Energetics Flashcards
What is enthalpy
The energy content of a substance
What is enthalpy change
The energy transferred to or from the number of moles specified in the equation
What is an exothermic reaction
Energy transferred from the reactants to the surroundings
ΔH is negative
The surroundings become hotter
What is an endothermic reaction
Energy is transferred from the surroundings to the reactants
ΔH is positive
The surroundings become cooler
What are the standard enthalpy changes measured under?
Standard conditions of
100 KPa
298 K
All substances in their standard states
What is the standard enthalpy of combustion
Is the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions, with all reactants and products in their standard states
What is the enthalpy of formation
The enthalpy change when 1 mole of a substance is formed from its elements. All reactants and products are in their standard states at 100KPa and 298 K
What is calorimetry used for
Used to determine enthalpy changes in the laboratory
The energy released is usually transferred to a known mass of water and the temperature change used to calculate the enthalpy change
Energy transferred equation
Energy transferred = specific heat capacity of water X mass of water X temp change
What is specific heat capacity
The energy needed to raise the temperature of 1g of a substance by 1K
the specific heat capacity of water is 4.2 J g-1 K-1
What can Hess’s Law be used to calculate
Enthalpy changes
State Hess’s Law
If a reaction takes place by more than one route, the enthalpy change of the reaction is the same, regardless of the route providing that the conditions are the same
What is bond enthalpy
The enthalpy change when one mole of bonds in gaseous molecules are broken, under standard conditions
What do bond enthalpies depend on
The molecules environment of the bond, and the values quotes in data books are mean bond enthalpies
What are bond enthalpies used to calculate
Enthalpy changes fro reactions by considering the total enthalpy change when the bonds are broken and the total enthalpy change when the bonds are made
