8What is the heat of a reaction? Write the answer
Heat of reaction, the amount of heat that must be added or removed during a chemical reaction in order to keep all of the substances present at the same temperature. If the pressure in the vessel containing the reacting system is kept at a constant value, the measured heat of reaction also represents the change in the thermodynamic quantity called enthalpy, or heat content, accompanying the process—i.e., the difference between the enthalpy of the substances present at the end of the reaction and the enthalpy of the substances present at the start of the reaction. Thus, the heat of reaction determined at constant pressure is also designated the enthalpy of reaction, represented by the symbol ΔH. If the heat of reaction is positive, the reaction is said to be endothermic; if negative, exothermic.
Thermodynamic Definition of Enthalpy (H):
H = E + PV
E = energy of the system
P = pressure of the system
V = volume of the system
Recall, by definition a change in energy equals heat transferred (q) plus work (w):
DE = q + w Consider a process carried out at constant pressure. At constant pressure, work involves only a change in volume. We can then substitute -PDV for w.
DE = qp - PDV
Then if we want to solve for the heat transferred, qp, at constant pressure, we simply rearrange the equation.
qp = DE + PDV
If DH <0, then qp >0 The reaction is Exothermic Heat goes from the system into the surroundings
If DH >0, then qp >0 The reaction is Endothermic. Heat goes from the surroundings into the system.
9What are the compression and expansion? What are the exothermic and endothermic processes?
Expansion and Compression of a Gas In an adiabatic process, there is no heat transferred to or from the system i.e. dQ = 0. The first law of thermodynamics then reduces to dQ = dW + dU ⇒ dU = −dW (1) where dW is the amount of work done by the gas and dU is the change in internal energy. Since the gas is expanding, the work being done by the gas is positive. Eqn. (1) then implies that the change in internal energy of the system must be negative, that is, the internal energy is decreasing. The only true statements are then: • The work done by the gas is positive, and no heat exchange occurs. • The internal energy of the system decreases. Since the system undergoes a cyclic process, i.e. a process in which the initial and final states are the same, the change in internal energy must be zero. This allows for the reduction of the first law of thermodynamics to dQ = dW. (2) The only true statements are then • The total change in internal energy of the system after the entire process of expansion and compression must be zero. •
10What are the exothermic and endothermic processes?
If DH <0, then qp >0 The reaction is Exothermic Heat goes from the system into the surroundings
If DH >0, then qp >0 The reaction is Endothermic. Heat goes from the surroundings into the system.
Endothermic
process: a
change (e.g. a chemical reaction) that requires (or absorbs) heat.
Endothermic
reaction, heat taken in & temperature of the substance drops
Exothermic process: a change (e.g. a chemical reaction) that releases heat.
Exothermic reaction, heat given off & temperature of substance rises.
EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration Ex Combustion of fuels
Yeast & Hydrogen Peroxide
Epson salts & water
ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesisEndothermic
Photosynthesis
Acedic Acid & Sodium Bicarbonate