Chapter 9 - enthalpy key terms Flashcards
What is Enthalpy (h) a measure of?
heat energy in a chemical system
Enthalpy change (△H)
the difference between the enthalpy of the products and the enthalpy of the reactants.
heat
flow of thermal energy from one object to another (kj) or
Temperature
the direction of energy transfer is determined by the temperature of the objects - measured in k or °C
system
the substances directly involved in a chemical reaction
surroundings
rest of the universe that isn’t the system
Exothermic reaction
chemical reaction where the enthalpy of the products is smaller than the enthalpy of the reactants due to heat loss (energy given out) to the surroundings. Causes temperature of the surroundings to go up.
Endothermic reactions
Enthalpy of products is greater than the enthalpy of the reactants (heat energy taken in from the surroundings)
Standard Enthalpy change of reaction △hr
the enthalpy change for a reaction as shown by the molar quantities in the chemical equation, measured under standard conditions.
standard conditions
Pressure = 100KPa
Temperature = 298 K
Concentration = 1 mol/dm³
standard states of all substances
Standard enthalpy change of combustion △hc
Enthalpy change when 1 mole of a substance reacts completely with excess oxygen under standard conditions.
Standard enthalpy change of formation △hf
The enthalpy change when 1 mol of a substance is formed from its elements under standard conditions
Standard enthalpy change of neutralisation △neutH
The enthalpy change when 1 mole of water is produced in a neutralisation reaction.
Enthalpy change (equation)
sum of bonds broken - sum of bonds made
△H = s(bond enthalpies of bonds broken) - s(bond enthalpies of bonds made)
Average bond enthalpy
the average enthalpy change that takes place when breaking by homylitic fission 1 mol of a given type of bond in the molecules of a gaseous species.
