Ever thought about why chemicals react the way they do? It all comes down to understanding energy, and that's where the first law of thermodynamics comes into play. This law tells us that energy doesn't just pop in or out during chemical reactions—it's all about changes. Specifically, let's explore enthalpy changes to figure out how substances behave in the world of chemistry.

🌀 First Law of Thermodynamics and Energy Exchange

System and Surroundings

In thermochemistry, we divide the universe into two parts:

• The system: The part we are studying, usually a chemical reaction or physical process.
• The surroundings: Everything outside the system.

Imagine our system as a beaker where a reaction occurs. The lab around the beaker is the surroundings.

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Heat (q) and Work (w)

In chemical reactions, energy can move as heat (q) or work (w) between the system and surroundings:

• Heat is moves due to temperature changes.
• Work here often means gases expanding or compressing in the system.

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🔥 Enthalpy Changes in Reactions

Defining Enthalpy (H)

Enthalpy is determined by $H=U+PV$ and it measures how much potential energy there is, and how much space it takes up under constant pressure.

Types of Reactions

Different types of reactions—combustion, formation, dissolution—have special ΔH values:

• Combustion: Often highly exothermic; imagine lighting up a grill.
• Formation: Making new compounds from elements; some take in heat while others release it.
• Dissolution: Dissolving salt might seem uneventful but involves subtle heats of solution.

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There are different methods for calculating energy changes which include:

• Hess’s Law, which looks at overall reaction steps
• $ΔH_f°$, which adds up individual pieces from scratch
• Bond enthalpies, which provide a detailed view at the individual bond level

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📊 Calculations Energy Changes

Hess’s Law

Hess's Law tells us that total enthalpy change doesn't depend on pathway taken—it only cares about start and finish lines! Here’s how to use Hess’s Law step-by-step:

1. Write down each step with known ΔH values leading to final reaction.
2. Add or subtract these steps ΔH values just like you would with math equations, until you get your overall reaction.

Standard Enthalpies

Standard Enthalpies of Formation ($ΔH_f°$) involves calculating total enthalpy changes for making compounds from their elements at standard states:

1. Find $ΔH_f°$values for each reactant/product from tables.
2. Use equation:

✏️ Practice Problem

Consider a reaction where 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of water vapor:

We know the following $ΔH_f°$ values:

$ΔH_f° (H_2O(g)) = -241.8$

$ΔH_f° (O_2(g)) = 0$

$ΔH_f° (H_2(g)) = 0$

Now, applying Hess's Law, we can calculate the ΔH for the given reaction:

$ΔH_{reaction} = Σ(ΔH_f°{products})-Σ(ΔH_f°{reactants})$

$ΔH_{reaction} = [2*(-241.8{kJ/mol})] - [2*0{kJ/mol} + 1*0{kJ/mol}]$

$ΔH_{{reaction}} = -483.6{kJ}$

🔗 Bond Enthalpies & Reaction Enthalpies

Bond enthalpies tell us how much energy we need to break or make bonds:

1. Recognize the bonds broken or formed in a reaction.
2. Add up the energy needed to break existing bonds (+) and the energy released when new bonds form (-).

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🌡️ Calorimetry & Measuring Heat Transfer

Calorimetry is a method for measuring the exact amount of heat exchanged, and it uses specific heat capacities with the equation:

To practice calorimetry:

1. Set up calorimeter according to procedure ensuring no heat escapes/is gained from surroundings.
2. Measure the temperature change accurately after adding or reacting substances within the calorimeter.
3. Apply above formula remembering m=mass, c=specific heat capacity, ΔT=temperature change.

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✏️ Practice Problem

If you have 100 grams of water, and you want to heat it up from 20°C to 50°C using a heater, where the specific heat capacity of water is 4.18 J/g°C, how much heat (q) is needed?

Using the formula:

Plug in the values given above.

$q = 100{g} *4.18{J/g°C} * (50°C - 20°C)$

$q = 100 {g} * 4.18 {J/g°C} * 30°C$

$q = 12540{J}$

So, 12,540 joules of heat are needed to raise the temperature of 100 grams of water from 20°C to 50°C.

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✨ Thermochemistry in Real Life

Energy production relies a lot on thermochemical principles, especially when we burn fossil fuels. This process releases a bunch of stored energy as heat, which we can use. But, it comes with a downside – it produces greenhouse gases that contribute to global warming.

In the industrial world we can use it to:

1. Understand whether reactions are endothermic or exothermic which influences how we design reactors, making sure they're safe and efficient.
2. Figure out costs involves knowing about the energy in reactions, helping us make smart economic choices.
3. To get the most out of a reaction, we need to understand how it behaves under different conditions like pressure and temperature.

Now you've got the basics of the first law of thermodynamics and enthalpy changes! Keep practicing using these equations and exploring because you're well on your way to mastering thermodynamics! 😄