Thermochemical Equations - Online Tutor, Practice Problems & Exam Prep

Thermochemical equations combine stoichiometry with the thermodynamics of chemical reactions, specifically focusing on the enthalpy change (${\mathrm{\Delta H}}_{\mathrm{rxn}}$). These equations include a balanced chemical equation alongside the ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ of reaction, providing a quantitative measure of the heat absorbed or released during the reaction. To analyze these reactions, a thermochemical stoichiometric chart is employed, which allows for the conversion between different quantities—such as moles, grams, or molecules—and the associated enthalpy change. The key aspect of these equations is establishing a relationship between ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ and the moles of a compound within the reaction. This enables the calculation of unknown quantities by using the coefficients from the balanced equation to perform mole-to-mole comparisons, ultimately allowing for the determination of the enthalpy change for an unknown component in the reaction. Understanding thermochemical equations is essential for predicting the energy changes that accompany chemical processes.

Thermochemical Equations

Now recall that stoichiometry deals with the numerical relationship between compounds and a balanced chemical equation. Now with a thermochemical equation, we're going to deal with chemical reactions that include an enthalpy of reaction, which is ΔHRXN. So ΔHRXN of reaction.

Here with thermochemical equations, we're going to be introduced to our thermochemical stoichiometric chart. Now here the chart uses the given quantity of a compound to determine the unknown quantity of another compound. So here with the thermochemical equation we have our balanced chemical equation and to the side of it you'll see your ΔHRXN of reaction.

It's our job to make a connection between your enthalpy of reaction and either moles, grams, molecules, what have you in terms of the chemical reaction. So in a thermochemical equation, what we're trying to do is not a multimal comparison. We're trying to do a ΔHRXN to mole comparison and that's the key difference with the thermochemical equation.

Now that we've seen this, let's move onwards and talk about more with thermochemical equations.

Thermochemical Equations

Now, if you've seen my stoichiometric chart under stoichiometry or solution chemistry or gas stoichiometry, this should be pretty familiar to you. But if this is the first time you're seeing this documentary chart, let's go through it all right. So what we need to realize here is that with thermochemical equations, sometimes it's pretty common to be given the ΔH of our chemical reaction.

So we'll start out with ΔH of Given. And like I said before, in stoichiometry we're used to doing a multiple comparison, which can still happen here. But the more important thing is that we establish a connection between the ΔH of reaction and one of the moles for one of the compounds within our chemical reaction. So here we go from ΔH of Given to moles of given.

And what we can say here is that besides going from ΔH you're given to moles of Given, we can go from grams of Given to moles of Given or we can go from ions, atoms, formula units or molecules of given to moles of given. Once we get there, we have to go to moles of our unknown. To do this requires a leap of faith in a sense because you're going from an area where you know information to an area where you know nothing at all.

We call this the jump. When you make this jump, in order to do it correctly you have to do a multiple comparison and use the coefficients in the balanced equation. From this point you are at moles of unknown and from here you can go in any way you want. You can go from moles of unknown to ions, atoms, formula units or molecules. You can go to grams or you can go to a new ΔH of unknown.

So just realize what the thermochemical equation will have. A balanced chemical equation which introduces the idea of stoichiometry before to be a thermochemical equation, will also have the ΔH of reaction present.

Thermochemical Equations Example 1

Here it says consider the following thermochemical reaction. We have two moles of magnesium solid reacting with one mole of oxygen gas to produce 2 moles of magnesium oxide solid. It gives us an enthalpy of reaction equal to -1204 kilojoules. Here we're asked how many grams of magnesium oxide are produced during enthalpy change of -375 kilojoules.

So what we have to do here is we have to convert the given quantity into moles of given. All right. So they're giving us -375 kilojoules of energy and what we need to do is establish a relationship between magnesium oxide and this value of -375. Well, according to my balanced equation for Step 2, it says we need to do a mole to mole comparison to convert moles of given into moles of unknown here because it's a thermochemical equation. It's going to be moles of given relating to ΔH of reaction.

So we're going to say for every two moles of magnesium oxide, the energy involved or enthalpy involved is - 1204   kJ 2   mol . So we've just found our moles. In terms of magnesium oxide steps Three says if necessary convert the moles into desired units. Here they want grams, not moles. So we're going to do one more step and say for every one mole of magnesium oxide, the mass of magnesium oxide, one magnesium is 24.31g. According to periodic table, one oxygen is 16 grams. Multiply and then add those numbers together. Gives us 40.31 grams.

Moles cancel out and now I'm going to have 25.11 grams of magnesium oxide. Step 4 isn't needed here because in Step 4, if we have to calculate more than one final amount, then we must compare them to determine the theoretical yield. Here we're only given one given amount of -375 kilojoules and using that helped us to determine the final answer of 25.11 grams of magnesium oxide.

Nitromethane (CH₃NO₂), sometimes used as a fuel for drag racing, burns according to the following reaction: 

4 CH₃NO₂ (l) + 7 O₂ (g) → 4 CO₂ (g) + 6 H₂O (g) + 4 NO₂ (g)      ∆Hº = – 2441.6 kJ

How much heat is released by burning 125.0 g of nitromethane (MW:61.044 g/mol)? 

• A. 1.250 x 10³ kJ
• B. -1.250 x 10³ kJ
• C. 2.440 x 10³ kJ
• D. -2.440 x 10³ kJ

Consider the following reaction: 

2 C₆H₆ (l) + 15 O₂ (g) → 12 CO₂ (g) + 6 H₂­O (g)  ∆Hº = – 6278 kJ

What volume of benzene (C₆H₆, d = 0.880 g/mL, molar mass = 78.11 g/mol) is necessary to evolve 5.19 x 10⁹ kJ of heat? 

• A. 1.47 x 10⁸ mL
• B. 5.19 x 10⁹ mL
• C. 7.34 x 10⁷ mL
• D. 9.51 x 10⁹ mL

The creation of liquid methanol is accomplished by the hydrogenation of carbon monoxide: 

CO (g) + 2 H₂ (g) → CH₃OH (l)       ∆Hº = – 128.1 kJ

How much heat (in kJ) is released when 125.0 g CO reacts with 2.32 x 10² g H₂?

• A. 275 kJ
• B. -572 kJ
• C. -128 kJ
• D. 281 kJ

Here’s what students ask on this topic:

What are thermochemical equations?

Thermochemical equations are a form of chemical equations that not only show the balance between reactants and products but also include the change in enthalpy (ΔH) associated with the reaction. Enthalpy is a measure of the total energy of a thermodynamic system, and it includes both internal energy and the product of its volume and pressure.

In a thermochemical equation, the enthalpy change indicates whether the reaction is exothermic (releasing heat, $\Delta H$ is negative) or endothermic (absorbing heat, $\Delta H$ is positive). The coefficients in the equation represent the stoichiometry of the reaction and also the ratio in which the energy is absorbed or released. For example, if the equation includes a $\Delta H$ value with a negative sign, it tells us that the reaction releases that specific amount of energy as heat for the given stoichiometric amounts of reactants and products.

These equations are crucial for understanding the energy implications of chemical reactions, which is important in fields such as chemistry, materials science, and engineering. They help predict the energy changes that occur during chemical processes, which is essential for designing industrial chemical processes and understanding natural phenomena.

How do you calculate the enthalpy of reaction?

To calculate the enthalpy of reaction, also known as the heat of reaction, you typically use Hess's Law or the standard enthalpy of formation for the reactants and products involved in the reaction. Here's a step-by-step guide:

1. Write down the balanced chemical equation for the reaction.
2. Determine the standard enthalpy of formation (${\mathrm{\Delta H}}_f°$) values for each reactant and product from a data table. These values represent the enthalpy change when one mole of a substance is formed from its elements in their standard states.
3. Use the formula:
${\mathrm{\Delta H}}_{\mathrm{reaction}}°=\sum ({\mathrm{\Delta H}}_f°\text{of products})-\sum ({\mathrm{\Delta H}}_f°\text{of reactants})$
4. Multiply the standard enthalpy of formation for each substance by the number of moles in the balanced equation.
5. Subtract the total enthalpy of the reactants from the total enthalpy of the products to find the enthalpy of reaction.

Remember that the standard enthalpy of formation for an element in its standard state is zero. The enthalpy of reaction is typically expressed in kilojoules per mole (kJ/mol).

What does the enthalpy of reaction measure?

The enthalpy of reaction, often denoted as ${\mathrm{\Delta H}}_{\mathrm{rxn}}$, measures the total heat content change during a chemical reaction. It's the difference between the enthalpy of the products and the reactants. If ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ is negative, the reaction is exothermic, meaning it releases heat to the surroundings. Conversely, if ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ is positive, the reaction is endothermic, indicating that it absorbs heat from the surroundings. The enthalpy of reaction is a crucial concept in thermodynamics and is used to predict whether a reaction will occur spontaneously, as well as to understand the energy changes involved in chemical processes. It's typically measured in units of joules per mole (J/mol) or kilojoules per mole (kJ/mol).

What is the enthalpy of reaction?

The enthalpy of reaction, often denoted as ${\mathrm{\Delta H}}_{\mathrm{rxn}}$, is a thermodynamic quantity that measures the total heat content change during a chemical reaction. It is the difference between the enthalpy (heat content) of the products and the reactants. If ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ is negative, the reaction is exothermic, meaning it releases heat into the surroundings. Conversely, if ${\mathrm{\Delta H}}_{\mathrm{rxn}}$ is positive, the reaction is endothermic, meaning it absorbs heat from the surroundings.

To calculate the enthalpy of reaction, you can use the enthalpies of formation of the reactants and products, which are the enthalpies required to form one mole of a substance from its elements in their standard states. The formula is:

${\mathrm{\Delta H}}_{\mathrm{rxn}}=\sum {\mathrm{\Delta H}}_f\left(\text{products}\right)-\sum {\mathrm{\Delta H}}_f\left(\text{reactants}\right)$

Where ∑ represents the sum over all reactants and products, and ΔHf is the standard enthalpy of formation for each substance. This value is crucial for understanding the energy changes involved in chemical processes and for the design of chemical reactors and processes.