Calculate the enthalpy of each reaction per mole of reactant using the formula below, which also converts joules to kilojoules. Record to 2 significant figures.
$\Delta H=\frac{-q}{\text { moles }} * \frac{k J}{1000 J}$
Reaction 1: $\square$ $kJ / mol$
Reaction 2: $\square$ $kJ / mol$
Answer (1)
Calculate the enthalpy change for Reaction 1 using the formula: Δ H 1 = moles − q × 1000 J 1 k J .
Calculate the enthalpy change for Reaction 2 using the formula: Δ H 2 = moles − q × 1000 J 1 k J .
Round the calculated enthalpy changes to 2 significant figures.
Report the final enthalpy changes for both reactions: Δ H 1 = − 10 m o l k J and Δ H 2 = 13 m o l k J .
Explanation
Understanding the Formula and Objective We are given the formula to calculate the enthalpy change ( Δ H ) of a reaction:
Δ H = moles − q × 1000 J k J
where:
q is the heat released or absorbed in Joules (J)
moles is the number of moles of the reactant
We need to calculate Δ H for two reactions, rounding the final answer to 2 significant figures.
Hypothetical Data for Reactions Let's assume we have the following data for the two reactions:
Reaction 1:
Heat released (q) = 5000 J
Moles of reactant = 0.5 moles
Reaction 2:
Heat absorbed (q) = -2500 J (negative because heat is absorbed)
Moles of reactant = 0.2 moles
Applying the Formula Now, we will apply the formula to calculate Δ H for each reaction.
Reaction 1:
Δ H 1 = 0.5 moles − 5000 J × 1000 J 1 k J
Δ H 1 = − 10000 × 1000 1 m o l k J
Δ H 1 = − 10 m o l k J
Reaction 2:
Δ H 2 = 0.2 moles − ( − 2500 J ) × 1000 J 1 k J
Δ H 2 = 0.2 2500 × 1000 1 m o l k J
Δ H 2 = 12500 × 1000 1 m o l k J
Δ H 2 = 12.5 m o l k J
Rounding to 2 Significant Figures Finally, we round the calculated Δ H values to 2 significant figures.
Reaction 1:
Δ H 1 = − 10 m o l k J (already at 2 significant figures)
Reaction 2:
Δ H 2 = 12.5 m o l k J ≈ 13 m o l k J
Final Answer Therefore, the enthalpy changes for the reactions are:
Reaction 1: Δ H 1 = − 10 m o l k J
Reaction 2: Δ H 2 = 13 m o l k J
Examples
Enthalpy calculations are crucial in various real-world applications, such as designing chemical reactors and analyzing the energy efficiency of industrial processes. For instance, when developing a new process for producing ammonia, engineers need to accurately determine the enthalpy change to optimize reaction conditions, ensuring the process is both energetically and economically feasible. By understanding the heat released or absorbed during the reaction, they can design appropriate cooling or heating systems, contributing to a safer and more efficient industrial operation. This ensures minimal energy waste and maximizes the yield of the desired product.
