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Fire

Chapter 4 - What Drives Chemical Reactions?

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Atom Symbol

Chapter 1 - Models Of The Particulate Nature Of Matter

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Chapter 2 - Models Of Bonding & Structure

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 Globe with Meridians

Chapter 3 - Classification Of Matter

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Rocket

Chapter 5 - How Much, How Fast & How Far?

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Test Tube

Chapter 6 - What Are The Mechanisms Of Chemical Change?

Imagine you&#39;re in New York City&#39;s subway system. You want to travel from Times Square (point A) to Central Park (point B). You can go directly or make a stop at Rockefeller Center (point C) and then proceed to Central Park. Whether you take the direct route or stop at Rockefeller, the total distance you travel remains the same. This is the essence of Hess&rsquo;s law!

<ul>
	<li>A chemical reaction can be like a journey with multiple routes.</li>
	<li>Just like in a subway system, there can be different paths to transform reactants into products.</li>
	<li>The change in energy, or enthalpy (&Delta;H), of the entire journey remains constant, regardless of the route taken.</li>
</ul>

Overall vs. Stepwise Reactions

<ul>
	<li>Example: Transforming solid sulfur (S) into sulfur trioxide (SO3).

	<ul>
		<li>Direct Route: S(s) + \(\frac{3}{2}\) O2(g) &rarr; SO3(l)</li>
		<li>Two-Step Journey:
		<ul>
			<li>S(s) + O2(g) &rarr; SO2(g)</li>
			<li>SO2(g) + \(\frac{1}{2}\) O2(g) &rarr; SO3(l)</li>
		</ul>
		</li>
	</ul>
	</li>
</ul>

Hess&#39;s Law Explained

<ul>
	<li>The sum of the energy changes (enthalpies, &Delta;H) for each step in the two-step journey will equal the energy change of the direct route.</li>
	<li>In other words: No matter which subway line you take, the total journey energy remains the same as long as you start and end at the same stations.</li>
</ul>

Hess&#39;s Law = Conservation of Energy

<ul>
	<li>Energy can neither be created nor destroyed; it can only change forms.</li>
	<li>The energy in a closed system remains constant, be it in the form of heat, light, or chemical potential.</li>
</ul>

<ul>
	<li>
	Visualize Hess&#39;s Law

	<ul>
		<li>Picture three stations: A, B, and C.</li>
		<li>The energy change (&Delta;Hx) from A &rarr; B is the same as traveling from A &rarr; C and then C &rarr; B. So, &Delta;Hx = &Delta;Hy + &Delta;Hz.</li>
	</ul>
	</li>
	<li>
	Practical Use of Hess&rsquo;s Law

	<ul>
		<li>Hess&#39;s law allows chemists to determine the energy change (enthalpy) of reactions that are tough to measure directly.</li>
		<li>By using known reactions with known enthalpies, one can calculate the unknown enthalpy for a specific reaction.</li>
	</ul>
	</li>
</ul>

Think of cooking! Sometimes, recipes have a shortcut and a long route. For instance, you might have a 2-step method to make a sauce (like first caramelizing the onions and then blending them with tomatoes) or a direct one. No matter which method you choose, the final taste of the sauce should be the same! This is similar to Hess&#39;s Law: different routes, same final state.

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✅ Remember: Always keep track of your &#39;journey&#39; in chemical reactions. Sometimes the scenic route offers more insight! 🚇🔬🌍

Hess's Law: The Unchanging Route of Chemical Reactions

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Standard Level

Chemistry SL

Chapter 4 - What Drives Chemical Reactions?

Hess's Law: The Unchanging Route of Chemical Reactions

Table of content

Real-world analogy🚇

Main concept🔍

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Chapter 4 - What Drives Chemical Reactions?

Hess's Law: The Unchanging Route of Chemical Reactions

Table of content

Real-world analogy🚇

Main concept🔍

Unlock the Full Content!