6.7
Thermochemical Equations
For a chemical reaction (the system) carried out at constant pressure – with the only work done caused by expansion or contraction – the enthalpy of r…
Most chemical reactions occur at atmospheric pressure. Under conditions of constant pressure, the heat change associated with the reaction, ΔQ, equals the change in enthalpy, ΔH, also called the enthalpy or heat of reaction.
Enthalpy of the reaction is the difference between the enthalpies of the products and the reactants. When the enthalpy of the products is greater than the enthalpy of the reactants, ΔH is positive. Such reactions absorb heat and are endothermic.
On the contrary, if the enthalpy of the reactants is greater than the enthalpy of the products, ΔH is negative. Such reactions release heat and are exothermic.
For any chemical reaction, the magnitude of the accompanying enthalpy change depends on the stoichiometric amounts of reactants and products, as indicated by the coefficients of the balanced equation.
A balanced chemical equation that includes phase labels and enthalpy of reaction, ΔH, is called a thermochemical equation.
Consider the combustion of methane – a primary source of fuel. Burning methane releases heat to the surroundings. The exothermic nature of the reaction is indicated by the negative enthalpy change in the thermochemical equation.
The equation for combustion shows that when one mole of methane gas reacts with 2 moles of oxygen gas to yield 1 mole of carbon dioxide gas and 2 moles of liquid water, 890.8 kilojoules of heat are released to the surroundings.
The molar ratio between the reactants or products and the heat of reaction can be used as conversion factors to calculate the heat exchanged during the reaction.
If a gas cylinder contains 25.5 kilograms of methane, and all of the methane in the cylinder undergoes combustion, what amount of heat will be produced? In general, the conceptual plan is to convert the mass to moles and then moles to the heat of reaction.
To begin, 25.5 kilograms is multiplied by 1000 to find the mass in grams. Then, the mass of methane is divided by its molar mass – 16.0 grams per mol, to yield 1594 moles of methane.
Finally, using the conversion factor between moles of methane and heat of reaction, 1594 moles of methane releases 14.2 times ten to the 6 kilojoules of heat - which is the heat of the reaction. The answer is negative since the reaction is exothermic as heat evolves in the reaction.
Q1: What is a thermochemical equation and why does it include phase labels?
A thermochemical equation is a balanced chemical equation that includes phase labels (solid, liquid, gas, aqueous) and the enthalpy change, ΔH, for the reaction. Phase labels are essential because the physical state of reactants and products affects the amount of heat released or absorbed. For example, forming liquid water releases more heat than forming water vapor from the same reactants.
Q2: How do you determine if a reaction is exothermic or endothermic from a thermochemical equation?
The sign of ΔH in a thermochemical equation indicates whether a reaction is exothermic or endothermic. A negative ΔH value means the reaction is exothermic and releases heat to the surroundings. A positive ΔH value means the reaction is endothermic and absorbs heat from the surroundings. The magnitude of ΔH shows how much heat is involved.
Q3: Why does the enthalpy change depend on the stoichiometric coefficients in a balanced equation?
The enthalpy change is an extensive property that depends on the amounts of reactants and products involved in the reaction. The stoichiometric coefficients in a balanced equation represent the molar ratios of substances. If you double all coefficients, you must also double the ΔH value because twice as much reactant produces twice as much heat.
Q4: How can you use a thermochemical equation to calculate heat released during a reaction?
The molar ratio between reactants or products and the heat of reaction serves as a conversion factor. First, convert the mass of a substance to moles using its molar mass. Then, multiply moles by the ΔH value per mole to find total heat exchanged. For example, 1594 moles of methane combusting at -890.8 kJ/mol releases 14.2 × 10⁶ kilojoules of heat.
Q5: What is the relationship between enthalpy change and heat at constant pressure?
At constant pressure, the heat change associated with a reaction, ΔQ, equals the change in enthalpy, ΔH. This relationship allows chemists to measure enthalpy changes experimentally using constant pressure calorimetry. The enthalpy of reaction represents the difference between the enthalpies of products and reactants under these conditions.
Q6: How does combustion of methane illustrate exothermic reactions in thermochemical equations?
Methane combustion is a classic exothermic reaction where one mole of methane gas reacts with two moles of oxygen gas to produce carbon dioxide and liquid water, releasing 890.8 kilojoules of heat. The negative ΔH value in the thermochemical equation indicates heat is released to the surroundings, making it an ideal example of how energy is quantified in chemical reactions.
Q7: Why is it important to specify physical states in thermochemical equations?
Physical states matter because the same reaction can release different amounts of heat depending on whether products are solids, liquids, or gases. For instance, hydrogen and oxygen forming liquid water releases 286 kJ/mol, but forming water vapor releases only 242 kJ/mol. Including phase labels ensures the thermochemical equation accurately represents the specific reaction conditions and energy change.