Eletricalchemical Changes

Exchange of Electrons.

Two terms: Oxidation and Reduction. 

Oxidation: - Any reaction with oxygen;

-Loss of Electron.

Reduction: - Any reaction that produced metal from ores;

- Gain of Electrons. 

REDOX reaction: There is gaining and losing of electrons. 

Oxidizing agent (OA) is defined as the one entity that will donate electrons, therefore promoting oxidation on the other entity.

Reduction agent (RA) is defined as the one entity that will receive that electron, therefore promoting the other entity. 

DOUBLE REPLACEMENT, OR ACID AND BASE REACTIONS ARE REDOX

Thermal Stability

Thermal stability is the tendency of a compound to resist decomposition when heated. Which means that a substance that has the greates decomposition molar enthapy is the one with greates thermal stability. Remember that IF MORE NERGY IS RELEASE WHEN A COMPOUND IS FORMED, THAN IT TAKES MORE ENERGY TO DECOMPOSE IT.

Hess' Law

When you leave you house to go the gym, you may take root 1, or maybe you can take root 2. It does not matter how you get to the gym, as long as you do. Same applies to chemical reaction and enthalpy.

For hess' law, you must know to things:

- Whenever you switch products with reactants and vice versa, you must change your enthalpy sign and well.

-If you multiply the coefficient of the equation, you multiply the enthalpy change value as well. 

Hess' law modified 

Hess determined that you can actually find the enthalpy change of a reaction, by using the molar enthalpy of the components of the reaction.

When using the method, remember that: 

- A balanced equation is necessary;

- Molar enthalpy of formation of elements are equal to 0

- Water vapour is always produces, unless we are talking about photosynthesis, cellular respiration and closed system combustion. Which in the case, liquid water will be formed. 

r Ho

 =        

f Ho

pr Σoducts  ­        

f Ho Σ reactant

Calorimetry

Calorimetry is a way scientific expriment to figure the energy released or absorbed by a reaction. In calaorimetry, we use a calorimeter, that is composed by a system (chemical reaction) and surroundings (water or something like water).  When the reaction measures a made. and the asusmpition the THERMAL ENERGY CHANGE IS EQUAL TO THE CHANGE IN ENTHALPY, SINCE THE ENERGY LOST OR GAINED BY THE SYSTEM IS EQUAL TO THE ENERGY GAINED OR LOST BY THE SURROUNDINGS.
Don;t forget that when the question gives you the mass of the calorimeter, and its a metal. You must account it as one of the surroundings, which means that not only water is present.

Heat Transfer and Calorimetry

Kinetic energy and Potential energy are actually related! They also have specific calculation for each one of them. 

Kinetic energy is related to temperature. Therefore, Ek is related to thermal changes. To figure out what is the thermal change in a substance, consequently Ek.

Q = mc^t

Q means thermal change

M means mass

c mens specific heat capacity

^t means the change in temperature.

When we are talking about potential energy, we talk about enthalpy. Enthalpy is the total energy of a substance so Ek and Ep. When we say that there is a change in enthalpy, we talk about the difference between the Ep of the products and reactants.  REMEMBER THAT CHANGE IN ENTHALPY DOES NOT HAVE ANYTHING TO DO WITH CHANGE IN TEMPERATURE. 

Molar change in enthalpy - Change in the Ep per mole of a substance.

To figure the Enthalpy change, we always look at the calculalio ^^H= n^^Hm

^^^H means enthalpy change 

n means moles 

^^Hm means molar enthalpy change

The molar enthalpy of FORMATION will be listed on you r data booklet, The molar enthapy of DECOMPOSITION is the same of the formation, but with a reversed sign. For the other reaction, like combustion, it will be said in the question, or you will have to figured it out.  The molar enthalpy of elements is 0. 

Thermochemical Changes - Intro

Energy

The scientific definition of energy describes it as the ability to do work. Without energy, no reactions could occur, therefore, our lives would be impossible to continue. ALL ENERGY ORIGINATES FROM THE SUN! 

In this unit we will study the two types of energy that are extremely related to chemical changes and reactions: Kinetic and Potential Energy.

1) Kinetic:

It is an energy that is related to movement,  and in the case of chemistry, we will be talking about the movement of particles and molecules. Therefore, we will understand that when particles, or molecules, are moving faster, their kinetic energy is increasing. When moving slower, their kinetic energy will decrease. 

Kinetic Energy is very related to temperature. When a substance increases its temperature, it means that the particles are moving fast, therefore, we must have a large kinetic energy. The opposite occurs, when the temperature of a substance decreases.

As we know from previous studies, temperature gives us clues of heat. What is heat? Heat is the transfer of thermal energy between two substance or objects. 

2) Potential:

This energy is related to the bonds of molecules in a substance, when they are formed or broken down, potential energy is changed. In this units, we will focus on the potential energy in the intra molecular bonds, which are of course the bonds between atoms that form a molecule. When this bonds are broken down, or formed, that means that a chemical change, or we can say, a chemical reaction is occurring. 

There are two types of chemical reactions: Endothermic and Exothermic Reactions. 

Endothermic reactions are chemical reaction which energy is required in other to occur. The best example of endothermic reaction is PHOTOSYNTHESIS. Exothermic reaction in another hand is the chemical reaction that releases energy. The best example of exothermic reaction is CELLULAR RESPIRATION.