Heat

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Chem/Reaction

Cegep/2

Word count

226 words

Reading time

2 minutes

Flow of thermal energy from a hot object to a colder object
Sym. $q$

$$q=ms\mathrm{\Delta }T\text{ where }s=\text{specific heat capacity}$$

At constant pressure:

$$\begin{array}{rl}{q}_P& =\mathrm{\Delta }E+P\mathrm{\Delta }V=\mathrm{\Delta }E+RT\mathrm{\Delta }{n}_g\\ q_{rxn}& =-(\underset{\approx 0}{\underbrace{q_{cal}}}+q_{sol})=-m_{sol}{s}_{sol}\mathrm{\Delta }T\end{array}$$

At constant volume:

$$\begin{array}{rl}{q}_V& =\mathrm{\Delta }E\\ q_{rxn}& =-(q_{cal}+q_{water})=-(C_{cal}\mathrm{\Delta }T+m_{H_{2}O}{s}_{H_{2}O}\mathrm{\Delta }T)=-(C_{cal}+m_{H_{2}O}{s}_{H_{2}O})\mathrm{\Delta }T\end{array}$$

$1\pu{cal} = 4.184\pu{J}$

Examples

1.435 𝑔 of naphthalene (𝐶10𝐻8) was burned in a bomb calorimeter. Consequently, the temperature of the water rose from 20.17 °𝐶 to 25.84 °𝐶. If the mass of water surrounding the calorimeter was exactly 2000 𝑔 and the heat capacity of the bomb calorimeter was 1.80 𝑘𝐽 °𝐶 , calculate $\mathrm{\Delta }{E}_{rxn}$ of combustion of naphthalene.

$$\mathrm{\Delta }{E}_{rxn}=q_V=-(1.80\mathrm{kJ}{}^{\circ }\mathrm{C}+2000\mathrm{g}\cdot 4.184\frac{\mathrm{J}}{\mathrm{g}{}^{\circ }\mathrm{C}}\cdot \frac{1\mathrm{kJ}}{1000J})(25.84°C-20.17°C)\approx -57.6526\mathrm{kJ}$$

0.500 𝑔 of magnesium and 100.0 𝑚𝐿 of 1.00 𝑀 𝐻𝐶𝑙 are placed in a coffee-cup calorimeter. The temperature of the solution increases from 22.2 °𝐶 to 44.8 °𝐶 . What is the enthalpy change for this reaction per mole of 𝑀𝑔? (assume the specific heat capacity of the solution is 4.20 $\frac{J}{gK}$ and the density of 𝐻𝐶𝑙 is 1.0 $\frac{\mathrm{g}}{\mathrm{mL}}$). Assume that the heat capacity of the Styrofoam cup is negligible.

$$\mathrm{\Delta }H=q_P=-(100.0\mathrm{mL}\cdot 1.0\frac{\mathrm{g}}{\mathrm{mL}}+0.500\mathrm{g})4.20\frac{\mathrm{J}}{\mathrm{gK}}(44.8{}^{\circ }\mathrm{C}-22.2{}^{\circ }\mathrm{C})=9539.46\mathrm{J}$$

Contributors

Ajitani Hifumi

Changelog

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