3: Standard enthalpy changes Flashcards
Define standard conditions
Standard conditions are a pressure of 100KPa( 1 atmosphere), a stated temperature, usually 298K (25*) and a concentration of 1.0 mol dm^-3 (for reactions with aqueous solutions).
Define standard state
Standard state is the physical state of a substance under the standard conditions of 100KPa(1 atmosphere) and 298K(25*).
Define standard enthalpy change of reaction, ^Hr*
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard state.
Define the standard enthalpy change of combustion ^Hc*
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
Define standard enthalpy change of formation ^Hf*
The standard enthalpy change of formation ^Hf* of a compound is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
What are the three things needed for standard conditions?
- Pressure of 100KPa (1 atmosphere)
- A stated temperature which is usually 298K(25*C)
- A concentration of 1 mol dm^-3(for aqueous solution reactions)
What does this symbol mean ΔH*?
H- enthalpy
Δ- Change
*-(planet symbol) under standard conditions
What is needed for the standard enthalpy change of reaction?
- An equation-this gives the amounts,in mol, of reactants and products.
- An enthalpy change, KJ mol^-1 - this gives the enthalpy change for the molar quantities in the stated equation.
If we are forming an element such as H2, what is the enthalpy change?
The enthalpy change is zero.
What is the enthalpy change of formation of an element define as?
0 KJmol^-1
heat loss in a chemical system=
heat gain by surroundings
heat gain in a chemical system=
Heat loss by surroundings.
