{kJ}/\text{mol}$ .

Rezolvare:

$\begin{align*} &801,3\ \text{kJ}\dotsc\dotsc\dotsc\dotsc 1\ \text{mol}\ \text{acetilen\u a} \\ & 16026\ \text{kJ}\dotsc\dotsc\dotsc\dotsc\ x \ \text{moli}\ \text{acetilen\u a} \\ &\noindent\rule{7cm}{0.4pt} \end{align*}$

$\begin{align*}\Rightarrow x&=\frac{16026\cdot 1}{801,3}\\\\&=20 \end{align*}$

$\begin{align*}\Leftrightarrow x&=20\ \text{moli}\ \text{acetilen\u a} \end{align*}$ .

Atunci:

$\begin{align*} n=\frac{m}{M} \end{align*}$

$\begin{align*} \Rightarrow m=n\cdot M \end{align*}$

$\begin{align*} \Leftrightarrow m=20\cdot 26 \end{align*}$

$\begin{align*} \Leftrightarrow m=520 \end{align*}$ g acetilenă

$\begin{align*}\Leftrightarrow m_{_{C_2H_2}}=520\ \text{g} \end{align*}$ .

Calculaţi căldura, exprimată în Jouli, care se degajă la răcirea a $\text{kg}$ de apă, de la temperatura la temperatura . Se consideră că nu au loc pierderi de căldură.

Rezolvare:

Căldura specifică a apei:

$\begin{align*} c=4,18\ \frac{\text{kJ}}{\text{kg}\cdot \text{grad}} \end{align*}$

$\begin{align*} Q&=m\cdot c\cdot \Delta t\\\\ &=18\ \text{kg}\cdot 4,18\ \frac{\text{kJ}}{\text{kg}\cdot \text{grad}}\cdot 30\ \text{grad}\\\\ &=2257,2\ \text{kJ}\\\\ &=2257,2\ \text{J}\cdot 10^{3} \end{align*}$

$\begin{align*} \Leftrightarrow Q&=2257,2\cdot 10^{3}\ \text{J} \end{align*}$

Determinaţi variaţia de entalpie, $\Delta _rH$ , pentru reacţia reprezentată prin ecuaţia:

$CaO(s) +H_2O(l) \to Ca(OH)_2(s)$ , $\Delta _rH$

utilizând ecuaţiile termochimice:

$\begin{align*} &Ca(s)+\dfrac{1}{2}O_2(g)\to CaO(s) \quad\quad \Delta _rH_1=-634,94\ \text{kJ} \end{align*}$

$\begin{align*} &Ca(s)+\dfrac{1}{2}O_2(g)\to CaO(s) \quad\quad\quad\quad\quad\quad\quad\quad\quad\ \ \Delta _rH_1=-634,94\ \text{kJ}\\ &Ca(s)+2H_2O(I)\to Ca(OH)_2(s)+H_2(g) \quad\quad\quad \Delta _rH_2=-414,66\ \text{kJ} \\ &H_2(g)+\dfrac{1}{2}O_2(g)\to H_2O(I) \quad\quad\quad \quad\quad\quad \quad\quad\quad\ \ \Delta_r H_3=-285,50 \ \text{kJ} \end{align*}$

Rezolvare:

$\begin{align*} \Delta _rH&=(\Delta _rH_2+\Delta _rH_3)-\Delta _rH_1\\ &=\left [ -414,66+(-282,50) \right ]-(-634,94)\\ &=-700,16+634,94\\ &=-65,22 \end{align*}$

$\begin{align*} \Leftrightarrow \Delta _rH&=-65,22\ \text{kJ} \end{align*}$ .

Scrieţi următorii oxizi: , , în ordinea crescătoare a stabilităţii. Justificaţi răspunsul. Se cunosc entalpiile molare de formare standard:

$\Delta _fH^0_{Fe_2O_3(s)}=-821,4$ $\text{kJ}$ , $\Delta _fH^0_{Al_2O_3(s)}=-1670$ $\text{kJ}$ , $\Delta _fH^0_{MgO(s)}=-602$ $\text{kJ}$ .

Rezolvare:

Cu cât entalpia de formare este mai negativă, la formarea moleculei din atomii componenți se degajă o cantitate mai m...