Midterm 2 Flashcards
(63 cards)
What are the two types of processes that are identified in heating curves
Two types of processes are identified in the heating curve:
- Phase change → added heat does not change temperatureq = n∆H → Number of moles * enthalpy of the phase change
- Heating of substance → added heat changes temperature
q = mc∆T → mass * specific hat capacity * change in temperature
what are the Enthalpies of reaction
The enthalpy change for a change is ∆H = Hf - Hi
so in chemical reactions ∆H rxn = ∑H products - ∑H reactants
∆H rxn is called the enthalpy of reaction
Like U absolute values of H are difficult to determine so we can’t calculate ∆H reaction from this equation.
what is the enthalpy change for endothermic vs exothermic processes
In endothermic processes ∆H rxn > 0
In exothermic processes ∆H rxn < 0
what are thermochemical equations
A thermochemical equation is the balanced equation that includes the value of ∆H
- Magnitude of ∆H depends on amounts of reactants and products
- ∆H forward = -∆H reverse
- ∆H depends on the physical state of reactants and products
what is the standard enthalpy change for a chemical reaction
standard enthalpy change for a chemical reaction. ∆H rxn
the enthalpy change for a reaction with reactants and products in their standard states
what is the thermodynamic standard state
Thermodynamic standard state
A particular conditions of T and P that is used as a reference for the calculations of thermodynamic properties: 1 atm and 25ºC/298 K
what are the standard enthalpies of formation
the enthalpy change for the hypothetical formation of 1 mol of substance in its thermodynamic standard state from its constituent elements in their standard states
for pure elements in their most stable forms at standard conditions ∆Hºf = 0 by definition
How do you calculate the enthalpy change for a reaction
Using enthalpies of formation to calculate enthalpies of reaction steps:
- Decompose reactants into elements
- recombine elements into productsNote: it doesn’t actually matter that this isn’t the pathway of formation since H is a state function
∆Hºrxn = ∑V(product)∆Hºf(products) - ∑V(reactants)∆Hºf(reactants)
where V(product) and V(reactant) are the unitless stoichiometric coefficients in the reaction equation
what is Hess’ law
Tabulated ∆H values can be used to calculated the enthalpy change of reactions. ∆H depends on amounts and initial and final states of reactants and products
An unknown ∆Hºrxn can be calculated by manipulating known equations to construct a pathway with the same initial and final states as the unknown equation.
how does chemistry protect us against solar rays
Reactions are an outer defence against radiation and high energy particles.
Chemical processes absorb solar radiation → photodissociation (break bonds) and photoionization (ionize)
what is photodissociation
Molecule absorbs photons and bonds are broken.
incoming energy must be sufficient to break the bond.
If you know the dissociation enthalpy for a chemical bond you can calculate what energy or wavelength is needed to break one bond with these formulas:
E = ∆H * (1 mol / 6.02 * 10^23)
wavelength = hc/E = (6.63 * 10^-34)(2.98 * 10^8) / E
what is photoionization
Molecules absorbs a photon and loses an electron → ions are formed (ionization)
occurs at high elevations (~90km) lower thermosphere
The amount of energy required to ionize one mol of a gas → enthalpy of ionization. (aka ionization energy)
Photon must have enough energy to remove an electron
are completely filtered out
Higher energy process than photodissociation, shorter wavelength photons filtered.
what is chemical equilibrium
Chemical reactions have forwards and reverse directions.
At equilibrium, concentrations of reactants and products become constants (but not equal). Reactions continue in both directions, it is a dynamic process.
what is the state of a system at equilibrium (5 points)
At equilibrium:
- Concentrations of reactants and Products do not change
- Reactions continue in both directions
- The rate of the reaction in the forward direction equals the rate of the reaction in the reverse reaction. Kf = Kr
- Can originate from a mixture of reactants and products, or pure reactants or pure products.
- The equilibrium constant K is a ration between the concentrations (or partial pressures) of reactants and the concentrations (or partial pressures) of products at equilibrium.
How do you calculate Kc and Kp
For a reaction: aA+bB<–> cC+dD
Kc=[C]^c[D]^d/[A]^a[B]^b
Kp=[PC]^c[PD]^d/[PA]^a[PB]^b
KP=KC(RT)^∆V
∆V=(c+d)-(a+b)
R = 0.082 Latm / molK
what is the state of a system away from equilibrium
When a chemical system is away from equilibrium:
- Concentrations of reactants and products change towards those at equilibrium
- The rate of the reaction in the forward direction is different from the rate of the reaction in the reverse direction. Kf doesn’t = Kr
- The reaction quotient Q is a ratio of the concentrations (or partial pressures) of reactants and products at each specific non-equilibrium condition. Q is not a constant.
- Before equilibrium is reached, the value of Q changes with time.
- Once equilibria is reached the value of K does not change with time.
what is Q
Q is a measure of the progression of a reversible reaction that is not at equilibrium. It is a ration of concentrations or a ration of partial pressures of reactants and products to the power of their stoichiometry.
how do you calculate Qc and Qp
For a reaction: aA+bB<–> cC+dD
Qc=[C]^c[D]^d/[A]^a[B]^b
Qp=[PC]^c[PD]^d/[PA]^a[PB]^b
QP=QC(RT)^∆V
∆V=(c+d)-(a+b)
R = 0.082 Latm / molK
what is K
K is the unique ration of concentrations or a ration of partial pressures of reactants and products at equilibrium to the power of their stoichiometry (under specified conditions).
what is happening when Q is >, <, and = to K
When Q > K, reaction proceeds towards reactants to establish equilibrium.
When Q < K, reaction proceeds towards products to establish equilibrium.
When K = Q the reaction is at equilibrium.
what is the Equilibrium constant
K
Equilibrium constants have no units and are dependent on temperature.
The concentrations or partial pressures are understood to be a ration with respect to a standard reference state known as activity.
what does it mean when K >, <, and = to 1
K < 1 at equilibrium = more reactants than products
K ~ 1 at equilibrium = similar amounts of products and reactants
K > 1 at equilibrium = more products than reactants.
what phases are involved in K
For heterogeneous mixtures (reactants and products are not in the same phase)
Pure solids, pure liquids and solvents are not included in the equilibrium constant expression. Their concentrations/activities are considered constant and equal to 1.
what is Le Châtelier’s principle
When a reaction mixture at equilibrium is disturbed to non-equilibrium concentrations, the concentrations will change to counteract the disturbance and re-establish equilibrium.