A2 - Energetics
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Define enthalpy
a measure of the heat energy in a chemical system
Law of conservation of energy
energy cannot be created or destroyed
Equation for enthalpy change
enthalpy of products - enthalpy of reactants
Define exothermic
heat out of the system releases heat energy to the surroundings energy loss balanced by the same energy gain
Is the enthalpy change in an exothermic reaction positive or negative?
negative
Draw an enthalpy profile diagram for an exothermic reaction
reactants above products
Define endothermic
heat into the system takes in heat from the surroundings energy gain is balanced by energy lost
Is the enthalpy change in an endothermic reaction positive or negative?
positive
Draw an enthalpy profile diagram for an endothermic reaction
products above reactants
Define activation energy
the minimum energy required for a reaction to take place
What is meant by the term standard enthalpy changes?
standard conditions for physical measurements such as enthalpy changes
Standard pressure
100kPa
Standard temperature
298K - 25 degrees C
Standard concentration
1 mol/dm3
Standard state
physical state of a substance under standard conditions
Explain the term enthalpy change of reaction
enthalpy change for stated equation
Explain the term enthalpy change of formation
the enthalpy change that takes place when one mole of a compound is formed from its element under standard conditions with all reactants and products in their standard states
Explain the term enthalpy change of combustion
enthalpy change that takes place when one mol of substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
Explain the term enthalpy change of neutralisation
the standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l)
What does enthalpy change of reaction always refer to?
a stated equation and its value depends on the balancing numbers
Equation for Q
Q = mc x change in T
Factors affecting accuracy of enthalpy change of combustion experimental value: heat loss
heat loss to the surroundings other than the water –> includes the beaker but mainly the air surrounding the flame
Factors affecting accuracy of enthalpy change of combustion experimental value: incomplete
incomplete combustion of methanol –> carbon monoxide and carbon produced instead of carbon dioxide –> see carbon as a layer of soot on the beaker
Factors affecting accuracy of enthalpy change of combustion experimental value: evaporation
evaporation of methanol from the wick –> burner must be weighed as soon as possible after extinguishing the flame –> some methanol may have evaporated from the wick –> spirit burners usually have a cover to reduce this error