9.1 Enthalpy Changes
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What is enthalpy?
A measure of the heat energy in a chemical system
What is a chemical system?
Refers to the atoms, molecules or ions making up the chemicals
Formula for enthalpy change
H (products) - H (reactants)
What is the law of conservation of energy?
Energy cannot be created or destroyed, just transferred between stores
What is an exothermic energy transfer?
From the system to the surroundings
What is an endothermic energy transfer?
From the surroundings to the system
What is the system and the surroundings?
System - chemicals (reactants and products) Surroundings - apparatus, laboratory
What happens during an exothermic reaction?
The products have lower enthalpy than the reactants Energy change is negative Temperature of surroundings increase, as they gain energy
What happens during an endothermic reaction?
Products have a higher enthalpy than the reactants Enthalpy change is positive Temperature of the surroundings decreases as they lose energy
What is activation energy?
The minimum amount of energy required for a reaction to take place
What are standard conditions?
Standard pressure - 100 kPa Standard temperature - 298K (25oC) Standard concentration - 1mol/dm3 Standard state - physical state of a substance under standard conditions
What is the standard enthalpy change of reaction?
Enthalpy change in a reaction in the molar quantities shown in a chemical equation at 100 kPa, 298K, with all reactants and products in their standard states
What is standard enthalpy change of formation?
Enthalpy change that takes place when one mole of a compound is formed from its elements at 100kPa, 298K, with all reactants and products in their standard states
What is enthalpy change of combustion?
Enthalpy change that takes place when one mole of a substance reacts completely with oxygen at 100 kPa, 298K, with all reactants and products in their standard states
What is standard enthalpy change of neutralisation?
Energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all reactants and products in their standard states