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Energies

Energies. Chapter 5 Tiers 1,2 & 3. TIER 1:REMEMBER DEFINE: Energy Joule Temperature Heat Calorimetry Calorimeter Specific heat capacity Endothermic reactions Exothermic reactions Standard enthalpy change of reaction Average bond enthalpy. ENERGY:

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Energies

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  1. Energies Chapter 5 Tiers 1,2 & 3

  2. TIER 1:REMEMBER DEFINE: • Energy • Joule • Temperature • Heat • Calorimetry • Calorimeter • Specific heat capacity • Endothermic reactions • Exothermic reactions • Standard enthalpy change of reaction • Average bond enthalpy

  3. ENERGY: • is the measure of the ability to do work. • it comes in many forms • Heat • Light • Sound • Electricity • Chemical energy JOULE is the unit for energy. It is defined as the heat required to raise the temperature of 1 g of water by 1oC

  4. TEMPERATURE and HEAT HEAT is the measure of the total energy in a given amount of a substance TEMPERTURE is the measure of the average kinetic energy of a substance

  5. CALORIMETRY Calorimetry is the quantitative measurement of the heat required or evolved during a chemical process. A calorimeter is an instrument for measuring the heat of a reaction during a well defined process. The following diagram depicts a constant volume or 'bomb' calorimeter (left) and a much simpler, but less accurate “coffee cup” calorimeter (right) often use in the laboratory.

  6. SPECIFIC HEAT CAPACITY • The specific heat capacity of a substance (liquid or solid) • is defined as the amount of heat needed to raise the unit mass of a substance by a single degree of temperature. • it is generally symbolized by displaying a 'C' or a 'c' depending upon how the heat is measured. • the common unit is J g-1oC-1 which is Joule per gram per degree Celsius

  7. CHEMICAL ENERGY • CHEMICAL ENERGY is energy that is released or absorbed during a chemical reaction. • ENDOTHERMIC REACTIONS • involve reactions where the energy is absorbed • the temperature of the reaction decreases • a cold pack used for sports injuries use endothermic reactions • EXOTHERMIC REACTIONS • involve reactions where the energy is released. • the temperature of the reaction increases • examples include combustion and neutralization reactions • a heat pack used for sports injuries or to keep warm use exothermic reactions

  8. STANDARD ENTHALPY CHANGE OF REACTION • Is a change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. The symbol is ΔHƟ and the unit is kJ dm-3 • STANDARD CONDITIONS FOR ENTHALPY CHANGES ARE: • A temperature of 298K or 25oC • A pressure of 101.3 kPa (1 atm) • Concentrations of 1 mol dm-3 for all solutions • All the substances in their standard states

  9. AVERAGE BOND ENTHALPY • Average bond enthalpy: • also called bond energy • is the energy needed to break a chemical bond. • the unit is also kJ dm-3 measured at 298K or 25oC. • enthalpy changes can also be calculated directly from bond enthalpies • The limitation to using average bond enthalpies is that it can only be used if all the reactants and products are in the gaseous state. • If water were a liquid product there would be even more heat evolved since the enthalpy change of vaporization would also need to be considered • The values have been obtained by considering a number of similar compounds. In practice, the energy of a particular bond would be slightly different under different circumstances due to the affects of neighboring atoms

  10. TIER 2:UNDERSTAND • Distinguish between heat and temperature • Distinguish between the system and the surroundings in a chemical reaction • State that combustion and most neutralization reactions are exothermic processes

  11. TEMPERATURE and HEAT Since heat is the measure of the total energy in a given amount of a substance whereas temperature is the measure of the average kinetic energy of a substance, heat depends on the amount of a substance but temperature does not. Sometimes heat is calculated by the equation: q = cmΔt where q is heat, c is specific heat capacity, m is mass & Δt is change is temperature. q is in Joules, c is in J g-1 oC-1, m is in g and Δt is in oC

  12. SYSTEM VS SURROUNDING In chemistry, the system is referred to as the area of interest which is most times the actual reaction and the surroundings is considered everything else. system system surrounding surrounding EXOTHERMIC Energy is given off by the system to the surroundings ENDOTHERMIC Energy is absorbed by the system from the surroundings

  13. EXAMPLES OF EXOTHERMIC REACTIONS • Combustion reactions of fuels • Adding concentrated acid to water {neutralization} • Burning of a substance • Adding water to anhydrous • Copper(II) sulfate • The thermitereaction • Reactions between lime and aluminum • Corrosion of metals • Respiration • Decomposition of vegetable matter into compost An exothermic thermitereaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.

  14. NEUTRALIZATION REACTIONS Often, neutralization reactions are exothemic(the enthalpy of neutralization). For example, the reaction of sodium hydroxide and hydrochloric acid or adding concentrated acid to water {neutralization} However, forms of endothermic neutralization do exist, such as the reaction between sodium bicarbonate(baking soda) and acetic acid (vinegar) Neutralization of sodium hydroxide (analyte) with hydrochloric acid (titrant) using phenolphthalein as the indicator

  15. TIER 3:APPLY • Apply the relationship between temperature change, enthalpy changes and the classification of a reaction as endothermic or exothermic. • Explain in terms of average bond enthalpies, why some reactions are exothermic and others are endothermic

  16. EXOTHERMIC VS ENDOTHERMIC REACTIONS • EXOTHERMIC • if the bonds in the products are stronger than the bonds in the reactants, heat is given off to the surroundings • the enthalpy change for the products are less than that of the reactants • ΔH is negative • ENDOTHERMIC • if the bonds in the reactants are stronger than the bonds in the products, heat is absorbed by the system • the enthalpy change for the products are greater than that of the reactants • ΔH is positive

  17. EXOTHERMIC VS ENDOTHERMIC REACTIONS The quantity and the sign of ΔH can be derived from the equation: ΔH = ΔH(products) + ΔH(reactants ) EXOTHERMIC REACTIONS: ΔH(products) < ΔH(reactants ) THEREFORE ΔH IS NEGATIVE ENDOTHERMIC REACTIONS: ΔH(products) >ΔH(reactants ) THEREFORE ΔH IS POSITIVE

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