a. How many kilojoules are released when 125 g of Cl2 reacts with silicon? Si(s)+2 Cl2(g) ⟶SiCl

step 1 To determine the amount of energy associated with the process, we'll first reference the balance

A. How many kilojoules are released when 125 g of Cl2...

a. How many kilojoules are released when $125 \mathrm{~g}$ of $\mathrm{Cl}_{2}$ reacts with silicon? $$ \mathrm{Si}(s)+2 \mathrm{Cl}_{2}(g) \longrightarrow \mathrm{SiCl}_{4}(g) \quad \Delta H=-657 \mathrm{~kJ} $$ b. How many kilojoules are absorbed when $278 \mathrm{~g}$ of $\mathrm{PCl}_{5}$ reacts? $$ \mathrm{PCl}_{5}(g) \longrightarrow \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g) \quad \Delta H=+67 \mathrm{~kJ} $$

a. How many kilojoules are released when $125 \mathrm{~g}$ of $\mathrm{Cl}_{2}$ reacts with silicon?
$$
\mathrm{Si}(s)+2 \mathrm{Cl}_{2}(g) \longrightarrow \mathrm{SiCl}_{4}(g) \quad \Delta H=-657 \mathrm{~kJ}
$$
b. How many kilojoules are absorbed when $278 \mathrm{~g}$ of $\mathrm{PCl}_{5}$ reacts?
$$
\mathrm{PCl}_{5}(g) \longrightarrow \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g) \quad \Delta H=+67 \mathrm{~kJ}
$$

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Key Concepts

Exothermic and Endothermic Reactions

Exothermic and endothermic reactions are distinguished by the sign of their enthalpy change. An exothermic reaction (negative ?H) releases energy into the surroundings, while an endothermic reaction (positive ?H) absorbs energy, which is a key concept in thermochemistry.

Enthalpy Change (?H)

Enthalpy change represents the heat absorbed or released during a chemical reaction at constant pressure. It is used to quantify the energy exchange per mole of reaction and allows predictions of the thermal effect when reactants are converted to products.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced equation. It involves converting between masses, moles, and energy changes according to the proportions given by the reaction's coefficients, allowing prediction of the amounts of substances consumed and produced.

Molar Mass Conversion

Molar mass conversion is the process of converting the mass of a substance to the number of moles using its molar mass. This step is essential in stoichiometric calculations because chemical equations are balanced on a mole-to-mole basis rather than mass.

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