Chapter 9
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What is enthalpy H?
a measure of the heat energy in a chemical system
What does the chemical system refer to?
the atoms, molecules, or ions making up the chemicals
Can enthalpy be measured?
no
Can enthalpy changes be measured?
yes
What is the enthalpy change ΔH?
the difference in enthalpies
ΔH =
H(products) - H(reactants)
Is ΔH positive or negative?
it can be either
What does the value of ΔH depend on?
whether the products contain more or less energy than the reactants
What is the law of conservation of energy?
energy cannot be created or destroyed
In a chemical reaction involving an enthalpy change, where is heat energy transferred between?
the system and the surroundings
What is the system?
the chemicals - the reactants and products
What are the surroundings?
the apparatus, the laboratory, and everything that is not the chemical; system
What is the universe?
everything, including the system and surroundings
Can an enthalpy change ΔH be determined experimentally?
yes
How can an enthalpy change ΔH be determined experimentally?
by measuring the energy transfer between the sustem and the surroundings
What two directions can energy transfer be?
1) exothermic - from the system to the surroundings 2) endothermic - from the surroundings to the system
What do enthalpy profile diagrams show?
1) the relative enthalpies of the reactants and products 2) enthalpy chaneg ΔH
What are the characteristics of an exothermic energy change?
1) chemical system releases heat energy to surroundings 2) energy loss by chemical system is balanced by same energy gain in surroundings 3) ΔH is negative 4) temperature of the surroundings increase as they gain energy
What are the characteristics of an exothermic energy change?
1) chemical system takes in heat energy from surroundings 2) energy gain in chemical system is balanced by same energy loss by surroundings 3) ΔH is positive 4) temperature of surroundings decreases as they lose energy
What are atoms and ions held together by?
chemical bonds
What are the bonds in chemical reactions broken by?
an input of energy
What is activation energy Eₐ?
the minimum energy required for a reaction to take place
Why do reactions with small activation energies take place very quickly?
the energy needed to break bonds is readily available from the surroundings
What does a standard enthalpy change ΔH⦵ refer to?
an enthalpy H change Δ under standard conditions ⦵