4 - Energetics Flashcards
What is thermochemistry?
The study of thermal changes during chemical reactions.
What are exothermic reactions?
When thermal energy is given out (-ΔH)
What are endothermic reactions?
When thermal energy is absorbed (+ΔH)
Is breaking bonds exo or endo thermic?
Endothermic.
Is making bonds exo or endo thermic?
Exothermic.
What is the unit of energy?
kJ mol-1
kilojoules per mole
What is enthalpy change?
Measuring a thermal energy change at constant pressure.
What are the standard conditions for measuring enthalpy changes?
100kPa (1 atm)
298K (25 °C)
What is standard state?
The state in which an element exists at standard conditions.
What is standard molar enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions.
What is standard molar enthalpy of combustion?
The standard enthalpy change when one mole of a substance is completely burnt in excess oxygen.
What is temperature?
The average kinetic energy of particles in a system.
What is heat?
A measure of the total energy transferred, due to a temperature difference.
What is specific heat capacity?
The amount of heat needed to raise the temperature of 1g of a substance by 1K.
Units = Jg-1K-1
What is the formula for enthalpy change?
q = mcΔT
mass x specific heat capacity x temperature change.
How to reduce heat loss of measuring the enthalpy change?
Use a polystyrene cup around the beaker as they are good insulators. (Prevent heat loss via conduction)
Use a lid on calorimeter to prevent heat loss via convection.
Use larger volumes of reactants as the smaller the effect of heat loss.
Stir the mixture to ensure uniform temperature.
What does Hess’ Law state?
The enthalpy change for a chemical reaction is the same regardless of the route taken, given states are constant.
Why can the enthalpy change of some reactions not be measured directly?
Slow or complex reaction
Unstable intermediates
High temp or pressure
Phase changes
What is a thermochemical cycle?
Diagram that represents Hess’ Law.
Which way do arrows point for Hess’s cycle when working out enthalpy of formation?
Down
Which way do arrows point for Hess’s cycle when working out enthalpy of combustion?
Down
What is the enthalpy of elements in their standard states?
0
What is the definition of Bond dissociation enthalpy?
The enthalpy change required to break a covalent bond with all species in the gaseous state.
What is the definition of Mean bond enthalpy?
The average energy required, per mole of gaseous compound, to break a particular covalent bond to form gaseous atoms.
What is the formula for overall bond enthalpy change?
Bonds broken - bonds formed.
(reactants - products)
