Chapter 7: Enthalpy Change
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What is energy?
The ability to do work or move an object against an opposing force.
What are some forms of energy?
Heat, light, sound, electricity, and chemical energy.
What is heat in terms of energy transfer?
Heat is a mode of energy transfer due to a temperature difference, increasing disorder.
How does heat affect molecular behavior?
It increases the average kinetic energy of molecules in a disordered fashion.
What is enthalpy (H)?
A measure of the heat energy contained in a substance, stored as potential energy.
What is enthalpy change?
The energy exchange between a chemical reaction and its surroundings at constant pressure.
Can enthalpy be directly measured?
No, only enthalpy change can be measured.
What happens to the surroundings during an exothermic reaction?
The temperature of the surroundings increases.
What happens to the surroundings during an endothermic reaction?
The temperature of the surroundings decreases.
What is the sign of ∆H for exothermic reactions?
Negative (∆H < 0).
What is the sign of ∆H for endothermic reactions?
Positive (∆H > 0).
Give examples of exothermic reactions.
Combustion, respiration, reaction of water with calcium oxide.
Give examples of endothermic reactions.
Decomposition of limestone, photosynthesis, dissolving certain ammonium salts.
What is the standard enthalpy change?
The enthalpy change measured under standard conditions.
What is Hess’s Law?
The total enthalpy change of a reaction is independent of the pathway taken.
How can Hess’s Law be used in calculations?
By constructing an enthalpy cycle.
What is bond energy?
The energy required to break a specific covalent bond.
Is bond energy always positive or negative?
Positive, as it involves breaking bonds.
What is the relationship between bond breaking and energy?
Bond breaking is endothermic.
What is the relationship between bond making and energy?
Bond forming is exothermic.
How do you determine if a reaction is exothermic using bond energies?
If energy released during bond formation is greater than energy absorbed during bond breaking.
How do you determine if a reaction is endothermic using bond energies?
If energy absorbed during bond breaking is greater than energy released during bond formation.
What is the formula for enthalpy change using bond energies?
Enthalpy change = Energy to break bonds - Energy to form bonds.
Why are average bond energies used?
Bond energies vary between compounds, so averages are used for calculations.