ThermoChemistry - blogs.polson.k12.mt.us

ThermoChemistry - blogs.polson.k12.mt.us

hermochemistry Thermochemistry: The study of energy during physical and chemical changes. Objectives: Be able to define and correctly use energy-related terminology. Identify and understand endothermic and exothermic

processes. Thermochemistry: The study of energy during physical and chemical changes. Energy: Ability to do work (push things) Chemical Bonds: Store energy

Chemical potential energy Heat and Thermal Energy: Heat: represented by (q) Thermal energy that flows from warmer to cooler areas. Thermal energy: total KE of particles in a substance(depends on #g and

temp.) Enthalpy: (H) The total amount of energy stored in a system at constant pressure. (KE+PE) Heat transfer: conduction colliding particles

radiation electromagnetic waves convection currents Heat Transfer: Exothermic Process

System releases energy and the surroundings tend to get warmer. Surroundings Enthalpy Decreas es -H EN GY R E System

Heat Transfer: Endothermic Process System gains energy from its surroundings. (surroundings get colder) Y G R E

EN System Surroundin gs Enthalp y Increase s +H Objectives

Be able to define and correctly use the common units of thermal energy. Be able to define and understand the concept of specific heat. Be able to make calculations related to thermal energy and temperature changes. Calorimetry

Precise calculation for measuring heat transfer Heat flow is measured in two common units, the calorie (cal) and joule (J). 1 J= 0.2390 cal 1 kg of water by 1oC. (4 J for 1 g) 4.184 J = 1 cal It takes about 4000 J to heat

Specific Heat: heat capacity the amount of energy required to raise 1.00 g of a substance by 1.00oC. Molecules such as water, have a high specific heat (C). Metals have relatively low C, change temperature (T) quickly.

Energy and Temperature Temperature is a property of matter whereas heat is transferred energy from one object to another. Temperature changes involve KE. Q = mCT T= Tf-Ti Q = enthalpy (#J) +Q = enthalpy increase = endothermic

-Q = enthalpy decrease = exothermic Problems Calorimetry Example: What is the change in enthalpy when a cup of water (227 g) cools from boiling to room temperature (97oC to 22oC)? Example: A wedding ring absorbs 16.4 J of energy when it is placed on a finger (the temperature rises from 21oC to 38oC). If the mass of the ring is 4.80 g, what is the specific heat of the metal?

Objectives Understand the concept of latent heat and how it corresponds to potential energy. Be able to make latent heat calculations. Latent Heat: Changes in state involve changes in Potential Energy.

This stored energy is called latent heat or the quantity of heat absorbed or released by a substance undergoing change of state. KE (temperature) is constant during a phase change. Draw diagram. Latent Heat Values: Size of value depends on the strength of intermolecular bonds Flathead Cherries Cherries are

sprayed with water to protect them from freezing, Why? Latent Heat Calculations Temperature remains constant, so we use: Q = (m/M) H o How much energy is needed to boil 19.75 g of ethanol (CH3CH2OH)?

o How much water (at 0oC) is freezing if 2.5 kJ of energy is released? Objectives Be able to draw a heating curve or cooling curve for a substance. Be able to correctly label the regions where KE and PE are occurring on a heating or cooling

curve. Heating and Cooling Curves PE = molecules pulled apart when boiling changes of state Q = m/M H PE = molecules pulled apart when

melting KE = molecules speed up heating or cooling Q = m T C KE = molecules speed up KE = molecules speed up Objectives

Understand the concept of a standard heat of formation. Be able to calculate the heat of reaction using Hesss Law and determine if a reaction is endothermic or exothermic. Standard Heat of Formation standard heat of formation (Hf0): change in enthalpy that accompanies the formation of one mole of a compound from its elements at 25oC and 101.3kPa.

Hf0 for any uncombined element in its normal state = 0 kJ/mol Hesss Law heat of reaction (Q): the change in enthalpy (energy lost or gained) in a chemical reaction Use Hesss Law to calculate the heat of reaction:

Q = S(nHf0)products - S(nHf0)reactants

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