Chapter 9 - Enthalpy Flashcards
Enthalpy changes, Measuring enthalpy changes, Bond enthalpies and Hess' law and enthalpy changes.
What is the law of the conservation of energy?
Energy cannot be created or destroyed, only converted from one form to another
Define enthalpy:
The total heat content that is stored in a chemical system
Define enthalpy change:
• The heat energy exchanged with the surroundings during a reaction
• (The difference between the enthalpy of the products and the reactants)
Define exothermic reaction:
• A reaction in which heat energy is released to the surroundings
• (The enthalpy of the products is smaller than the enthalpy of the reactants)
Define endothermic reaction
• A reaction in which heat energy is absorbed from the surroundings
• (The enthalpy of the products is greater than the enthalpy of the reactants)
Define activation energy:
The minimum energy required for a reaction to take place
What are the standard conditions?
• 101kPa
• 298K
• 1mol/dm^3
• Standard state (the physical state of a substance under standard conditions)
Define the standard enthalpy change of reaction:
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
Define the standard enthalpy change of formation:
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
Define the standard enthalpy change of combustion:
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
What is the equation used to measure an energy change?
q = mcΔT
• q - energy change with surroundings (J)
• m - mass (g)
• c - specific heat capacity (Jkg^-1K^-1)
• ΔT - change in temperature (K)
How is enthalpy change worked out from the energy change?
ΔH = q/n
• ΔH - enthalpy change (kJmol^-1)
• q - energy change with surroundings (kJ)
• n - number of moles (mol)
• Note: Questions usually ask for ΔH in kJmol^-1 so qx10^3
Define the standard enthalpy change of neutralisation:
The enthalpy change that accompanies the reaction of an acid and a base to form one mole of H2O under standard conditions
What are the causes for less energy being transferred than expected when calculating ΔHc?
• Heat loss to the surroundings
• Incomplete combustion
• Evaporation
• Non-standard conditions
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the curve
Define average bond enthalpy:
The energy required to break one mole of a specified type of bond in a gaseous molecule
How is energy transferred when bonds are broken?
Energy is released
What is Hess’ law?
The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same
How is enthalpy change calculated from average bond enthalpies?
ΔH = Σ Reactant’s bonds’ enthalpies - Σ Product’s bonds’ enthalpies
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