8 Flashcards
Enthalpy (h)
Enthalpy is the heat energy stored within a system
Enthalpy change (ΔH)
The heat energy exchange with the surroundings at standard temperature and pressure
Standard conditions
Temperature: 298 Kelvin
Pressure: 100 kPa
Solutions at 1mol dm-3
Exothermic reaction
In an exothermic change energy is transferred from the system (chemicals) to the surroundings.
This causes the temperature (of surroundings) to increase
The products have less energy than the reactants
In an exothermic reaction the ∆H is negative
Endothermic reaction
In an endothermic change, energy is transferred from the surroundings to the system (chemicals).
This causes the temperatures (of the surroundings) to decrease
They require an input of heat energy
The products have more energy than the reactants
In an endothermic reaction the ∆H is positive
Enthalpy change of reaction
The enthalpy change of a reaction at standard temperature and pressure
ΔHr
Enthalpy change of formation
The enthalpy change where I mole of the compound is formed from its constituent elements in their standard states
ΔHf
Enthalpy change of combustion
The enthalpy change that occurs when one mole of a substance is burnt in excess oxygen under standard conditions.
all reactants and products being in their standard states
ΔHc
Enthalpy change of neutralisation
The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.
ΔHn
Energy profile diagram
Show the enthalpy changes that occur during a chemical reaction
Calorimetry
Process that allows us to calculate the energy/ enthalpy change of a reaction by measuring temperature change
The energy change of a reaction is monitored by measuring the change in temperature of a known volume of water/ solution
How to calculate energy change
Q = -m x c x ΔT
Energy change = -mass of water/solution x specific heat capacity x change in temperature
ΔH can only be worked out from Q if you know the number of moles involved
