3.1.4 Energetics Flashcards
Enthalpy change [definition]:
Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure.
What is the enthalpy change in an exothermic reaction?
In an exothermic reaction the
∆H is negative
What is the enthalpy change in an endothermic reaction?
In an endothermic reaction
the ∆H is positive
Exothermic reaction [2]:
- Energy is transferred from the system (chemicals) to the surroundings.
- The products have less energy than the reactants.
Endothermic reaction [3]:
- Energy is transferred from the surroundings to the system (chemicals).
- They require an input of heat energy e.g. thermal
decomposition of calcium carbonate. - The products have more energy than the reactants
What are the standard conditions of standard enthalpy changes? [4]:
- 100 kPa pressure
- 298 K (room temperature or 25oC)
- Solutions at 1mol dm-3
- all substances should have their normal state at 298K
Standard enthalpy change of formation [definition]:
The enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K and 100kpa), all reactants and products being in their standard states
- Symbol fH
Standard enthalpy of combustion [definition]:
The enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states.
- Symbol cH
What is the enthalpy of formation of an element?
0 kJ mol-1
Why is incomplete combustion less exothermic than complete combustion?
Incomplete combustion will lead to soot (carbon), carbon monoxide and water. It will be less exothermic than
complete combustion.
What is the equation for heat change in a reaction?
q = mc∆T
energy change = mass of solution x heat capacity x temperature change
Q (J) = m (g) x c(J g⁻¹ K⁻¹) x T ( K)
Hess’ law
elements compound in standard state
gaseous atoms
