1.4 Energetics Flashcards
Exothermic definition
Energy is released from the chemical to the surroundings
Endothermic definition
Energy is taken in from the surroundings
Enthalpy change definition
The change in heat energy at constant pressure
What is the enthalpy change for exothermic processes?
Negative
What is the enthalpy change for endothermic processes?
Positive
What are the standard conditions?
100kPa
25C (298K)
1.0 mol dm-3 concentration
What happens to the bonds in a reaction?
The bonds in the reactants are broken
The bonds in the products are formed
What type of reaction is breaking bonds?
Endothermic as energy is required
What type of reaction is forming bonds?
Exothermic as it releases energy
Activation energy definition
The minimum energy needed to start a reaction
Mean bond enthalpy definition
The energy required to break one mole of covalent bond into gaseous atoms averaged over a range of different compounds
What type pf process is a mean bond enthalpy reaction?
Endothermic (positive)
Standard enthalpy of combustion definition
Enthaply change when one mole of substance is completely burnt in excess oxygen under standard conditions, all reactants and products in their standard states
What substances have a zero enthalpy of combustion?
Water and carbon dioxides
Why may the enthalpy of combustion be difficult to measure?
Incomplete combustion may occur
Why may some enthalpy be difficult to measure?
Reaction may have a very high activation energy
Reaction may be too slow
Possibility of competing reactions
Hess’ Law definition
States that the enthalpy change for a chemical reaction is the same whatever the route is taken from reactants to products
What are the types hess’ cycles
Bond enthalpy, formation and combustion
Enthalpy of formation
The enthalpy change when one mole of substance is produced from its constituent elements in standard conditions
Enthalpy of combustion
The enthalpy change when one mole of substance is completely burnt in excess oxygen under standard conditions
Calorimetry equation
Q=mcDeltaT
Q=mcDeltaT explained
Q=energy change
m=mass
c=specific heat capacity
DeltaT= temperature change
Specific heat capacity definition
The energy required to raise 1g of substance by 1K without a change of state
Enthalpy change per mole equation
DeltaH=q/moles
Direction of arrows in enthalpy of formation
Bottom up
Direction of arrows in enthalpy of combustion
Both down to water and carbon dioxide
Bond dissociation enthalpy
The energy required to break one mole of gaseous bonds to form gaseous atoms
