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. Chemical Bonds: Ability to do work (push things) 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 Decreases -∆H ENERGY System

Heat Transfer: Endothermic Process System gains energy from its surroundings. (surroundings get colder) Surroundings ENERGY System Enthalpy Increases +∆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 It takes about 4000 J to heat 1 kg of water by 1oC. (4 J for 1 g) 4.184 J = 1 cal

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 = m•C•∆T ∆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 How much energy is needed to boil 19.75 g of ethanol (CH3CH2OH)? 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 ∆KE = molecules speed up ∆ PE = molecules pulled apart when melting ∆ KE = molecules speed up Imagine heating an ice cube. What energy changes take place as it is continually heated? heating or cooling Q = m · ∆T · C ∆KE = molecules speed up

Objectives Understand the concept of a standard heat of formation. Be able to calculate the heat of reaction using Hess’s 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

Q = S(n·∆Hf0)products - S(n·∆Hf0)reactants Hess’s Law heat of reaction (Q): the change in enthalpy (energy lost or gained) in a chemical reaction Use Hess’s Law to calculate the heat of reaction: Q = S(n·∆Hf0)products - S(n·∆Hf0)reactants