CL 9- enthalpy Flashcards
activation energy
definition
the minimum energy required for a reaction to take place
standard conditions
pressure of 100kPa, temperature of 298K, and a concentration of 1moldm⁻³
standard states
definition
the physical state of a substance under standard conditions
standard enthalpy change of reaction
definition
the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation, under standard conditions, with all reactants and products in their standard states
standard enthalpy change of formation
definition
the enthalpy change that takes place when 1 mole of a compound is formed from it’s elements, under standard conditions, with all reactants and products in their standard states
standard enthalpy change of combustion
definition
the enthalpy change that takes place when 1 mole of a substance completely reacts with oxygen, under standard conditions, with all reactants and products in their standard states
standard enthalpy change of neutralisation
definition
the enthalpy change that accompanies the reaction of an acid by a base to form 1 mole of water, under standard conditions, with all reactants and products in their standard states
equation for ΔH
Q=mcΔT
enthalpy change=mass x SHC x change in temp
average bond enthalpy
definition
the energy required to break 1 mole of a specified type of bond in a gaseous molecule
why are bond enthalpies always positive?
they are endothermic- energy is required to break a bond
in terms of bonds breaking/making
explain an exothermic reaction
energy released when making bonds in the products is greater than the energy required to break the bonds in the reactants
in terms of bonds breaking/making
explain an endothermic reaction
the energy required to break the bonds in the reactants is greater than the energy released when new bonds are made in the products
enthalpy change of neutralisation method
- measure 25cm³ of acid and 25cm³ of alkali in separate measuring cylinders
- set up a polystyrene cup in a beaker (to keep it upright) and a thermometer, and add the acid
- record the inital temperature, and then continue to record temp every 30 seconds
- at the 3 minute mark, add alkali and record temp
- continue recording temp every 30 seconds until you reach 10 minutes, or reach a final max temp
- calculate energy exchanged with Q=mcΔT
enthalpy change is negative
enthalpy change of combustion method
- measure 100cm³ of water with a measuring cylinder and pour into calorimeter
- take an initial reading of the water temperature with a thermometer
- weigh the spirit burner with ethanol and the lid on, and record the mass
- place spirit burner underneath calorimeter and light the wick (don’t move spirit burner once it’s been lit)
- stir the water with a glass rod
- when water reaches 70°C, put out the flame by placing the lid back on the spirit burner with tongs
- measure the mass of the spirit burner, lid and ethanol
difference in masses= mass of alcohol used in reaction(used to calculate moles) - use Q=mcΔT to determine the enthalpy change
sources of error for enthalpy change of combustion
- heat loss to the surroundings from spirit burner, wick and calorimeter
- evaporation of water
- evaporation of alcohol
- incomplete combustion of alcohol
- reaction is unlikely to occur under standard conditions
