CHAPTER 9: ENTHALPY Flashcards
What is enthalpy?
The measure of heat energy in a chemical system
What is enthalpy change?
Enthalpy(products) - enthalpy(reactants)
How can you tell when a reaction is exothermic?
The chemical system releases heat energy to the surroundings, so the temperature of the surroundings increases as it loses energy. The enthalpy change is negative.
How can you tell when a reaction is endothermic?
The chemical system absorbs heat energy from the surroundings, so the temperature of the surroundings decreases as it gains energy. The enthalpy change is positive.
What is activation energy (Ea)?
The minimum energy needed for a reaction to take place.
What are standard conditions?
100kPa, 298K, 1moldm-3 and in standard states.
What is the enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.
What is the enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.
What is the enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
What is the enthalpy change of neutralisation?
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H20, under standard conditions, with all reactants and products in their standard states.
What is absolute zero (0K) equivalent to?
-273 degrees celsius
What is Q=MCdeltaT
energy change = mass x specific heat capacity (4.18) x temperate change
Why may an experimental value be different to the data book value?
- There could have been heat loss to the surroundings.
- There may have been incomplete combustion.
- Non-standard conditions may have been used.
What is average bond enthalpy?
The energy required to break one mole of a specified type of bond into a gaseous molecule.
What is an exothermic reaction in terms of making and breaking bonds?
If more heat energy is released in forming bonds in the products than is absorbed when breaking bonds in the reactants.
