Why wouldn't you do this reaction directly in a lab (and would rather use Hess' Law to determine the enthalpy of formation)?:
1/2N2(g) + 2H2(g)+ 1/2Cl2(g) -> NH4Cl(s)
I don't think the reactants are that dangerous...? It's still pretty safe to do in a lab, isn't it? Are they too dangerous?
Is it because they're all gases and it's hard to work with gases in a lab...?
Nitrogen does not readily react with hydrogen, you would have great difficulty driving this reaction to completion, in fact, NH4CL in a solid state is difficult to stabilize. Why is ammonium chloride used in fireworks?
There are several reasons why one might choose to use Hess' Law to determine the enthalpy of formation instead of directly carrying out the reaction in a lab. Let's break it down:
1. Safety: While the reactants in the given equation (1/2N2(g) + 2H2(g) + 1/2Cl2(g)) may not be highly dangerous individually, it's important to consider the hazards posed by working with these gases in a lab setting. Gases can be challenging to handle, control, and contain, especially in large quantities. There is a risk of leaks, explosions, or other hazardous incidents, which could pose harm to the researchers, the lab, and the environment.
2. Reactant availability: It might not be practical or cost-effective to obtain pure gases like N2, H2, and Cl2 in a lab. These gases are typically stored and transported in pressurized cylinders, which require specialized equipment and handling procedures. Moreover, there might be regulatory restrictions on the storage, use, or disposal of certain gases.
3. Experimental limitations: Even if the gases were available, performing the reaction in a lab could be technically challenging. Gaseous reactions are generally more challenging to control and monitor accurately compared to reactions involving solids or liquids. Measuring the precise quantities, ensuring complete reactant conversion, and minimizing losses during transfer can introduce errors and uncertainties in the experimental data.
Using Hess' Law provides an alternative approach that avoids these challenges. It allows us to determine the enthalpy change of a reaction indirectly by combining known enthalpy changes of other reactions. This method relies on the principle that enthalpy is a state function—the total change in enthalpy is independent of the pathway taken. By strategically choosing other reactions with known enthalpies, we can cancel out unwanted components and calculate the enthalpy change of the desired reaction. This approach provides a safer, more controlled, and often more practical means of determining the enthalpy of formation.