Chemical Energetics Flashcards
Define
enthalpy change
The energy transferred in a chemical reaction (symbol ΔH)
Define
exothermic
The term used to describe a reaction in which energy is released to the surroundings.
ΔH is negative, surroundings warmer
energy of reactants > energy of products
Examples: bond making, metal + acid or water, neutralization, combustion, condensation & freezing
Define
endothermic
The term used to describe a reaction in which energy is abrobed from the surroundings.
ΔH is positive; surroundings cooler
energy of reactants < energy of products
Examples: bond breaking, thermal decomposition, dissolving salts,photosynthesis,boiling & evaporation
Define
standard conditions
Conditions of temperature and pressure that must be the same in order to compare moles of gases or enthalpy changes accurately.
105 pascals (100 kPa) and 298 K (25oC)
Also, standard enthalpy changes are with reactants and products in their standard states.
Define
standard enthalpy change of reaction
ΔH⊖r
The enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions.
Define
standard enthalpy change of formation
ΔH⊖f
The enthalpy change when one mole of a compound is formed from its elements under standard conditions.
For elements, this is zero by definition. For compounds, it is usually exothermic (but can be endothermic).
There should only be one mole on the right hand side of the equation.
Define
standard enthalpy of combustion
ΔH⊖c
The enthalpy change when one mole of a substance is burned in excess oxygen under standard conditions.
This is always exothermic.
There should only be one mole on the left hand side of the equation.
Define
standard enthalpy change of hydration
ΔH⊖hyd
The enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions.
Define
standard enthalpy change of solution
ΔH⊖sol
The enthalpy change when when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions.
An infinitely dilute solution does not produce any further enthalpy change when more solvent is added.
Define
standard enthalpy change of neutralisation
ΔH⊖n
The enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.
Define
standard enthalpy change of atomisation
ΔH⊖at
The enthalpy change when one mole of gaseous atoms is formed from its element under standard conditions.
This is always endothermic.
Define
bond energy
The energy needed to break one mole of a particular bond in one mole of gaseous molecules.
Define
calorimetry
and the equation used to calculate energy change
A technique used to measure the enthalpy change of some reactions, where the temperature change of the surroundings is measured.
ΔH = mcΔT
State
Hess’ Law
The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes places as long as the initial and final conditions are the same.
Draw a Hess Law cycle involving enthalpy change of reaction ΔHr and enthalpy change of formation ΔHf.
Draw a Hess Law cycle involving enthalpy change of reaction ΔHr and enthalpy change of combustion ΔHc.
Draw a Hess Law cycle involving enthalpy change of hydration ΔHhyd and enthalpy change of solution ΔHsol.
Draw a Hess Law cycle involving enthalpy change of hydration ΔHr and bond energies.
Draw a reaction pathway diagram for an exothermic and an endothermic reaction, labelling the activation energy and enthalpy change.
