Energy & Chemical Reactions

A + B => C + heat, the potential energy of the products as compared with the potential energy of the reactants is

less and the reaction = exothermic

A piece of Mg(s) ribbon held in a Bunsen burner flame begins to burn according to the equation 2Mg(s) + O2 (g) => 2MgO (s). The rxn begins bc the reactants ... 1) are activated by heat from the Burden burner flame 2)are activated by heat from the burning Mg 3) underwent an ^ in entropy 4) underwent a decrease in entropy

1

Which change is exothermic? 1) freezing of water 2) melting of iron 3) vaporization of ethanol 4) sublimation of Iodine

1, less potential energy in solid water than liquid?

In a reversible rxn, the diff. bet. the activation energy of the forward rxn & the activation energy of the reverse rxn= .... 1) activated complex 2) heat of reaction 3) potential energy of reactants 4) potential energy of products

2) heat of reaction

A + B => AB + 50kJ . If an activation energy of 5 kJ is required for the forward rxn, the activation energy of the reverse reaction is

55 kJ. (50 = released energy, or potential energy of products, + 5 = act. energy between reactants and products, together = bottom to top of reactants so act. energy)

Exothermic

Energy is transferred out of a system as a result of a temp. difference, reported as a - #, aka heat is released

How is the enthalpy of products and reactants, the sign of the change in H, the energy absorption/releasement, curve, and more for exothermic reactions?

Enthalpy of products < enthalpy of reactants & sign of H is -. Goes from a higher reactants to lower products. Same process of absorbing and losing energy, but more energy is released than absorbed, products= at lower state than reactants, difference bet. energy of products & energy of reactants = heat of reaction. Many exothermic reactions = self-sustaining bc heat they let out provides the activation energy for reactants.

How is the enthalpy of products and reactants, the sign of the change in H, the energy absorption/releasement, curve, and more for endothermic reactions?

Enthalpy of products > Enthalpy of reactants & change of H = +. Goes from a lower reactants to a higher (more energy) products. Energy is absorbed by reactants to break certain bonds and start reaction (activation energy). Since final forms are made, energy is released to create these new bonds. Less energy is released than absorbed, so products = at a higher energy state than reactants.

Hess' Law

G.H.Hess found in the 19th century that when >=2 chemical rxns are combined/added, heat of reaction of combined rxn = sum of indiv. heats of reactions, basically an application of the first law of thermodynamics, basically a statement of the law of conservation of energy applied to chemical rxns. When a compound is present in = amounts on opposite sides of the indiv. rxns, it is not present in the combined rxn.

Are heat and temp not the same? Why?

Heat energy is related to the total amount of matter present in a system, while temp. is not. Temp = kinetic energy of the system's particles.

Potential energy diagram

Illustrates the progress of a chemical reaction & rovides qualitative info abt energy changes within a reaction.

Thermal energy

Main component of internal energy; Energy associated with random molecular motions

Chemical energy

Main component of internal energy; energy associated with chemical bonds and attractions between the particles in a system

Activation energy/energy of activation

Minimum energy needed by reactants in order for a reaction to occur

Specific Heat Formula & Constant for Water

Q = mC (T2-T1), use when something along the lines of "how much heat is needed to raise the temp. of ____ g of liquid water from ___ to ___? Water constant for C: on Table B, K in denominator can be C too.

Boiling and Melting Heat Formulas

Q=mHv, Q=mHf; constants on Ref. Table B

How does the energy allow a reaction to occur spontaneously?

Systems tend to change from higher to lower energy states. Many chemical reactions release energy, with the result that the products have less energy than the reactants. There is a strong tendency for a reaction to occur spontaneously when it's exothermic, aka when it's -.

How does the disorder factor allow a reaction to occur spontaneously?

Systems tend to reach a state of higher disorder, measured in entropy, and represented by symbol S. When a system achieves greater disorder, its entropy increases.

How can a system reach a higher disorder?

Temp ^ = ^ of random motion of the particles; phase change closer to gas (most disorder); products of chemical reaction = simpler in structure than reactants; more products than reactants in reaction; substance= placed in solution

First Law of Thermodynamics

The change in internal energy of a system = sum total of the heat absorbed by system & work done on the system. Heat & work = + #s if absorbed, if heat = released or work is done by the system, the 2 quantities = - #'s.

Which 2 factors enable a reaction to occur spontaneously?

The energy factor (exo/endothermic) and the disorder factor (entropy).

Internal energy

Total energy within a system

Enthalpy

Transfer of energy. If heat absorbed by system at constant pressure, enthalpy ^ (>0), if heat = released by system at constant pressure, decrease (change of H < 0 ). Usually assumed chem. reactions occur under constant pressure.

What is change of H in degrees symbol mean?

When a heat of reaction is measured under standard thermodynamic conditions (pressure = 100 kPa, pure states of reactants & products).

When a substance was dissolved in H2O, the temo. of the H2O decreases. Process = ...

endothermic, w/ the absorption of energy

N2(g) + 3H2(g) => 2NH3 (g) + heat

exothermic, with a decrease in entropy

An endothermic reaction has products that have ____ potential energy than the reactants and a change in H that is ___

higher, +

Systems in nature tend to undergo changes toward

lower entropy & higher entropy

Exothermic always (absorb/release) heat

release

Heat of reaction

When applied to a chemical reaction, change of H (triangle G); the difference in enthalpy bet. reactants & products. (Potential energy of products - PE of reactants)

Endothermic

When energy is transferred into a system as a result of a temp. difference, reported as a + #; aka heat is absorbed

Heat

energy transferred bet. a system & its surroundings as a result of a temp. difference