Energy Changes in Chemical and Physical Processes 1

Energy The ability to –do WORK (an object is pushed or pulled for a distance) –CAUSE CHANGE –MOVE MATTER Involved in every natural process

Two general categories Kinetic –Energy of motion –Depends on mass and velocity of object KE=1/2 mv 2 Potential –Stored energy due to position or condition

Different forms Mechanical –Associated with the motion or position of an object Thermal –Total energy (kinetic and potential) of all the particles in an object Chemical –Potential energy related to ability to bond chemically Electrical –Produced by moving electric charges Electromagnetic –Produced by the motion of charged particles within atoms Nuclear –Related to the forces within the nucleus of atoms

Conservation of Energy and Mass Energy conversions –One form of energy can be converted to any other form Law of conservation of energy –Energy cannot be created or destroyed; the total amount of energy is the same before and after every process –This is also known as the First Law of Thermodynamics Law of conservation of mass –The total amount of mass is the same before and after every process

Temperature Kinetic Theory –Matter is made up of particles that are always in motion (have kinetic energy) Temperature –A measure of the average kinetic energy of the particles in substance –Changes when particles gain or lose energy –3 common temperature scales (F, C, K)

Temperature scales Kelvin (K) –SI unit –Directly proportional to Kinetic energy of particles Example- If you double the Kelvin temperature, you have doubled the kinetic energy of the particles –Absolute zero 0 K, the temperature at which the kinetic energy of particles is zero and molecular motion ceases –At normal atmospheric pressure, water freezes/melts at 273K and boils/condenses at 373K

Celsius –Commonly used in lab –At normal atmospheric pressure, water freezes/melts at 0C and boils/condenses at 100C Fahrenheit –Commonly used in everyday situations –At normal atmospheric pressure, water freezes/melts at 32F and boils/condenses at 212F

We can convert between temperature scales K=C or C= K – 273 C=5/9 (F-32) or F= 9/5 C + 32

Temperature and thermal energy Thermal energy –total energy of all the particles in an object –Found by adding the energy of each particle –Depends on the total number of particles in the sample Temperature –Measure of the average kinetic energy –does not depend on the number of particles in the sample Example- 100mL of boiling water has the same temperature as 1000mL of boiling water, but the 1000mL of boiling water would have more thermal energy

Temperature and heat Heat (q) –The movement of thermal energy from a substance at a higher temperature to a substance at a lower temperature –Has to do with transfer of energy –The “flow” of energy from one substance or object to another

System vs Surroundings In studying energy changes, we define a system as the part of the universe on which we focus our attention to study. The surroundings include everything else in the universe.

A few more terms… Endothermic –A process that requires energy –energy is absorbed or gained by the system –Energy is lost by the surroundings –Usually evidenced by a decrease in temperature of the surroundings Exothermic –A process that releases energy –Energy is given off or emitted by the system –Energy is gained by the surroundings –Usually evidenced by an increase in the temperature of the surroundings

Heat flow is defined from the point of view of the system. – In an endothermic process, heat flows into the system from the surroundings q has a positive value for the system q has a negative value for the surroundings –In an exothermic process, heat flows from the system to the surroundings. q has a negative value for the system q has a positive value for the surroundings

Energy is conserved! q sys = -q surr

Potential Energy Diagram Endothermic Reaction a) Activation Energy for forward reaction b) Activation energy for reverse reaction c) ΔH rxn (enthalpy or energy change for the reaction) Note that for an endothermic rxn, ΔH is always positive

Potential Energy Diagram Exothermic Reaction Note that for an exothermic reaction, ΔH is always negative. (reactants have more potential energy than the products); energy is released into the surroundings All reactions, even exothermic reactions, require some initial addition of energy. This energy is required to reach the unstable, high energy state known as the activated complex.

Units for Measuring Heat Flow Heat flow is measured in two common units, the calorie and the joule. 1 cal = 4.184J The energy in food is usually expressed in Calories. 17.1

Heat Capacity and Specific Heat The heat capacity of an object depends on both its mass and its chemical composition. –The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object. 17.1

The specific heat capacity, or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C. 17.1

Calorimetry Calorimetry is the precise measurement of the heat flow into or out of a system for chemical and physical processes. 17.2

In calorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings. The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system. 17.2

The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter In a simple constant-pressure calorimeter, a thermometer records the temperature change as chemicals react in water. The substances reacting in solution constitute the system. The water constitutes the surroundings.