Hemet High · Honors Chemistry
16 · Reaction Energy
Notes
Ch 16.1 – Thermochemistry
· : the study of the transfer of energy as heat that occurs during chemical reactions and changes in state.
· : q, is energy transferred from one object to another because of a temperature difference between them.
· Heat always flows from a object to a object and will continue to flow until they are in .
· : one that absorbs heat from the surroundings (+q).
· : one that releases heat to it’s surroundings (-q).
· : the insulated device used to measure the absorption or release of heat in chemical or physical processes.
Specific Heat
· Heat flow is measure in (J).
· One joule of heat raises the temperature of 1g of pure water
· : the amount of heat needed to increase the temp of 1 g of the substance 1°C or 1 Kelvin.
· Water’s specific heat is
Calculating Specific Heat (C)
· Divide the heat input, q (Joules) by the temperature change, ΔT (°C or K) times the mass of the substance, m (g).
C = q = heat m x ΔT mass x change in temp
Example
· The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper is absorbs 849J of heat. What is copper’s specific heat (C)?
Example
· How much heat, q, is required to raise the temperature of 400.0 g of silver 45°C? Look at Table 17.1 on page 508 for specific heat of silver.
Enthalpy of Reaction
· is the amount of energy absorbed by a system as during a process at constant .
o The enthalpy change is always the between the enthalpies of the products and the reactants and is called Enthalpy of Reaction.
∆H = Hproducts - Hreactants
· ∆H is for an exothermic reaction because the system heat.
· ∆H is for an endothermic reaction because the system heat.
Example
· Enthalpy of Reaction for the formation of water vapor.
o What we already know:
o This equation does not tell us that energy is as heat during the reaction.
o Thermochemical Equation:
o Writing with ∆H: 2H2(g) + O2(g) à 2H2O(g) ∆H =
· Enthalpy of Reaction for the decomposition of water vapor.
o What we already know:
o This equation does not tell us that energy as heat is during the reaction.
o Thermochemical Equation:
o Writing with ∆H: 2H2O(g) à 2H2(g) + O2(g) ∆H =
· Compounds that release a amount of energy as heat when they are formed are very .
· Compounds that release a very amount of energy as heat or absorb a large amount of energy as heat when they are formed are sometimes and may decompose or react violently.