M3 B5 Enthalpy Changes
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What is enthalpy?
Enthalpy is the heat energy stored in a chemical system
Define ‘activation energy’
Activation energy is the minimum energy required to start a reaction by the breaking of bonds.
What happens to the reactants in an exothermic reaction?
Reactants lose energy to the surroundings, causing temperature to increase.
What happens to the reactants in an endothermic reaction?
Reactants gain energy from the surroundings, causing temperature to decrease.
What is the symbol for enthalpy change?
ΔH (capital delta, H)
Name two common types of exothermic reactions
• Combustion of fuels • Respiration (oxidation of carbohydrates) (glucose + oxygen)
Name two common types of endothermic reactions
• Thermal decomposition (of calcium carbonate) • Photosynthesis (plants)
What is a standard enthalpy change? (ΔH ⦵)
Enthalpy changes that occur under a given set of ‘standard’ conditions.
State the standard conditions
(ΔH ⦵) - 298 Kelvin (25 degrees Celcius) - 100KPa (100,000 Pascals) or 1 atmosphere - 1 mol per dm cubed for concentrations of solutions
Define ΔrH (enthalpy change of reaction)
The enthalpy change associated with a stated equation. (if ⦵ then add ‘in their standard states under standard conditions’ to the definition)
Define ΔfH (enthalpy change of formation)
The enthalpy change when ONE mole of a compound is made from its elements. (if ⦵ then add ‘in their standard states under standard conditions’ to the definition)
Define ΔcH (enthalpy change of combustion)
The enthalpy change when ONE mole of a substance is completely combusted with oxygen. (if ⦵ then add ‘in their standard states under standard conditions’ to the definition)
Define ΔneutH (enthalpy change of neutralisation)
The enthalpy change when an AQUEOUS acid is neutralised by an aqueous base, forming ONE mole of WATER. (if ⦵ then add ‘in their standard states under standard conditions’ to the definition)
What are the 3 methods used to calculate enthalpy changes?
• Experimental data (temperature changes) • Average bond enthalpies • Other enthalpy changes (Hess cycles)
Experimental data: What is the equation for calculating enthalpy changes?
Q= -m c ΔT (ΔT = delta T = Change in temp)
What is the equation for ΔH (change in enthalpy) per mole?
ΔH (delta H) = Q divided by n (n= moles)
What does Q stand for when calculating enthalpy changes?
Q= heat energy change (In Joules)
Why is the enthalpy value calculated experimentally less than those determined theoretically? (3 reasons)
• Due to heat loss to the surroundings • Incomplete combustion • Non-standard conditions
Where do heat losses occur and how can they be minimised (2 for each)
• Loss through top of open container- minimised using a lid • Loss through the material of the container- minimised through insulating the material
When bonds are broken, what type of energy change is involved?
Endothermic- Energy required to break bonds
When bonds are made, what type of energy change is involved?
Exothermic- Energy is released
If more energy is required to break the bonds in the reactants, than is released when bonds are made in the products, is the overall energy change endo or exothermic?
Endothermic
If more energy is released when bonds are made in the products, than was required to break the bonds in the reactants, is the overall energy change endo or exothermic?
Exothermic
Define ‘average bond enthalpy’
Average bond enthalpy is the enthalpy change when breaking 1 mole of a given type of bond in the molecules of a gaseous species.