Energy
There are two main types of energy:
- Both kinetic energy and thermal energy refer to the kinetic energy of moving particles of matter. However, temperature is the average kinetic energy of the particles in a sample of matter, while thermal energy is the total kinetic energy of the particles in a sample of matter.
- potential energy – energy stored by an object, due to it position or situation (chemical potential energy is due to breaking or forming of bonds)
Chemical Energy
Up to this point, we have balanced equations and done stoichiometric calculations with no interest in the quantitative energy changes which accompany the reaction.
N2(g) + 3H2(g) → 2NH3(g) + heat
*exothermic – release of thermal energy from system – kinetic energy is released from system
heat + 2H2O(l) → 2H2(g) + O2(g)
*endothermic – absorption of thermal energy into system – kinetic energy is absorbed by system
Thermochemistry involves the energy changes that accompany
physical, chemical and nuclear transformations. Using the concept of thermochemistry, we can now look at the energy associated with a physical/chemical/nuclear change.
Heat & Energy Changes
- chemical system – all the substances undergoing the physical, chemical or nuclear change
- surroundings – all matter not contained in the system
- quantity of heat (q) – when a reaction occurs, heat is transferred between objects (from hot to cool object)
- isolated system – a system that does not allow energy or matter to flow in/out of it (for instance, a sealed flask)
- open system - a system that does allow energy or matter to flow in/out of it (for instance, an open beaker)
Calorimetry
Calorimetry is a technique used to measure the amount of heat transferred between system and surroundings (exothermic: system to surroundings; endothermic: surroundings to system). The amount of heat depends on the temperature change (ΔT), the masses of the substances in the system (m) and on the specific heat capacity (c) of the substance in the system.
q = m·ΔT·c
where q is the quantity of heat transferred (kJ), m is the mass (g), ΔT is the temperature change (°C or K) and c is the specific heat capacity (J/g°C or J/gK)
- specific heat capacity is the amount of heat required to raise 1 g of a substance by 1°C
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| ***We will use 4.184 J/gC for water. |
TryIt! When 250 mL of water is heated from 22°C to 95°C, how much heat does the water absorb?
Heat Transfer & Enthalpy Change
Chemists are interested in the enthalpy change for a reaction.
- enthalpy (ΔH) – the heat lost or gained by a system when a process occurs under constant pressure
The total energy of the system and surroundings remains the same, there is just a transfer of energy from one to the other (Law of Conservation of Energy – First Law of Thermodynamics).
Let’s examine a typical exothermic reaction:
Initially, the chemical potential energy in the system is converted to kinetic energy of the particles in the system. Then, through collisions, the kinetic energy is transferred to the particles in the surroundings. Thus, as the potential energy of the system decreases, the kinetic energy of the surroundings increases proportionately. So the enthalpy change in the system is equal to the heat released to the surroundings.
ΔH = positive value
(Hfinal > Hinitial), system has gained heat from the surroundings (endothermic)
572 kJ + 2H2O(l) → 2H2(g) + O2(g) or ΔH = +572 kJ
ΔH = negative value
(Hfinal < Hinitial), system has lost heat to the surroundings (exothermic)
C(s) + O2(g) → CO2(g) + 394 kJ or ΔH = -394 kJ
The
enthalpy of a system is considered to be a state
function since it
only depends on the initial and final circumstances of the system, not on how the
change occurs. For the physicists in the crowd, this is just like displacement.
TryIt! Answer:
V = 250 mL m = dV q = m ΔT c
T1 = 22°C =(1.00 g/mL)(250 mL) =(250 g)(95°C-22°C)(4.184 J/g°C)
T2 = 95°C = 250 g = 76 285 J
c = 4.184 J/g°C = 76 kJ
∴ the amount of heat absorbed is 76 kJ.
Homework: p.297 #1-3
Answers:
