3.2.1 Enthalpy Changes Flashcards
What is enthalpy and how is it measured?
- heat content of a chemical system
- measured by the energy change
What are exothermic and endothermic reactions?
- exo
- energy is given out
- ΔH is negative
-e.g.combustion of fuels - endo
- heat energy is absorbed
ΔH is positive - e.g. thermal decomposition of CaCO3
Enthalpy change in exothermic reactions
- heat loss from system to surroundings
- ΔH is negative because heat is lost by system
- enthalpy of products is smaller than enthalpy of reactants
Exothermic energy level diagram
See notes
Enthalpy change in endothermic reactions
- require an input of heat energy
- ΔH is positive because heat is gained by chemical system
- enthalpy of products is greater than enthalpy of reactants
Enthalpy level diagram for endothermic reaction
See notes
What is the equation for photosynthesis?
6CO2 + 6H2O -> C6H12O6 + 6O2
What is activation energy?
The minimum energy required for a reaction to take place
What are standard conditions?
Temperature - 298K
Pressure - 100kPa
Solutions - 1 moldm-3
Standard states
What is the standard enthalpy change of reaction?
The enthalpy change that occurs between molar quantities of reactants as shown in the stated equation, under standard conditions to give products in their standard state.
ΔH°r
What is the standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions both compound and elements are in their standard states
ΔH°f
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What is the standard enthalpy change of combustion?
The enthalpy change when one mole of an element or compound reacts by complete combustion with excess oxygen under standard conditions
ΔH°c
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What is the standard enthalpy change of neutralisation?
The enthalpy change when 1 mol of water is formed from a neutralisation reaction
ΔH°n
What is the equation to calculate heat energy given out?
Q = mcΔt
Q is in Joules
M is mass of solution in grams
How to calculate enthalpy change from heat energy?
Convert to kJ
kJ / mol
Find mol of reactants (not normally solution)
Why do enthalpy changes take place?
- bonds are broken and formed
- energy (in the form of heat) is needed to overcome attractive forces between atoms
What is bond breaking and bond making?
- bond breaking is endothermic
- bond making is exothermic
What is exact bond enthalpy?
- the amount of energy needed to break one mole of specific covalent bonds in the gaseous state
What is the difference between average bond enthalpy and exact bond enthalpy?
- bond energies are affected by other atoms in the molecule
- an average of a number of the same type of bond but in different environments is calculated
What is average bond enthalpy?
- the mean energy needed for the breaking of 1 mol of bonds in gaseous molecules
How to calculate enthalpy change from bond energies?
ΔH = E (bond enthalpies of reactants) - E(bond enthalpies of products)
What is Hess’ law?
- the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs
- it is an indirect way of measuring enthalpy change
Draw a Hess’ cycle for enthalpy of formation (rough)
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Draw a Hess’ cycle for enthalpy of combustion (rough)
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Draw and solve a Hess’ cycle for combustion
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What is the link between strong and weak acids and enthalpy change?
Strong acids: all negative values since complete dissociation occurs, reactions have high values
Weak acids: all values are slightly lower
Describe how a simple calorimeter can be used to calculate enthalpy change
- measured directly by monitoring temperature change
- experiments carried out in polystyrene beaker which are good insulators and prevent heat loss and gain
- calculate heat energy exchanged using Q = mcΔt
- calculate enthalpy by dividing by moles
Arrows for Hess’s cycle of combustion
Point down
Arrows for Hess’s cycle of formation
Point up
