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An exothermic reaction is one that releases energy into the surroundings, usually in the form of heat. This energy release occurs because the total energy required to break bonds in the reactants is less than the energy released when new bonds are formed in the products. In the case of the combustion of propane, the reaction equation is: \[ \mathrm{C}_{3} \mathrm{H}_{8}(g) + 5 \mathrm{O}_{2}(g) \rightarrow 3 \mathrm{CO}_{2}(g) + 4 \mathrm{H}_{2} \mathrm{O}(g) \]The enthalpy change (\(\Delta H\)) is negative, \(-2043\,\text{kJ}\), indicating that energy is released as the propane reacts with oxygen. This makes it an exothermic process. Such reactions are crucial in fuels, as they provide the heat energy necessary for power applications.
Whenever a chemical reaction has a negative enthalpy change, it is a sign of an exothermic reaction. Always remember, the more exothermic a reaction, the larger the negative value of \(\Delta H\).

• Negative \(\Delta H\) signifies an exothermic reaction.
• Combustion reactions often fall under exothermic categories.

Stoichiometry is the study of quantitative relationships or ratios in chemical reactions. It's like a recipe that tells you how much of each ingredient (or reactant) you need to get a desired amount of product. In the case of this exercise, we used stoichiometry to figure out how much propane is needed to release a certain amount of heat. Using the balanced equation: \[ \mathrm{C}_{3} \mathrm{H}_{8}(g) + 5 \mathrm{O}_{2}(g) \rightarrow 3 \mathrm{CO}_{2}(g) + 4 \mathrm{H}_{2} \mathrm{O}(g) \]We know that 1 mole of \(\mathrm{C}_{3} \mathrm{H}_{8}\) releases \(2043\,\text{kJ}\) of heat. We calculated that to produce \(369\,\text{kJ}\) of heat, approximately 0.18 moles of propane are required. This kind of calculation is common in chemistry and is essential for converting quantities of one substance into another, particularly when calculating how much of a compound is needed or produced in a reaction.

• Relies on balanced chemical equations.
• Helps in converting between moles, mass, and number of particles.

Molar mass is a crucial concept that links chemical formulas to measurable quantities. It denotes the mass of one mole of a substance and is calculated using the atomic masses of the elements in the molecule. For propane \(\mathrm{C}_{3} \mathrm{H}_{8}\), we must sum the masses of all the carbon and hydrogen atoms in the molecule.Propane consists of 3 carbon atoms and 8 hydrogen atoms: \[3 \times 12.01\,\text{g/mol} + 8 \times 1.01\,\text{g/mol} = 44.11\,\text{g/mol} \]This tells us that one mole of propane weighs 44.11 grams. Using this information alongside stoichiometry, we calculated that the 0.18 moles of propane needed to release \(369\,\text{kJ}\) of energy corresponds to approximately 7.94 grams. This calculation is vital in laboratory settings where precise amounts of chemicals are necessary.

• Calculated by summing atomic masses from the periodic table.
• Used to convert between moles and grams.