Topic 8 Energetics 1 Flashcards
What is enthalpy change?
The change in ‘heat’ energy of a substance at a constant pressure.
What are the trends in enthalpy change for exothermic and endothermic reactions?
Exo - Negative
Endo - Positive
What are the standard conditions?
Temperature - 298 K
Pressure - 100 KPa
Concentration - 1.0 mol/dm^3
What is the definition of standard enthalpy change of reaction?
The energy change when the molar quantities shown in the balanced equations react completely together to from products at standard conditions.
What is the definition of standard enthalpy change of formation?
The enthalpy change when ONE MOLE of substance is FORMED from its ELEMENTS IN THEIR STANDARD STATES at standard conditions.
What is the definition of standard enthalpy change of combustion?
The energy PRODUCED when ONE MOLE of substance is BURNED IN AN EXCESS OF OXYGEN measured at standard conditions.
What is the definition of standard enthalpy change of neutralisation?
The energy RELEASED when ONE MOLE OF WATER is formed by the reaction of an acid and alkali measured at standard conditions.
What are some key points to do with enthalpy change definitions?
- You are encouraged to ALWAYS give an equation with a definition in a test.
- State symbols are CRUCIAL.
What are the units for enthalpy change?
Kjmol^-1 (Kj/mol)
What is the specific heat capacity of water?
4.18 J (4.2 J if specified)
What is the equation used in the calorimetry experiment?
q = mcΔT energy = mass (of water) x spc (of water) x temp change
What is the equation for enthalpy change?
ΔH = q / n
Enthalpy change = energy / number of moles
What is Hess’ law?
The enthalpy change for a reaction is independent of the route taken.
How is enthalpy change measured?
Enthalpy change is not measured directly and instead temperature change is. The enthalpy change can then be found by using some equations.
What experiment can be used to find the enthalpy change in a reaction?
Calorimetry is the experiment used to calculate enthalpy change. In it the energy given out / taken in is used to heat / cool a known mass of water. The temperature change is recorded from this and can be used in equations to find the enthalpy change.
