Define enthalpy
Measure of hat energy in a system (chemical)
Define chemical system
Atoms, molecules or ions making up the chemicals
Define enthalpy change
= H(products)-H(reactants)
It cannot be measured directly
What is the law of conservation of energy?
Energy cannot be created or destroyed. It is always transferred
Where is energy transferred between in a chemical reaction with a change in energy?
Between system and surrounding
Define
i) System
ii) Surrounding
iii) Universe
i) Reactants & products
ii) Everything not in the chemical system
iii) System + surrounding
What are the features of an exothermic reaction?
- More energy released when making bonds than taken in when breaking them
- Heat transferred to surroundings
- Energy loss in system, energy gain in surroundings
- Delta H is -ve
- Surrounding temperature increases
What are the characteristics of an exothermic enthalpy profile diagram?
- Delta H arrow goes down (from reactant line to products)
- E(a) is from reactant line to top of peak
What are features of an endothermic reaction?
- Energy transferred from surroundings to system
- Delta H is +ve
- Energy gain in system balanced by energy loss in surroundings
- Surrounding temperature decreases
- More energy taken in to break bonds than released to form bonds
What are the characteristics of an endothermic enthalpy profile diagram?
- Reactants line lower than products
- Delta H arrow from reactants line UP to products line
- E(a) from reactants line up to peak of graph (NOT products line)
Define activation energy.
- Bonds that exist between atoms and ions in reactants need to be broken for a reaction to occur
- E(a) is the minimum amount of energy input required to break these bonds and start a reaction
What are standard enthalpy changes and what are the standard conditions?
Standard Enthalpy Changes: Enthalpy changes are measured under standard conditions
Standard Conditions: 100kPa 25 degrees C Conc.: 1mol/dm3 (states: physical states of substance under standard conditions)
Define enthalpy change of reaction.
Enthalpy change of a given reaction in the molar quantities stated in the reaction with all reactants and products in standard states
(State standard conditions)
Define enthalpy change of formation.
Enthalpy change when 1 mole of a compound is formed from its constituent elements under standard conditions with all elements in standard states.
(State standard conditions)
What is the enthalpy change of formation of an element?
0kg/mole as it is the formation of an element from the same element.
Define enthalpy change of combustion.
Enthalpy change when 1 mol of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states
(State standard conditions)
Define enthalpy change of neutralisation.
Enthalpy change of the reaction of an acid with a base to from one mole of water
(Standard conditions and states)
