Enthalpy Flashcards
What is enthalpy?
H is a measure of the heat energy in a chemical system, which is the atoms, molecules or ions making up the chemicals.
What is enthalpy change?
The difference in enthalpies.
ΔH = H (products) - H (reactants)
ΔH can be positive or negative, depending on whether the products contain more or less energy than the reactants.
What is the Law of Conservation of Energy?
States that energy cannot be created or destroyed.
When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings.
The system is the reactants and products.
The surrounding are the apparatus, lab, and anything not in the system.
The universe involves both.
What is the direction of energy transfer?
From the system to the surroundings - exothermic change.
From the surroundings to the system - endothermic change.
What is an exothermic energy transfer?
The chemical system releases heat energy to the surroundings.
Any energy loss by the system is balanced by the same energy gain by the surroundings.
ΔH is negative.
The temperature of the surroundings increases.
What is the enthalpy profile diagram for an exothermic energy change?
The reactants are above the products.
The arrow showing ΔH points down to show it is negative.
What is an endothermic energy transfer?
The chemical system takes in heat energy from the surroundings.
Any energy gain by the chemical system is balanced by the same energy loss by the surroundings.
ΔH is positive.
The temperature of the surroundings decrease as they lose energy.
What is the enthalpy profile diagram for an endothermic energy change?
The reactants are below the products.
The arrow showing ΔH points up to show it is positive.
What is activation energy?
The minimum energy required to break the bonds and for a reaction to take place.
Reactions with small activation energy take place very rapidly, because the energy needed to break the bonds is readily available from the surroundings.
What are standard conditions?
Standard pressure is 100kPa.
Standard temperure is 298k or 25°C.
Standard concentration is 1 moldm^-3.
Standard state is the physical state of a substance under standard conditions.
ΔH° shows enthalpy change under standard conditions.
What is the standard enthalpy change of reaction?
ΔrH° is the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation under standard conditions, with all the reactants and products in their standard states.
E.g. Mg + 1/2 O2 –> MgO or 2Mg + O2 –> 2MgO (the kJ mol-1 value would be doubled for this).
What is the standard enthalpy change of formation?
ΔfH° is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all the reactants and products in their standard states.
E.g. Mg + 1/2 O2 –> MgO
All elements have an enthalpy change of formation of 0 kJmol-1.
What is the standard enthalpy change of combustion?
ΔcH° is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard states.
E.g. C4H10 + 6 1/2 O2 –> 4CO2 + 5H2O
What is the standard enthalpy change of neutralisation?
ΔneutH° is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all the reactants and products in their standard states.
E.g. HCl + NaOH –> H2O + NaCl
The value is the same for all neutralisation reactions (-57kJmol-1)
What is the equation for calculating energy change?
q = mcΔT
q = heat energy in j
m = mass of surroundings in g
c = specific heat capacity of water 4.18Jg-1K-1
ΔT = temperature change in k