3.2.1 Enthalpy Changes Flashcards
(16 cards)
Define ‘system’ in terms of a chemical reaction.
System - the atoms and bonds involved in a chemical reaction.
What is the law of conservation?
The amount of energy in an isolated system remains the same. Energy cannot be created or destroyed, it can only be transferred.
Which energy change breaks bonds?
Energy is taken in from the surroundings in order to break bonds, therefore it is endothermic.
Which energy change makes bonds?
Energy is released to make bonds, therefore it is exothermic.
What is an endothermic reaction?
-A reaction with a positive enthalpy change (+ΔH)
-Enthalpy of products > enthalpy of reactants
What is an exothermic reaction?
-A reaction with a negative enthalpy change (-ΔH)
-Enthalpy of reactants > enthalpy of products
What does an exothermic enthalpy change diagram look like?
-Enthalpy of reactants is higher than enthalpy of products
-ΔH > activation energy
What does an endothermic enthalpy change diagram look like?
-Enthalpy of reactants is lower than enthalpy of products
-Activation energy > ΔH
Define activation energy.
Activation energy - the minimum energy required for a reaction to take place.
What are the standard conditions?
-298K (temperature)
-100kPa (pressure)
-1.00 mol dm⁻³ (concentration)
Define ‘in standard state’.
The state of an element/compound exists at in standard conditions.
Define enthalpy change of reaction.
The enthalpy associated with a stated equation.
Define enthalpy change of combustion.
The enthalpy change when one mole of substance combusts completely with oxygen measured under standard conditions.
Define enthalpy of formation.
The enthalpy change when one mole of a substance is formed from its constituent elements measured under standard conditions.
What is the standard enthalpy of formation of any given element and why?
ΔfH = 0 for every element, because the reactant is the exact same as the product, therefore no bonds have been made or broken.
Define standard enthalpy of neutralisation.
The enthalpy change when an aqueous acid reacts with an aqueous base to form one mole of water, measured under standard conditions.
