ENERGY AND ENERGY CHANGE

ENERGY AND ENERGY CHANGE

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Energy and Energy Changes
Chemical changes occur on the molecular level. A chemical change results in the formation of a new substance.

 

Examples of Chemical Changes include:

• Combustion or burning e.g. burning wood

• Dissolution of salt in water

• combination of acid and base

• digestion of food

• cooking of an egg,

• rusting of an iron or metal object

• Combination of hydrochloric acid and sodium hydroxide to produce salt and water.

Physical Changes
Physical changes deals with energy and states of matter. A physical change unlike the chemical change does not lead to the formation of a fresh substance. Changes in state such as melting, freezing, vaporization, condensation, sublimation are all physical changes.

Examples of physical changes include:

• crumpling a sheet of paper

• melting an ice cube

• casting silver in a mold

• breaking a bottle

• Crushing a can.

How to know a Chemical Change and a physical change

A chemical change results to the formation of a substance which was not there previously. You may be able to ascertain a chemical change through some indicators like light, heat, color alteration, gas formation, odor, or sound. The reactant and the product of a physical change are the same, although they may appear to be variable.

A physical change may have occurred if the changes that are associated with a chemical change are not found. It can be hard to tell this in some reactions such as when sugar is dissolved in water. In this case the content is still the same chemically although the sugar has dissolved. The sugar is now present in the mixture as molecules of sucrose .Nevertheless, when you dissolve salt in water, the salt dissociates into its ions of Na+ and Cl- resulting to a chemical change. In the two scenarios, a white solid (salt) is dissolved into a clear liquid and in the two scenarios the reactant can be recovered by taking away the water. Irrespective of this, the two reactions are different.

Endothermic and Exothermic Reactions
Scores of chemical reactions discharge energy in the form of heat, light, or sound. These types of reactions are termed exothermic reactions. Exothermic reactions may occur instinctively and lead to an increased randomness or entropy (ΔS > 0) of the reacting system. They are designated by a negative heat flow meaning that heat is expelled to the surroundings and decrease in enthalpy (ΔH < 0). Exothermic reactions generate heat and may be explosive when performed in the lab.

The second group of chemical reaction rather than give out heat to the surroundings absorbs heat from the surrounding in order to occur. These types of chemical reactions are referred to as endothermic reactions. Endothermic reactions are not spontaneous reactions. Work must be performed to be able to cause the reaction to take place. When endothermic reactions take up energy, a drop in temperature is calculated and noted on the course of the reaction. Endothermic reactions are denoted by positive heat flow and a rise in enthalpy (+ΔH).

Examples of Endothermic and Exothermic Processes
Photosynthesis is one example of an endothermic chemical reaction. During photosynthesis, plants make use of the energy obtained from the sun in the conversion of carbon dioxide and water into glucose and oxygen. The process of photosynthesis takes up 15MJ of energy (sunlight) in order to produce one kilogram of glucose as exemplified in the equation below:

Sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

One example of an exothermic reaction is the reaction between sodium and chlorine to form table salt. The formation of the table salt reaction gives off 411 kJ of energy for one mole of salt formed as exemplified below:

Na(s) + 0.5Cl2(s) = NaCl(s)

Exothermic- The term exothermic explains the process that gives out energy in the form of heat.

Formation of a chemical bond usually leads to the release of an energy to the surrounding and can therefore be termed an exothermic process. Exothermic reactions frequently feel hot due to the fact that it is releasing energy to the surrounding.

Endothermic – is used to denote a process or reaction that absorbs energy in the form of heat before it could occur.

Breaking a chemical bond needs energy and is consequently regarded as Endothermic. Endothermic reactions frequently feel cold because they absorb heat from the surrounding.

Examples of exothermic Processes Examples of endothermic Processes

• freezing of water

• solidification of solid salts

• condensation of water vapor

• formation of a hydrate from an anhydrous salt

• formation of an anion from an atom in gaseous state

• Total destruction of matter E=mc2

• division of an atom

• melting of ice cubes

• melting of solid salts

• evaporation of liquid water

• production of an anhydrous salt from a hydrate

• producing a cation from an atom in the gaseous state

• breaking up of a gas molecule

• separation of ion pairs

• boiling of an egg

• baking of bread

Examples of Exothermic Reactions Examples of Endothermic Reactions

• burning of hydrogen

• liquefaction of lithium chloride in water

• combustion of propane

• drying out the moisture of sugar with sulfuric acid

• thermite

• disintegration of hydrogen peroxide

• disintegration of ammonium dichromate

• halogenation of acetylene

• Reaction of barium hydroxide octahydrate crystals with dry ammonium chloride

• melting ammonium chloride in water

• reacting thionyl chloride (SOCl2) with cobalt(II) sulfate heptahydrate

• mixture of water and ammonium nitrate

• mixture of water and potassium chloride

• reaction between ethanoic acid and sodium carbonate

• Photosynthesis (chlorophyll is used in the reaction of carbon dioxide, water and energy to produce glucose and oxygen.

Energy-level profile diagrams
An energy diagram can be employed to demonstrate the energy movements in these reactions and temperature can be made use of to evaluate them visibly.

Every chemical process is characterized by changes in the energy. Before a reaction could occur it can either release or absorb energy. Exothermic reactions usually occur spontaneously and make their surroundings to heat up. That is the entropy or disorder of the environment is increased.

The energy state of any chemical reaction can be denoted by Gibbs free energy.

The Enthalpy
Enthalpy is described in thermodynamics as an evaluation of the heat content of a chemical or physical system. Enthalpy (H) is an estimation of the total energy of a system and regularly denotes and demonstrates in a simpler way energy transfer between the reacting systems. A positive change in enthalpy denotes an endothermic reaction for the reason that energy is absorbed. A negative change in enthalpy denotes an exothermic reaction for the fact that the system has lost some energy to the environment.

The Entropy
A thermodynamic property that is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work.

The Free energy
The free energy of a reacting system is the difference between the internal energy of a system and the product of its entropy and absolute temperature.

Gibbs free energy
The difference between the enthalpy of a system and the product of its entropy plus absolute temperature is referred to as Gibbs free energy. It is a determination of the useful work obtained from a thermodynamic system at constant temperature and pressure.

chemistry
Heat
Heat is defined as the energy transferred from one system to another by thermal interaction.

Law of conservation of energy:

The law of the conservation of energy states that energy can neither be destroyed nor created but can be altered from one form to another. The total energy of a reacting system and the surroundings remains constant or unchanged.

Change in Enthalpy is the expression that is used to explain the energy swap over that occurs with the surroundings at a constant pressure. It is denoted with the symbol ΔH.

Enthalpy is the total energy content of the reactants. It is denoted with the symbol, H.

ΔH = ΔH products - ΔH reactants

The units of enthalpy are kilojoules per mole (kJmol-1)

An exothermic enthalpy change is constantly represented with a negative value, due to the fact that energy is expelled to the surroundings.

ΔH = -xkJmol-1

An endothermic enthalpy change is constantly represented with a positive value, since the energy is absorbed by the system from the surroundings.

ΔH = + ykJmol-1.

Standard enthalpy changes: standard conditions

When we want to place the enthalpy changes of different types of reactions side by side each other for comparison, we must make use of standard conditions like known temperatures, pressures, amounts and concentrations of reactants or products.

The standard conditions are listed below:

• A pressure of 100 kilopascals (102kPa)

• A temperature of 298K (25oC )

• Reactants and products in physical states, typical for conditions above.

• A concentration of 1.0mol dm-3 for solutions.

The o sign is used to denote a standard condition.

Standard enthalpy change of reaction

ΔHor

The standard enthalpy change of reaction is the enthalpy change of that reaction when the amounts of reactants shown in the equation for the reaction, react under standard conditions to form the products in their standard states.

Standard enthalpy change of formation

ΔHof

The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions. This means that both compound and elements are in their standard states.

The Standard enthalpy change of combustion

ΔHoc

The standard enthalpy change of combustion is the enthalpy change when one mole of an element or compound is completely reacted with oxygen under standard conditions.

The Energy Content of Fuels

Energy content is a significant property of both food and matter that is made use of during the process of heating. The energy that our body utilizes for day to day activities like sleep, walk, talk etc comes from the food that we eat, for instance the candy bar. The energy that is generated a fuel is burned is a crucial quantity, and we'd like to be capable of measuring the effectiveness of fuel.

The energy content is the amount of heat produced by the combustion of 1 gram of a substance and is measured in joules per gram (J/g). Heat is a form of energy or in reality a flow of energy flow and it is usually calculated in calories.1 cal = 4.186 J.

Every now and then it is tricky to accurately measure the amount of heat that is produced by a substance. The measurement is made easier by burning a particular quantity of the fuel to heat up water. The energy expelled by the fuel can then be estimated by calculating the heat absorbed by the water as calculated by the change in temperature of the water. Heat gained by water can be denoted by

Q = change T * m * c

whereT is the temperature change, m is the mass, and c is the specific heat capacity constant of water.

A brief summary of energy flow through the body

Food that we ingest contains energy. The maximum amount of energy contained in the food we eat is a measure of the heat that is released after a total combustion of the food to carbon dioxide (CO2) and water in a bomb calorimeter. This energy is termed ingested energy (IE) or gross energy (GE).