3.9: enthalpy Flashcards
what is enthalpy?
a measure of the heat energy in a chemical system under standard conditions
what is a chemical system and the surroundings?
a chemical system refers to the atoms, molecules or ions making up the chemicals
the surroundings are everything else e.g. the thermometer, air around the chemical
what is enthalpy change?
the energy absorbed or released per mole
the heat energy change under standard conditions
why do we measure energy transfers and not enthalpy directly?
because enthalpy can’t be measured but enthalpy changes can - the enthalpy change can be positive or negative depending in whether the product contains more or less energy than the reactants
what is an exothermic reactions?
energy transfer from the system to the surroundings (energy lost from system is balanced by the same energy gain by surroundings)
the temperature of the surroundings increase and gain energy
the enthalpy change is always negative
what is an endothermic reaction?
energy transfer from the surroundings to the system (energy gained by the system balanced by same energy loss by surroundings)
the temperature of the surroundings decrease and lose energy
the enthalpy change is positive
what is activation energy?
the minimum energy required for a reaction to occur by the breaking of bonds
definition of the standard enthalpy of formation?
the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions with all reactants/products in their standard states (can be exothermic or endothermic)
definition of standard enthalpy of combustion?
the enthalpy change when 1 mole of a substance reacts completely with excess oxygen under standard conditions with all reactants/products in their standard states (always exothermic)
definition of standard enthalpy of neutralisation?
the enthalpy change that occurs when 1 mole of water is produced in the reaction of an acid with an alkali under standard conditions (exothermic)
