Standard States And Standard Enthalpy Changes MDCAT Quiz: MDCAT students need to understand the energy changes accompanying chemical reactions. These concepts enable one to calculate the enthalpy changes of reactions under standard conditions—something often used in MDCAT Quiz questions on heat transfer and reaction energetics.
Understanding Standard States
A standard state is the most stable form of a substance at a given temperature, usually 298 K, and pressure, 1 atm. For instance, oxygen in its standard state is O₂ gas, and water in its standard state is liquid at 298 K and 1 atm. Standard states are important in the comparison of enthalpies between different substances because they provide a common reference point. In thermodynamics, standard conditions are defined to simplify the calculation of thermodynamic properties, especially useful in calculating standard enthalpy changes (ΔH°).
Standard Enthalpy Change (ΔH°)
The standard enthalpy change (ΔH°) is the heat change that takes place when one mole of a substance undergoes a reaction in standard conditions—298 K and 1 atm pressure. It is a key quantity for predicting the heat released or absorbed during chemical reactions.
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For exothermic reactions, ΔH° is negative, meaning that heat is released to the surroundings.
For endothermic reactions, ΔH° is positive, so this indicates that heat is absorbed from the surroundings.
You get standard changes in enthalpy from MDCAT Quiz items when you need to calculate an overall enthalpy change using Hess’s Law, or where you have given standard enthalpy values of reactants and products. Use free flashcard tools in an attempt to remember the standard enthalpy values of common reactions and substances.
Mastering standard states and standard enthalpy changes is critical to the performance of MDCAT students in the thermodynamics section. Using MDCAT Quiz and Free Flashcard resources, you will be able to strengthen your understanding of enthalpy changes and be able to apply this knowledge in solving related problems in the exam.
The standard state of oxygen at 25°C is __________.
O₂ (gas)
The standard enthalpy of formation for an element in its most stable form is __________.
zero
The standard state of water at 25°C is __________.
liquid
The standard enthalpy change of a reaction is the enthalpy change when __________.
the reaction occurs under standard conditions
The standard enthalpy change of formation is __________ for elemental oxygen in its standard state.
zero
The enthalpy change for a reaction is independent of __________.
the pathway taken
Standard enthalpy changes are measured at a temperature of __________.
298 K (25°C)
In standard conditions, one mole of a substance is measured at __________.
1 bar of pressure
The standard enthalpy of formation of any compound is defined as the enthalpy change when __________.
1 mole of compound is formed from elements in their standard states
Standard enthalpy changes are expressed in units of __________.
kJ/mol
For the standard enthalpy of reaction, the reactants and products must be in their __________.
standard states
The enthalpy change for the combustion of one mole of a substance in standard conditions is called __________.
standard enthalpy of combustion
Standard states refer to the state of a substance at __________.
298 K and 1 bar pressure
The standard enthalpy change of formation for a compound in the gas phase is __________ compared to its liquid phase.
higher
The standard enthalpy change for a reaction is usually determined by measuring __________.
temperature changes
The standard enthalpy of formation of a compound is __________ if the compound is formed from elements in their standard states.
zero
When calculating standard enthalpy changes, the substances must be in their __________.
standard states
The standard enthalpy of combustion for carbon is __________.
-393.5 kJ/mol
The enthalpy change for a reaction is dependent on __________.
the temperature and pressure
The standard enthalpy of vaporization is __________ for a substance at its boiling point.
positive
The standard enthalpy of fusion is __________ at the melting point of a substance.
positive
The enthalpy change for the formation of a compound is __________.
calculated by subtracting the enthalpies of products from reactants
Standard conditions refer to a temperature of __________.
298 K (25°C)
The standard enthalpy change of formation for a compound is __________.
zero if the compound is in its elemental form
The standard state of hydrogen at 25°C is __________.
H₂ (gas)
The enthalpy change of a reaction is __________ for a process occurring at constant pressure.
equal to the heat added or released
The enthalpy of formation of sodium chloride in its solid form is __________.
negative
In an exothermic reaction, the standard enthalpy change is __________.
negative
The standard enthalpy of combustion is defined as the enthalpy change when __________.
one mole of a substance reacts completely with oxygen
Standard enthalpies of formation for elements in their standard state are always __________.
zero
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