Video: Standard Enthalpy Changes Chemistry • First Year of Secondary School

Video Transcript

In this video, we will learn how to describe different types of standard enthalpy changes and define them. Something that can be confusing is what exactly the word standard means. In this video, we’ll see standard used in three different ways: standard conditions, like standard temperature and pressure, standard states, and standard enthalpy changes. In detail, standard means something slightly different for each. But in general, standard means that there is agreement about how these things are defined. Let’s dive in.

An enthalpy change is simply the change in the energy of the system we’re looking at. If an exothermic reaction happens in our system, bonds are usually broken and new bonds are formed. Chemical potential energy is converted into other forms of energy like heat, and this energy will leave the system and pass to the surroundings. This means that the enthalpy of our system goes down. In an endothermic reaction, other forms of energy, like heat, are converted into chemical potential energy in our system. Energy from the surroundings flows into the system, increasing the enthalpy of the system. We use the symbol Δ𝐻 to indicate a change in enthalpy.

This all sounds great, but there’s a little problem. Enthalpy changes can vary with temperature and pressure. For instance, the enthalpy change for a reaction occurring at 25 degrees Celsius might be a little different to a measurement for the same reaction at 50 degrees Celsius. This is why standard enthalpy changes were invented. A standard enthalpy change is simply the expected change in enthalpy that would occur for a particular reaction or process under standard conditions. You’ll often see standard enthalpy rather than standard enthalpy change is often used to mean the same thing. We’ll always be looking at enthalpy change when talking about standard enthalpies.

When standard conditions have been used, this character, called a Plimsoll, is added to the top right of the enthalpy change symbol. A standard condition is simply an agreed-upon value for a condition when we make a measurement. One of the most common standard conditions is that pressure is fixed at a value of one atmosphere. The very close value of one bar is also sometimes used. One atmosphere is equivalent to 1.01325 bar. There isn’t a single agreed-upon temperature, but most standard enthalpies will be stated at 25 degrees Celsius, which is equivalent to 298.15 kelvin. For most standard enthalpies, you can assume that the temperature is 25 degrees Celsius unless otherwise stated.

For enthalpy changes involving solutes, typically we use a concentration of one molar. When looking around at different data, you may see enthalpy changes measured under different standard conditions than these. Take care to investigate which ones are being used so you don’t mix them up. When talking about standard enthalpy changes, you’ll often hear about standard states. But what does this mean? Since enthalpies are hard to measure directly, scientists came up with a special system with a special reference point for each chemical. This is a way of keeping track of the changes in enthalpy during reactions that everyone can agree on.

For most chemicals, the standard state is simple. It’s the typical state of that chemical at one atmosphere of pressure and 25 degrees Celsius. The standard state for water H2O is H2O liquid because water is typically a liquid at one atmosphere of pressure and 25 degrees Celsius. The choices of standard state for elements aren’t all simple unfortunately. For some elements, you’ll just have to remember. For most elements like hydrogen, we choose their typical state. It’s very easy. The standard state for hydrogen is H2 gas. It’s also clear for elements like helium, He gas, O2 gas, solid sodium, and liquid mercury since these are the states we commonly find these pure elements at 25 degrees Celsius and one atmosphere.

Some elements have more than one common form or allotrope, so scientists had to agree which one to pick. For instance, carbon has many stable forms at one atmosphere and 25 degrees Celsius that have slight differences in their enthalpies. So scientists agreed on graphite as the standard state for carbon. If we’re being picky, writing carbon solid isn’t really enough when doing a standard enthalpy change because it’s not clear what form the solid carbon is in.

Next, we’re going to go through a few types of standard enthalpy change. First, we’ll look at the enthalpy changes associated with a substance transforming from a solid into a liquid or a liquid into a gas. The first item is the standard enthalpy of fusion, which is the enthalpy change when a substance transforms from a solid to a liquid by melting. The standard pressure for this process will be one atmosphere, but the temperature will be the temperature for the phase transition, the temperature at which the substance normally melts.

For instance, the standard enthalpy of fusion of water is 6.0 kilojoules per mole. We can write the equation for this process like this. We have solid water on the left turning to liquid water on the right at one atmosphere and zero degrees Celsius. We use zero degrees Celsius specifically because it’s the melting point of water. Melting is an endothermic process. We require energy to turn a solid into a liquid, so the enthalpy change is positive. This is because it takes energy to overcome the intermolecular forces in a solid to turn something into a liquid.

The reverse process is solidification or freezing, turning from a liquid to a solid. We can describe the standard enthalpy of solidification as the enthalpy change when a substance freezes at standard pressure at the melting point of the substance per mole of substance. For a substance like water, the melting point and the freezing point are the same, and we’d still be using one atmosphere as our standard pressure. The standard enthalpy of solidification or with a negative is the standard enthalpy of fusion. So the standard enthalpy of solidification of water is negative, and the process is exothermic since the formation of intermolecular bonds releases energy to the surroundings. We’re dealing with the exact same process but in reverse.

The standard enthalpy change associated with a substance boiling is called the standard enthalpy of vaporization. A standard enthalpy of vaporization is the enthalpy change when a substance boils at standard pressure at the boiling point of the substance per mole of substance. The pressure will be one atmosphere, and the boiling point will depend on the substance. The standard enthalpy of vaporization of water is 40.7 kilojoules per mole. This again is endothermic because we’re having to introduce energy to break into molecular bonds. The chemical equation has its turning liquid water into gaseous water at one atmosphere and 100 degrees Celsius, the boiling point of water.

The reverse of vaporization is condensation. A standard enthalpy of condensation is the enthalpy change when a substance condenses at standard pressure at the boiling point of the substance per mole of substance. The pressure is one atmosphere, and the boiling point will depend on the substance and can be interchanged with the condensation point because they’re the same. The standard enthalpy of condensation of water is the negative of this standard enthalpy of vaporization of water. So it’s negative 40.7 kilojoules per mole. This is exothermic since energy is being given out as intermolecular forces between the gas molecules form as water turns into a liquid. The temperature during this process is held at a constant 100 degrees Celsius, the boiling point of water.

There are enthalpy changes associated with sublimation and deposition, but we won’t be looking at them in this video. Instead, we’ll take a brief look at heat curves, one of the ways we can figure out what the enthalpy change associated with a state change actually is. For this example, we’re going to imagine we’re applying heat to water and measuring the temperature. The melting point of water is zero degrees Celsius, so up until that point, it will be a solid. The boiling point of water is 100 degrees Celsius, so between zero and 100 degrees Celsius, it will be a liquid. Above 100 degrees Celsius, it will be a gas.

We’re going to be measuring our energy relative to the amount of water we have in moles. You may come across situations where you have the mass or volume of water, in which case you need to calculate the amount per mole by doing the appropriate calculations. This is how the curve would appear in ideal circumstances. Heating ice doesn’t take that much energy, so the slope of this portion of the graph is very steep. At the melting point of ice, instead of putting energy into increasing the temperature, we’re putting energy into breaking into molecular forces. The difference in energy in kilojoules per mole between the beginning and the end of this section is equivalent to the standard enthalpy of fusion of water.

In the next section, we’ll be adding energy to liquid water, heating it up. But once we reach the boiling point of water, we’ll need to add in energy to break the intermolecular forces between water molecules to turn them into a gas. We can then calculate the standard enthalpy of vaporization of water from the length of this section. And at the end point of this section, extra energy will contribute to the temperature of the steam. You may see versions of this graph where certain elements are contracted or exaggerated in order to make them easier to read.

Next, we’re going to look at the enthalpy changes associated with substances dissolving. When a substance dissolves in a solvent, there is an associated change in enthalpy. This is called the standard enthalpy of solution or dissolution. These mean the same thing. When one mole of sodium chloride dissolves in water at 25 degrees Celsius, the enthalpy change is 3.9 kilojoules. For sodium chloride, dissolving is an endothermic process. The overall process can be broken down into three separate stages with associated enthalpy changes. In the first step, we have our solid sodium chloride crystal and our water molecules, and we have to make space between the water molecules for the sodium and chloride ions. This requires energy. We can label the enthalpy change for this endothermic process as Δ𝐻 one.

In the second step, we need to separate the crystal lattice into sodium and chloride ions. This process is also endothermic. In the third step, we’re bringing together water molecules and sodium and chloride ions, allowing them to form bonds between each other. This process is exothermic. For sodium chloride, the balance of these energy contributions means that, overall, energy is required. The energy released when water molecules and sodium and chloride ions come together is not enough to compensate for the energy required to separate the water molecules in the solvent and separate the sodium and chloride ions in the crystal lattice.

The full definition of this standard enthalpy of solution is the enthalpy change when a substance dissolves in a solvent to form an infinitely dilute solution per mole of substance. An infinitely dilute solution is a theoretical concept where there are no solute–solute interactions. Practically, it’s impossible to make a measurement like this, so we can calculate the value based on data from real solutions. There’s also a standard enthalpy of dilution associated with real solutions, taking them from their current concentration to an infinitely dilute form.

When we dissolve hydrogen chloride gas in water, the enthalpy change associated with the formation of an infinitely dilute solution is negative 74.8 kilojoules per mole. But if, for instance, we have one mole of HCl dissolved in one mole of water, the enthalpy of dilution is only negative 29.2 kilojoules per mole. So we get most of the energy out when we form a concentrated solution, and we get the rest by infinite dilution. So the standard enthalpy of dilution is the enthalpy change when a solution of a substance is diluted infinitely per mole of substance.

Next, we have combustion. If we burn methane completely in oxygen at one atmosphere and 25 degrees Celsius, the enthalpy change is negative 882.0 kilojoules per mole of methane. The standard enthalpy of combustion of a substance is the enthalpy change when the substance burns completely in oxygen at standard pressure and temperature per mole of substance. And we assume substances will be in their standard states unless otherwise specified. Methane, oxygen, and carbon dioxide will be gasses, and water will be a liquid. Remember, we’re assuming the temperature is constant, so any heat generated by the experiment is released to the surroundings and we’ll have liquid water rather than gaseous water.

And the last type of standard enthalpy change we’ll be looking at is the standard enthalpy of formation. A standard enthalpy of formation is the enthalpy change when a substance forms from its elements at standard conditions per mole of substance, and elements are assumed to be in their standard states. As usual, the assumed standard pressure will be one atmosphere, and the assumed standard temperature will be 25 degrees Celsius. This is how we’d construct the chemical equation for the formation of methane. Methane standard state at 25 degrees Celsius is a gas, and methane is composed of atoms of carbon and hydrogen. The standard state of carbon is solid graphite, and the standard state of hydrogen is H2 gas, so we’ll need two molecules for each molecule of methane.

The standard enthalpy of formation of methane is negative 74.9 kilojoules per mole. This is exothermic, meaning that methane is more stable than its constituent elements in their standard states. Enthalpy of formation can be endothermic, indicating that the product is less stable than the constituent elements in their standard states. By definition, the standard enthalpy of formation of an element already in its standard state is zero. After all, if nothing changes, there can be no enthalpy change.

That was a lot to cover, so let’s take some time to look at the key points. Firstly, a standard condition is simply any condition where we have a fixed agreed-upon value for making measurements. For instance, we commonly fix pressures at one atmosphere, temperatures at 25 degrees Celsius, and concentrations at one molar. Standard conditions often produce the reference states we call standard states for a substance. A standard state is an agreed reference state, typically based on the state of the substance at one atmosphere and 25 degrees Celsius. Any standard enthalpy change is simply an enthalpy change established under standard conditions like pressure and temperature with substances assumed to be in their standard states unless stated otherwise. And we express these per mole of substance, usually in kilojoules per mole.

We have standard enthalpy changes of state like standard enthalpies of fusion going from a solid to a liquid, the reverse, which is solidification, vaporization, which is turning from a liquid to a gas, and condensation, which is the reverse. We have the standard enthalpy change of solution, which is where we take a substance and dilute it until it’s infinitely dilute with a solvent. And then the standard enthalpy of dilution, which is the enthalpy change when a real solution is diluted infinitely. There’s the standard enthalpy of combustion, which is the enthalpy change when we completely react a substance with oxygen, and the standard enthalpy of formation where we form a substance from its elements in their standard states.