energetics I Flashcards
“What term describes the change in energy that occurs when bonds are broken and formed during a chemical reaction?”
“Enthalpy change.”
“What is enthalpy change, represented as ∆H (delta H), in a chemical reaction?”
“It is the heat energy change at constant pressure, with units of kJ mol^-1.”
“What does the notation ΔH indicate in a chemical reaction?”
“It indicates that the measurements were made under standard conditions, where the elements were in their standard states.”
“What are the standard conditions for measuring enthalpy change (ΔH)?”
“Standard conditions are 100 kPa pressure and a temperature of 298 K (25°C).”
“What is characteristic of exothermic reactions in terms of heat energy?”
“They give out heat energy, resulting in a negative value for ΔH.”
“What typically happens to the temperature in exothermic reactions?”
The temperature often increases.
“What is characteristic of endothermic reactions in terms of heat energy?”
“They absorb heat energy, resulting in a positive value for ΔH.”
“What typically happens to the temperature in endothermic reactions?”
“The temperature often decreases.”
“What do enthalpy (or energy) level diagrams depict in a reaction?”
“They show the relative energies of the reactants and products, with the difference in enthalpies representing the enthalpy change of the reaction.”
“How does the enthalpy of a substance relate to its stability?”
“The less enthalpy a substance has, the more stable it is.”
“What happens to the enthalpy of the products compared to the enthalpy of the reactants in an exothermic reaction?”
“The products have less enthalpy than the reactants.”
“What happens to the enthalpy of the products compared to the enthalpy of the reactants in an endothermic reaction?”
“The products have more enthalpy than the reactants.”
“What do reaction profile diagrams illustrate?”
“They show how the enthalpy changes during reactions.”
“What is activation energy (Ea) in a chemical reaction?”
“It is the minimum amount of energy needed to begin breaking reactant bonds and initiate the reaction.”
“Why is it not possible to directly measure the actual enthalpy of a system?”
“Because in practice, only enthalpy change is relevant.”
“How can enthalpy changes be determined?”
“Enthalpy changes can be determined either by experiment or by consulting data books.”
“What are standard enthalpy changes?”
“Enthalpy changes under standard conditions, typically at 100 kPa pressure and a specified temperature, often 298 K.”
“Why is it important to use standard conditions when discussing enthalpy changes?”
“Because changes in enthalpy are influenced by temperature and pressure, using standard conditions ensures that everyone understands exactly what the enthalpy change is describing.”
