The combustion of hydrogen-oxygen mixtures is used to...

The combustion of hydrogen-oxygen mixtures is used to produce very high temperatures (approximately $2500^{\circ} \mathrm{C}$ ) needed for certain types of welding operations. Consider the reaction to be $$\mathrm{H}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \Delta_{\mathrm{r}} H^{\circ}=-241.8 \mathrm{kJ} \mathrm{mol}^{-1}$$ What is the quantity of heat evolved, in kilojoules, when a $180 \mathrm{g}$ mixture containing equal parts of $\mathrm{H}_{2}$ and $\mathrm{O}_{2}$ by mass is burned?

The combustion of hydrogen-oxygen mixtures is used to produce very high temperatures (approximately $2500^{\circ} \mathrm{C}$ ) needed for certain types of welding operations. Consider the reaction to be
$$\mathrm{H}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g}) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \Delta_{\mathrm{r}} H^{\circ}=-241.8 \mathrm{kJ} \mathrm{mol}^{-1}$$
What is the quantity of heat evolved, in kilojoules, when a $180 \mathrm{g}$ mixture containing equal parts of $\mathrm{H}_{2}$ and $\mathrm{O}_{2}$ by mass is burned?

Key Concepts

Enthalpy Change of Reaction

The enthalpy change of a reaction, often expressed in kilojoules per mole, quantifies the heat absorbed or released during a chemical reaction at constant pressure. It is a key aspect in thermochemistry that connects chemical processes with energy changes.

Molar Mass Conversion

Molar mass conversion is the method of converting the mass of a substance into the number of moles using its molar mass. This step is crucial in relating the mass of reactants or products to the corresponding molar amounts needed for applying stoichiometric relationships.

Thermochemical Calculations

Thermochemical calculations combine the stoichiometric relationships from the balanced equation with the enthalpy change to determine the overall heat evolved or absorbed in a reaction. This process typically involves calculating the moles of the limiting reagent and then using the energy change per mole to find the total energy change.

Stoichiometry

Stoichiometry involves using the balanced chemical equation to determine the quantitative relationships between the reactants and products. It allows one to relate the number of moles of one substance to the number of moles of another, making it possible to calculate how much product is formed or reactant is required in a chemical reaction.

Limiting Reagent Determination

The concept of a limiting reagent is used to identify which reactant will be entirely consumed first, thereby determining the maximum amount of product that can be formed in a reaction. This process is essential when reactants are not mixed in their strictly stoichiometric ratios.

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