Kara Sears
Hess's Law, discovered by Germain Henri Hess in 1840, is a useful principle in thermochemistry. It is used to determine an unknown enthalpy change in a reaction, especially when direct measurement is not feasible. The law states that the total enthalpy change for a reaction remains constant, regardless of the number of steps involved. By manipulating and combining multiple chemical equations, Hess's Law allows us to calculate the enthalpy change of a reaction. This often involves reversing, multiplying, or dividing equations to match reactants and products appropriately. For example, when dealing with moles of reactants or products, it is crucial to multiply the equations and their corresponding enthalpy values correctly to ensure accurate calculations.
| Characteristics | Values |
|---|---|
| Use | To find out an unknown enthalpy change |
| Application | Particularly useful for enthalpy changes that cannot be measured directly by an experiment |
| Statement | "The total enthalpy change for a reaction is independent of the route taken" |
| Another way of saying this | No matter which route you take, the enthalpy change is always the same |
| Enthalpy of Combustion (ΔH⦵c) | The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions |
| Enthalpy of Formation (ΔH⦵f) | The enthalpy change when one mole of substance is formed from its elements under standard conditions |
| Multiplying factor | Doesn't have to be an integer value |
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What You'll Learn
Enthalpy of combustion
The enthalpy of combustion of a substance is defined as the heat energy given out when one mole of a substance burns completely in oxygen. It is a measure of the energy released in a combustion reaction and is an example of an enthalpy change. Enthalpy changes can be calculated from experimental data and are independent of the route taken, as described by Hess's Law.
The combustion of gasoline is a highly exothermic process. The enthalpy of combustion can be used to compare which fuels or substances release the most energy when burned. For example, the enthalpy of combustion of ethanol is -1366.8 kJ/mol, indicating the amount of heat produced when one mole of ethanol undergoes complete combustion at 25°C and 1-atmosphere pressure. The standard enthalpy of combustion is the enthalpy change when one mole of a substance burns under standard state conditions, which is sometimes referred to as the "heat of combustion."
There are two types of enthalpy of combustion: high(er) and low(er) heat(ing) values. The higher heating value (HHV) indicates the upper limit of the available thermal energy produced by the complete combustion of a fuel. It is measured as a unit of energy per unit mass or volume of substance and assumes that all water components are in a liquid state at the end of combustion. The lower heating value (LHV), on the other hand, is another measure of available thermal energy produced by the combustion of fuel. It considers energy losses, such as the energy used to vaporize water, and assumes that the water component is in a vapor state at the end of combustion.
The enthalpy of combustion can be calculated using a bomb calorimeter. A simplified version of this can be set up in a lab with a spirit burner and a metal can. The fuel is burned, and the temperature increase is measured. The mass of the fuel corresponding to the temperature increase can then be used to calculate the enthalpy change of the reaction, which, in turn, can be used to calculate the enthalpy of combustion of that fuel.
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Enthalpy of formation
In chemistry and thermodynamics, the standard enthalpy of formation, or standard heat of formation, is a measure of the change in enthalpy that occurs during the formation of one mole of a substance from its constituent elements in their reference state. This process occurs under standard conditions, specifically at a temperature of 25 °C or 298.15 K, and a pressure of 1 bar or 100 kPa. The standard enthalpy of formation is denoted by the symbol ΔHf and is typically measured in kilojoule per mole (kJ/mol).
The equation for the standard enthalpy change of formation is:
\[\co: 9<\Delta H_{reaction}^o = \sum {\Delta H_{f}^o(products)} - \sum {\Delta H_{f}^o(reactants)}\]
This equation states that the standard enthalpy change of formation is equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants. This equation can be used to determine the enthalpy change for a reaction, taking into account the enthalpies of formation of the reactants and products.
All elements in their reference states, such as oxygen gas (O2) and solid carbon in the form of graphite, have a standard enthalpy of formation of zero since there is no change involved in their formation. For elements with multiple allotropes, such as carbon, the reference state is typically the most stable form, which also corresponds to the lowest enthalpy. However, there are exceptions, such as phosphorus, where white phosphorus is chosen as the reference state despite being less stable than red phosphorus.
The standard enthalpy of formation can be applied to various compounds. For example, the standard enthalpy of formation of carbon dioxide at 298.15 K is ΔHf = -393.5 kJ/mol. In the case of ionic compounds, the standard enthalpy of formation can be determined by considering the sum of several terms within the Born-Haber cycle, including the enthalpy of atomization, ionization energy, bond energy, electron affinity, and lattice energy.
In summary, the standard enthalpy of formation provides a quantitative measure of the energy changes associated with the formation of one mole of a substance under standard conditions. It is a fundamental concept in chemistry and thermodynamics, allowing for the calculation of enthalpy changes in chemical reactions and providing insights into the stability and energy characteristics of different substances.
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Reversing equations
Hess's Law allows us to combine equations to generate new chemical reactions whose enthalpy changes can be calculated rather than directly measured. This is because the enthalpy of a given chemical reaction is constant, regardless of whether the reaction occurs in one step or many steps.
Reversing an equation means reversing the sign on the enthalpy value. This is because when an equation is reversed, the exothermic reaction becomes endothermic and vice versa. Therefore, the sign of the enthalpy must be changed. For example, if the original equation is CH4 + 2 O2 → CO2 + 2 H2O ΔH = −891 kJ, then reversing the equation would result in CO2 + 2 H2O → CH4 + 2 O2 ΔH = 891 kJ.
In some cases, it is necessary to reverse at least one equation when applying Hess's Law. For example, if the desired reaction has C2H4 as a reactant, but only one reaction from the data has C2H4 as a product, then the reaction must be reversed to change the sign on the ΔH.
It is important to note that no equation should be reversed if it is not necessary for the problem. For example, if NH3 and O2 are on the reactant side in both data equations, then reversing the equation would not be necessary.
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Multiplying by two
Hess's Law, also known as Hess's Law of Constant Heat Summation, was formulated by Russian chemist and doctor Germain Hess in 1840. It states that the total enthalpy change for a reaction is the sum of all changes, regardless of the number of steps or stages. In other words, the enthalpy of a given chemical reaction remains constant, regardless of whether the reaction occurs in a single step or multiple steps.
This law is based on the fact that enthalpy is a state function, which means that the overall change in enthalpy can be calculated by summing up the changes at each step of the reaction until the product is formed. All steps must occur at the same temperature, and the equations for each step must be balanced.
When applying Hess's Law to solve chemical problems, it is sometimes necessary to multiply the coefficients of the equation by a certain factor. This factor is often 2, as seen in several examples.
For instance, consider the reaction:
> 2H2 (g) + O2 (g) ----> 2H2O (l)
To find the enthalpy of formation, we multiply the ΔHformation by 2:
> ΔHformation: (-286 kJ/mol) * (2 mol) = -572 kJ
In another example, when dealing with the reaction:
> H2(g) + 1/2O2(g) ---> H20
We need to multiply the reaction by 2, which also leads to multiplying the delta H by two:
> 2H2 + O2 ----> 2H2O
In a different scenario, we may have multiple equations that need to be manipulated to get the desired products and reactants. For instance, consider the following three equations:
> eq 1 ---> do not flip, multiply by 2 (gets the 2C we need on the reactant side)
> eq 2 ---> leave untouched (keeps H2 on the reactant side and in the desired amount)
> eq 3 ---> flip, divide by 2 (puts C2H2 on the product side in the desired amount)
By multiplying the first equation by 2, we ensure that we have the correct number of moles of carbon (2C) on the reactant side. Leaving the second equation untouched maintains the desired amount of hydrogen (H2) on the reactant side. Finally, flipping and dividing the third equation by 2 results in the desired product, C2H2, on the product side.
In summary, multiplying by two in the context of Hess's Law involves adjusting the coefficients of chemical equations by a factor of 2 to achieve the desired number of moles of a substance on either the reactant or product side. This manipulation is often done to balance the equation or to ensure that specific reactants or products are present in the correct amounts.
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Cancelling variables
Hess's Law, formulated by Russian chemist and doctor Germain Hess in 1840, is a principle in thermochemistry that allows us to calculate the overall change in enthalpy by summing up the changes for each step of the reaction. This is because enthalpy is a state function, and the enthalpy of a given chemical reaction is constant regardless of whether the reaction occurs in one step or many steps.
For example, consider the reaction:
> H2(g) + 1/2O2(g) ---> H2O (l)
The standard enthalpy of formation for one mole of water is -286 kJ. To find the enthalpy for the formation of two moles of water, we multiply the equation by 2:
> 2H2(g) + O2(g) ---> 2H2O (l)
Now, the enthalpy is calculated as:
> ΔHformation: (-286 kJ/mol) * (2 mol) = -572 kJ
In this case, we multiplied the equation by 2, which resulted in the cancellation of the 1/2 in front of O2(g). This manipulation ensured that the equation was balanced, with the correct number of reactants and products.
Another example is provided by ChemTeam, where they apply Hess's Law to three equations involving the combustion of carbon and hydrogen. By manipulating the equations through multiplication, division, and flipping, they are able to cancel out specific variables to obtain the desired equation and enthalpy value. For instance, they multiply the second equation by 2 to cancel out 2S and obtain 2SO2 on the product side.
In summary, cancelling variables in Hess's Law involves manipulating chemical equations by multiplying, dividing, or flipping them to obtain the desired reactants and products. This manipulation also affects the associated enthalpy values, allowing us to calculate the overall change in enthalpy for a reaction.
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Frequently asked questions
Hess's Law, discovered by Germain Henri Hess in 1840, states that the total enthalpy change for a reaction is independent of the route taken. It is used to find out an unknown enthalpy change, especially when direct measurement is not possible.
Hess's Law is commonly used to address questions involving enthalpies of formation and combustion.
Yes, in one example, when forming carbon dioxide and water from multiple moles of reactants, you need to multiply the carbon value by two and the hydrogen value by three.
In some cases, such as when dealing with the formation of water, you need to multiply the reaction by 2 to ensure that the coefficients are balanced correctly.
No, the multiplying factor in Hess's Law does not have to be an integer value. It can be any number that ensures the equation is balanced correctly.
