Video Transcript
The symbol for standard enthalpy change is Δ𝐻⦵. Which of the following is not a standard condition? (A) A volume of one meter cubed. (B) A pressure of one bar for gases. (C) A concentration of one mole per cubic decimeter for solutions. (D) All components in their standard state.
The enthalpy, represented by the capital letter 𝐻, is the total internal energy of a system. There is no way to directly measure the energy that a system has, but we can measure an increase or decrease in energy as a result of a reaction or process. For example, many chemical reactions release energy over the course of the reaction. This energy is transferred to the environment in the form of heat. So one way to measure the change in energy during a chemical reaction would be to measure the change in temperature.
However, the conditions that we might perform this experiment under matter quite a bit. Under different conditions, the same measured change in temperature would represent a different change in enthalpy. To control for this, reactions are often performed under standard conditions. This symbol, which is called a plimsoll, indicates the reaction was performed under standard conditions.
Standard conditions are pressures of one bar, which is approximately equal to one atmosphere, temperatures of 25 degrees Celsius, solution concentrations of one mole per cubic decimeter, and all components should be in their standard states, which is the state the chemical species would take at 25 degrees Celsius and a pressure of one bar.
For example, oxygen and hydrogen both exist as gases when the temperature is 25 degrees Celsius and the pressure is one bar. So the standard state for both oxygen and hydrogen is gaseous. We can see that answer choices (B), (C), and (D) are all standard conditions. Of the conditions we were given, the only one that was not a standard condition is answer choice (A) a volume of one cubic meter.
