enthalpy changes Flashcards
define enthalpy change of a reaction
enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactant and products in their standard state
define enthalpy change of formation
The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions all reactants and products being in their standard states
define enthalpy change of combustion
The standard enthalpy of combustion of a substance is defined as the enthalpy change that occurs when one mole of a substance reacts completely with oxygen under standard conditions. (298K and 100kPa), all reactants and products being in their standard states
define enthalpy change of neutralisation
The standard enthalpy change of neutralisation is the enthalpy change that accompanies the reaction of an acid and a base under standard conditions to produce 1 mole of water with all reactants and products in their standard states
Why can ‘enthalpy change of formation’ not be measured directly?
Many different compounds can form from the same constituent elements
How is enthalpy change calculated
ΔH = ±q / n
Breaking bonds is … because
Endothermic.
Energy is put into the system.
Atoms become less stable as they have no bonds and lots of energy
Making bonds is … because
Exothermic.
Energy is released to the surroundings.
Despite the little energy needed to make a bond, the molecule becomes more stable when bonded and thus releases energy.
What is activation energy defined as?
The minimum energy required of particles to collide to start a reaction.
What does Hess’ Law state?
The enthalpy change of reaction is independent of the route taken.
What 2 things are required of particles to react?
● Minimum kinetic energy (i.e., activation energy). ● Correct orientation.
How does concentration affect the rate of reaction?
Increasing concentration ⇒ more particles per unit volume (1) ⇒ more frequent successful collisions (1) ⇒ increased rate of reaction (1).
If the question explicitly says double concentration, say double particles and thus double frequency of successful collisions.
How does temperature affect the rate of reaction?
Increasing temperature ⇒ higher % of particles with KE greater than EA (1) ⇒ more frequent successful collisions (1) ⇒ increased rate of reaction (1).
How does surface area affect the rate of reaction?
Increasing surface area ⇒ more sites are exposed (1) ⇒ more frequent successful collisions (1) ⇒ increased rate of reaction (1).
What are the limitations of calorimetry?
- Heat loss to surroundings.
- Non-standard conditions.
- Heat capacity of container not included.
- Contents of calorimeter may evaporate.
