Energetics Flashcards
Define Enthalpy
Enthalpy is the total chemical energy contained within a substance, often referred to as its heat content. It reflects the internal energy of the system, including both the energy stored in chemical bonds and the energy required to account for pressure and volume changes. During chemical reactions, the enthalpy can change, which is represented as ΔH (where Δ signifies a change).
Define Enthalpy Change
Enthalpy Change: The change in enthalpy during a chemical reaction or physical process, typically expressed as ΔH. It can be either positive (endothermic) or negative (exothermic), depending on whether the system absorbs or releases heat.
Enthalpy Level Diagrams
Enthalpy Level Diagrams: Diagrams that represent the relative enthalpy of reactants and products in a chemical reaction. The difference in enthalpy levels shows whether the reaction is endothermic or exothermic. The diagram typically shows reactants on the left and products on the right, with the enthalpy change (ΔH) indicated.
Define Standard Enthalpy Change of Combustion.
Afterwards, look at the examples.
Standard Enthalpy Change of Combustion: The enthalpy change when one mole of a substance undergoes complete combustion in oxygen under standard conditions (298 K, 1 atm pressure).
Example 1: CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)
Standard enthalpy change of combustion = -890 kJ/mol.
Example 2: C₈H₁₈ (l) + 12.5O₂ (g) → 8CO₂ (g) + 9H₂O (l)
Standard enthalpy change of combustion = -5470 kJ/mol.
Define Standard Enthalpy Change of Neutralisation.
Afterwards, look at the examples.
The enthalpy change when one mole of an acid reacts with one mole of a base to form one mole of water under standard conditions.
Example 1: HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
Standard enthalpy change of neutralisation = -57.3 kJ/mol.
Example 2: H₂SO₄ (aq) + 2NaOH (aq) → Na₂SO₄ (aq) + 2H₂O (l)
Standard enthalpy change of neutralisation = -56.0 kJ/mol.
Define Standard Enthalpy Change of Formation.
Afterwards, look at the examples.
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K, 1 atm pressure).
Example 1: C (s) + O₂ (g) → CO₂ (g)
Standard enthalpy change of formation = -393.5 kJ/mol.
Example 2: N₂ (g) + 3H₂ (g) → 2NH₃ (g)
Standard enthalpy change of formation = -45.9 kJ/mol.
Explain bond enthalpy
Afterwards, look at the example.
Bond Enthalpy: Bond enthalpy (also called bond dissociation enthalpy) is the energy required to break one mole of a specific covalent bond in a molecule in the gas phase, forming separate atoms or radicals. It is always positive because energy is required to break bonds.
Example: The bond enthalpy of the C-H bond in methane (CH₄) is the energy required to break one mole of C-H bonds in CH₄ molecules to form CH₃ and H atoms.
Explain Hess’s Law.
Afterwards, provide an example.
Hess’s Law: Hess’s Law states that the total enthalpy change of a chemical reaction is the same, regardless of the number of steps in the reaction, as long as the initial and final conditions are the same. This law is based on the principle that enthalpy is a state function, meaning it depends only on the initial and final states, not on the pathway taken.
Example: If a reaction can be broken down into multiple steps, the total enthalpy change for the reaction can be found by summing the enthalpy changes of the individual steps.
What’s the symbol and unit for standard enthalpy change of combustion?
Symbol: ΔH°_c (where ΔH° represents standard enthalpy change and _c indicates combustion)
Unit: The unit for standard enthalpy change of combustion is typically kJ/mol (kilojoules per mole).
Define Thermometric Titration.
A type of titration in which the temperature change is measured to determine the endpoint of the reaction. This method involves recording temperature changes during the titration, as heat is released or absorbed during the chemical reaction, allowing the endpoint to be detected by a noticeable temperature change. It is often used in reactions where significant heat is produced or absorbed, such as neutralisation reactions.
What’s the symbol for standard enthalpy change of formation?
Symbol: ΔH°_f (where ΔH° represents standard enthalpy change and _f indicates formation)
Explain mean bond enthalpy.
Afterwards, look at the example.
Mean Bond Enthalpy is the average energy required to break one mole of a specific type of bond in a molecule, averaged over several compounds that contain that bond. It represents the bond dissociation energy of a bond in a molecule in the gas phase and is usually quoted as an average value because bond strengths can vary slightly depending on the molecule in which the bond is found.
Unit: The unit is kJ/mol (kilojoules per mole).
For example, the mean bond enthalpy of the C-H bond is the average energy required to break the C-H bond in various compounds, such as methane (CH₄), ethane (C₂H₆), etc.
Define the standard enthalpy change of atomisation.
What’s the subol for standard enthalpy change of atomisation?
The standard enthalpy change of atomisation is the enthalpy change when one mole of atoms is formed from an element in its standard state in the gas phase under standard conditions (298 K, 1 atm pressure).
This process involves the breaking of bonds or the separation of the element into individual atoms.
Symbol: ΔH°_at (where ΔH° represents standard enthalpy change and _at indicates atomisation)
Unit: The unit is kJ/mol (kilojoules per mole).
For example, the standard enthalpy change of atomisation of sodium (Na) is the enthalpy change when one mole of sodium atoms is formed from solid sodium:
Na (s) → Na (g)
ΔH°_at = +108 kJ/mol
Define calorimetry
An experimental method for finding enthalpy change by measuring temperature change over time. When recorded and plotted ona graph, data can be extrapolated to given an accurate value for the change in temperature at the beginning of the reaction.
Define the term extrapolated
To extrapolate means to make an educated guess or prediction about something beyond the known data, based on the pattern or trend observed in that data.
For example, if you have data about how far a car has traveled over the first few hours, you can extrapolate to predict how far it will travel after a few more hours, even if you haven’t measured those extra hours yet. You’re extending the trend beyond the available information.
In simpler terms, it’s like using what you already know to predict what might happen next.
What are the sources of error using a calorimeter
Sources of error
ΔH values found using calorimetry
- never completely accurate as energy is easily lost from the system. **
- Heat loss can occur due to conduction, convection or inaccuracies in measuring temperatures.
- The heat loss to the surroundings can be reduced by putting a lid on the calorimeter and insulating the outsides of the calorimeter using an insulator like polystyrene. **
- The specific heat capacity of the solution is taken to be 4.18 kJ mol⁻¹, which is the value for water and not the actual solution.
- The** specific heat capacity of the calorimeter is not taken into account so this leads to inaccuracies in the calculation.**
Define the term Tabulated
Tabulated: This means information or data that is arranged in a table or chart, often for easier reference or comparison.
Defint the term solvating
Solvating: This refers to the process where a solvent molecules surround and interact with solute particles (atoms, ions, or molecules) to form a solution. It helps dissolve the solute in the solvent.
Define the term thermodynamic quantity
A thermodynamic quantity is a property of a system that can be measured or calculated in the study of thermodynamics. Examples include temperature, pressure, volume, internal energy, enthalpy, entropy, and Gibbs free energy. These quantities help describe the state and behavior of a system.
What does the study of thermodynamics mean?
The study of thermodynamics is the branch of physics that deals with the relationships between heat, work, energy, and the properties of matter. It focuses on how energy is transferred and transformed, and how systems respond to changes in temperature, pressure, and volume. It is essential for understanding natural processes, engines, refrigerators, and many other physical phenomena.
