Question

measurements show that the energy of a mixture of gaseous reactants increases by 281. kj during a certain chemical reaction, which is carried out at a constant pressure. furthermore, by carefully monitoring the volume change it is determined that 126. kj of work is done on the mixture during the reaction calculate the change in enthalpy of the gas mixture during the reaction. be sure your answer has the correct number of significant digits. is the reaction exothermic or endothermic? exothermic endothermic

Explanation:

Step1: Recall the first law of thermodynamics

The first law of thermodynamics is given by $\Delta U = q + w$, where $\Delta U$ is the change in internal energy, $q$ is the heat added to the system, and $w$ is the work done on the system. At constant pressure, the change in enthalpy $\Delta H$ is equal to the heat change $q_p$, so $\Delta H = q_p$. Also, we know that $\Delta U = q + w$. Here, $\Delta U$ (change in internal energy) is given as the increase in energy of the reactants, so $\Delta U = + 281\space kJ$ (positive because energy increases), and $w = + 126\space kJ$ (positive because work is done on the system).

First, we can find $q$ from $\Delta U = q + w$. Rearranging for $q$, we get $q=\Delta U - w$.

Step2: Calculate the heat change $q$

Substitute the values: $\Delta U = 281\space kJ$ and $w = 126\space kJ$ into the formula $q=\Delta U - w$.

$q = 281 - 126 = 155\space kJ$

Since at constant pressure $\Delta H = q_p$, so $\Delta H=155\space kJ$

Step3: Determine if the reaction is exothermic or endothermic

A reaction is endothermic if heat is absorbed by the system ( $q$ is positive) and exothermic if heat is released by the system ( $q$ is negative). Here, $q = 155\space kJ$ (positive), which means the system absorbs heat, so the reaction is endothermic.

Answer:

The change in enthalpy is $\boldsymbol{155}$ kJ. The reaction is endothermic (the option is: endothermic).