Question

Write thermochemical equations for the formation of one mole of the following compounds from the elements in their native states at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm} .\) (a) solid potassium chlorate (b) liquid carbon tetrachloride (c) gaseous hydrogen iodide (d) solid silver(I) oxide

Short answer

Question: Write thermochemical equations for the formation of one mole of the following compounds from their elements in their native states at 25°C and 1 atm: (a) solid potassium chlorate, (b) liquid carbon tetrachloride, (c) gaseous hydrogen iodide, and (d) solid silver(I) oxide. Answer: (a) 2 K(solid) + 3/2 Cl2(gas) + 3/2 O2(gas) -> KClO3(solid) (b) C(solid, graphite) + 2 Cl2(gas) -> CCl4(liquid) (c) 1/2 H2(gas) + 1/2 I2(solid) -> HI(gas) (d) 2 Ag(solid) + 1/2 O2(gas) -> Ag2O(solid)

Step-by-step solution

(a) solid potassium chlorate

Potassium chlorate (KClO3) is composed of potassium (K), chlorine (Cl), and oxygen (O) elements. At 25°C and 1 atm, the standard states of these elements are solid K, diatomic Cl2 gas, and diatomic O2 gas. To form one mole of potassium chlorate from these elements, the thermochemical equation is: 2 K(solid) + 3/2 Cl2(gas) + 3/2 O2(gas) -> KClO3(solid)

(b) liquid carbon tetrachloride

Carbon tetrachloride (CCl4) is made from carbon (C) and chlorine (Cl) elements. At 25°C and 1 atm, the standard states of these elements are solid graphite C and diatomic Cl2 gas. To form one mole of carbon tetrachloride from these elements, the thermochemical equation is: C(solid, graphite) + 2 Cl2(gas) -> CCl4(liquid)

(c) gaseous hydrogen iodide

Hydrogen iodide (HI) consists of hydrogen (H) and iodine (I) elements. At 25°C and 1 atm, the standard states of these elements are diatomic H2 gas and diatomic I2 solid. To form one mole of hydrogen iodide from these elements, the thermochemical equation is: 1/2 H2(gas) + 1/2 I2(solid) -> HI(gas)

(d) solid silver(I) oxide

Silver(I) oxide (Ag2O) is composed of silver (Ag) and oxygen (O) elements. At 25°C and 1 atm, the standard states of these elements are solid Ag and diatomic O2 gas. To form one mole of silver(I) oxide from these elements, the thermochemical equation is: 2 Ag(solid) + 1/2 O2(gas) -> Ag2O(solid)

Key concepts

Thermochemistry

Thermochemistry is a branch of thermodynamics focused on the study of the energy and heat associated with chemical reactions and phase transitions. The energy change during a chemical reaction is known as enthalpy (ΔH). A fundamental concept in thermochemistry is that a reaction is exothermic (releases heat) if ΔH is negative, and endothermic (absorbs heat) if ΔH is positive.

For example, when writing thermochemical equations, it's crucial to accurately represent not only the substances involved but also their physical states, such as solid (s), liquid (l), gas (g), or aqueous (aq). The accompanying enthalpy change directly ties to the stoichiometry of the reaction and whether heat is being absorbed or released. In the provided solutions, forming compounds like potassium chlorate or silver(I) oxide reflects changes in enthalpy. Understanding these equations helps students predict the heat exchange during the formation of different compounds.

Standard States of Elements

The standard state of an element is the reference state of a substance at a specified temperature and pressure, commonly set at 25°C (298 K) and 1 atmosphere (atm). These conditions are essential when calculating thermodynamic data like enthalpy of formation.

In the exercises, standard states were considered to determine the correct reactants. For instance, potassium is solid, chlorine and oxygen are gases under these conditions. Recognizing these states is pivotal when balancing thermochemical equations because they reflect the actual physical form in which each element is most stable at standard conditions. Noting that elements like iodine exist as a solid (I2) and hydrogen as a gas (H2) under standard conditions allows for correct equation formulation.

Formation of Compounds

When elements react to form compounds, they do so in a way that involves a rearrangement of atoms to create new chemical structures. The formation of compounds from their native elements, as seen in the exercise examples, is a fundamental reaction in chemistry. This is often represented through chemical equations which show how many moles of each element or compound are involved.

To write these equations correctly, one factors in the stoichiometry, demonstrates the conservation of mass, and ensures that the products reflect a real compound with correct formulas based on their chemical bonds and empirical data. This process can shed light on the type of bonding and the resulting properties of the compound, such as physical state and reactivity.

Chemical Stoichiometry

Chemical stoichiometry is the quantitative aspect of chemical equations, representing the relationships between the amounts of reactants and products in a chemical reaction. It adheres to the law of conservation of mass, meaning that the amount of each element must be the same on both sides of the equation.

By inspecting the balanced equations given in the solutions, you can notice that this balance is not always intuitive—fractions often appear to balance out the number of atoms of elements like oxygen or hydrogen, which might naturally occur as diatomic molecules. Before diving into the calculations, students need to ensure that they've correctly balanced the equation, since this will dictate the ratios of substances needed and formed, and by extension, the energy changes occurring during the reaction. Furthermore, understanding chemical stoichiometry allows students to predict the yields of products and the quantities of reactants required for a given reaction.