Level 1

1. Write down at least two definitions of the first law of thermodynamics

Thermodynamics is a branch of physics concerned with heat and temperature and their relation to energy and work. The behavior of these quantities is governed by the four laws of thermodynamics, irrespective of the composition or specific properties of the material or system in question.

Q = ΔU + W Internal Energy- Internal Energy: total energy of a system -.Involves translational, rotational, vibrational motions.-Cannot measure absolute internal energy.-Change in internal energy,

Relating DE to Heat(q) and Work(w)-Energy cannot be created or destroyed.-Energy of (system + surroundings) is constant.-Any energy transferred from a system must be transferred to the surroundings (and vice versa).-From the first law of thermodynamics:

Exothermic and Endothermic Processes-Endothermic: absorbs heat from the surroundings.-An endothermic reaction feels cold.-Exothermic: transfers heat to the surroundings.-An exothermic reaction feels hot.

2Define the reaction enthalpy! Why it does not fully equivalent to the heat of reaction?

The enthalpy change for a process, ∆H, is equal to the heat absorbed by the system if thatprocess is done under constant pressure conditions (and assuming that only P-V work is possi-ble). Since the enthalpy of a system, H = E + PV, is a state function, we can systematizeenthalpy calculations by considering a path whereby the compounds first turn into their con-stituent elements in their standard states (by convention at 25oC and 1 atm pressure) and thenrecombine to form the products. The enthalpy change in the latter step is just the enthalpy of for-mation of the products and the former is the enthalpy of destruction (i.e., the negative of theenthalpy of formation) of the reactants. Hence,

∆H =Σ∆H0f(products) − ∆H0f(reactants)

Since we are interested in calculating a difference, the absolute enthalpy of the elements intheir standard states is unimportant [it cancels out of Eq. (8.1)], and we adopt the convention thatthe enthalpy of formation of an element in its standard state is zero.Consider the following example (reduction of iron oxide):

Fe2O3(s) + 3H2(g)→2Fe(s) + 3H2O(l).

3Chemical equilibrium.General record of a chemical reaction.Write the answer?

Chemical Equilibrium is a state of dynamic balance where the rate of the forward reaction is equal to the rate of the reverse reaction. A « B Assume that the forward and reverse reactions are both elementary steps. In a chemical reaction, chemical equilibrium is the state in which both reactants and products are present in concentrations which have no further tendency to change with time.^[1]Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are generally not zero, but equal. Thus, there are no net changes in the concentrations of the reactant(s) and product(s). Such a state is known as dynamic equilibrium N₂(g) + 3H₂(g) ⇄ 2NH₃(g) : DH = - 92 kJ mol

• Rate (forward) = k_f [A]

• Rate (reverse) = k_r [B]

• At equilibrium: k_f [A] = k_r [B]

• N₂(g) + 3H₂(g) ⇄ 2NH₃(g) : DH = - 92 kJ mol

- Consider the following equilibrium system:

wA + xB ⇄ yC + zD K_c=

Homogeneous equilibria:

CH₄(g) + H₂O(g) ⇄ CO(g) + 3H₂(g);

CO(g) + H₂O(g) ⇄ CO₂(g) + H₂(g);

Heterogeneous equilibria:

CaCO₃(s) ⇄ CaO(s) + CO₂(g);

HF(aq) + H₂O(l) ⇄ H₃O⁺(aq) + F^-(aq);

PbCl₂(s) ⇄ Pb²⁺(aq) + 2 Cl^-(aq);