Thermodynamics Flashcards
(24 cards)
Whats the difference between themodynamics and kinetics?
Thermodynamics – What products will form and what will the equilibrium ratios of reactants to products be?
Kinetics – How fast will the reaction happen (reach equilibrium)?
What is the first law of thermodynamics?
Energy cannot be created or destroyed, only converted between forms.
What is an open system, a closed system, and an isolated system?
An open system interacts with its surroundings through exchanging both energy and matter, a closed system is only through energy and an isolated system is through neither.
What is heat (q) (transfer)?
Heat is the transfer of energy that causes disorderly molecular motion (thermal motion). Heat transfer from system to surroundings causes random motion of molecules in surroundings.
What is work (w) (transfer)?
Work is the transfer of energy that causes organised molecular motion. Work done by system on surroundings causes motion of molecules in the surroundings in the same direction.
What is internal energy (U)?
Internal energy is the total kinetic and potential energy of the constituents of a system but NOT the kinetic energy associated with motion of the whole system.
What is the equation for the first law of thermodynamics?
internal energy, work, heat
∆𝑈 = ∆𝑞 + ∆𝑤
What is expansion work?
and equation
How energy and temperature changes when gases expand. eg in a piston.
∆𝑤 = −𝑝𝑒𝑥𝑡∆𝑉
What is heat capacity (C)?
equation
The heat capacity of a substance tells us the amount of energy supplied as heat required to raise its temperature by a fixed amount,
e.g. how many Joules required to raise temperature by 1 °C.
It can be represented as:
𝐶 = 𝑑𝑞/𝑑𝑇
What is the equation fr enthalpy (H)?
is endo/exo greater/less than 0
𝐻 = 𝑈 + 𝑝𝑉
∆𝐻 = ∆𝑞 = ∆U + p∆𝑉 (at constant pressure)
> This means we can just measure temperature changes in a constant pressure system to determine changes in enthalpy. Exothermic Δ𝐻 < 0, endothermic Δ𝐻 > 0
How can internal energy changes be measured?
Internal energy changes can be measured using a bomb calorimeter (constant volume).
What is Hess’ law?
Standard enthalpy of reaction is equal to the standard enthalpy of any series of reactions between the same reactants and products.
What is the second law of thermodynamics?
The entropy (S) of an isolated system increases in the course of a spontaneous change.
∆𝑆 > 0
Δ𝑆𝑢𝑛𝑖𝑣𝑒𝑟𝑠𝑒 = Δ𝑆𝑠𝑦𝑠𝑡𝑒𝑚 + Δ𝑆𝑠𝑢𝑟𝑟𝑜𝑢𝑛𝑑𝑖𝑛𝑔𝑠 ≥ 0
System entropy can decrease, but only if entropy of surroundings increases by a greater amount.
What is the thiird law of thermodynamics?
𝑆(𝑇 = 0𝐾) = 0
The third law allows us to define the entropy of any substance at any temperature.
When do spontaneous changes happen?
When the total entropy (of the Universe) increases. Also when ∆𝐺 < 0.
What is gibbs energy?
equation
Gibbs energy is defined as 𝐺 = 𝐻 − 𝑇𝑆
What is the integrated Gibbs-Helmholtz equation?
what does it describe?
Integrated Gibbs-Helmholtz equation describes how Gibbs energy changes with temperature:
Δ𝐺1/𝑇1 = Δ𝐺0/𝑇0 + Δ𝐻( 1/𝑇1 − 1/𝑇0)
What are phase diagrams, and what is the equation for the gradient of the equilibrium line?
Phase diagrams display at what combination of temperature and pressure different phases exist (and where equilibria between phases are located). The gradient of an equilibrium line is given by the Clapeyron equation
What is chemical potential?
The chemical potential is the change in Gibbs energy with respect to the number of moles. If the system contains only one substance, the chemical potential is:
𝜇 = 𝐺/𝑛 = 𝐺𝑚
i.e. the chemical potential equals the molar Gibbs energy.
What does the Gibbs-Duhem equation give?
The Gibbs-Duhem equation tells us that if one μ changes, it affects all other values of μ.
What is the reaction quotient?
Reaction quotient 𝑄 = 𝑝𝐵/𝑝𝐴
What is Le Chatelier’s principle?
If the equilibrium of a system is disturbed by a change in one or more of the determining factors (as temperature, pressure, or concentration) the system tends to adjust itself to a new equilibrium by counteracting as far as possible the effect of the change.
Knowing le Chatelier’s principle, can see how changing T is counteracted by a change in reactant/product ratio.
If T is increased for an exothermic/endothermic reaction what happens to equilibrium?
changes in equilibrium with T
For an exothermic reaction, equilibrium moves to increase reactants. This is to balance the temperature increase by converting this heat to an enthalpy increase. For an endothermic reaction, equilibrium moves to increase products. This is to balance the temperature increase by absorbing the heat.
What are the effects of pressure on equilibrium?
Adding an ideal gas into the system at the same volume doesn’t change partial pressures, therefore no change in equilibrium. Compressing the system to smaller volume does change partial pressures, so 𝑝𝐴 must increase faster than 𝑝𝐵 to keep equilibrium constant the same.
