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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
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Tutti i libri di testoBrown 14th EditionCh.5 - ThermochemistryProblema 47b
Capitolo 5, Problema 47b

Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (b) Balance the forward reaction with whole-number coefficients. What is ΔH for the reaction represented by this equation?

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1
Write down the unbalanced chemical equation: CH3OH(l) + O2(g) → CO2(g) + H2O(l).
Balance the carbon atoms first. There is one carbon atom on both sides of the equation, so the CO2 coefficient is 1.
Next, balance the hydrogen atoms. There are four hydrogen atoms in CH3OH, so you need 2 water molecules (H2O) on the product side to have four hydrogen atoms.
Balance the oxygen atoms last. There are 2 oxygen atoms in CO2 and 2 more in the two H2O molecules, totaling 4 oxygen atoms in the products. Therefore, you need 2 O2 molecules on the reactant side to provide 4 oxygen atoms.
The balanced chemical equation is: CH3OH(l) + 2 O2(g) → CO2(g) + 2 H2O(l). The ΔH for the reaction remains the same, -726.5 kJ, as the stoichiometry of the reaction involving CH3OH has not changed.

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Combustion Reactions

Combustion reactions are exothermic processes where a substance reacts with oxygen to produce heat and light. In the case of methanol, the reaction with oxygen produces carbon dioxide and water, releasing energy. Understanding the nature of combustion is essential for analyzing the energy changes and products formed in such reactions.
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Combustion Apparatus

Balancing Chemical Equations

Balancing chemical equations involves ensuring that the number of atoms for each element is the same on both sides of the equation. This is crucial for obeying the law of conservation of mass. In the given reaction, balancing with whole-number coefficients allows for accurate stoichiometric calculations and understanding of the reaction's proportions.
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Balancing Chemical Equations

Enthalpy Change (ΔH)

Enthalpy change (ΔH) represents the heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH indicates an exothermic reaction, meaning energy is released, as seen in the combustion of methanol. Knowing how to interpret ΔH is vital for understanding the energy dynamics of chemical reactions.
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Enthalpy of Formation
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Consider the combustion of liquid methanol, CH3OH(l): CH3OH(l) + 3/2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -726.5 kJ (a) What is the enthalpy change for the reverse reaction?

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