Energy Changes in Chemical Reactions

Chemistry 12AP

Thermochemistry

ThermodynamicsThe study of energy and its interconversions.

Types of EnergyEnergy – the capacity to do workWork: energy used to cause an object with

mass to move against a force.

Types:Radiant energyThermal energyChemical energyPotential energy

Law of Conservation of Energy

Energy can be converted from one form to another.

Energy Changes in Chemical Reactions Almost all chemical reactions absorb or

produce (release) energy.Heat is the transfer of thermal energy

between two bodies that are at different temperatures. (sometimes referred to as heat flow)

Temperature is the average kinetic energy of the molecules.

Electrostatic Potential Energy

k – constant of proportionality, 8.99 x 109Jm/C2

When dealing with molecular-level objects, the electrical charges on Q1 and Q2 are typically on the order of magnitude of the charge of the electron. When Q1 and Q2 have the same sign (i.e. both positive or both negative) the two charges repel one another, pushing them apart; E would be positive. If they have opposite charges, then they would attract one another, making the E negative.

The lower the energy (E) of a system, the more stable it is. Thus, the more strongly opposite charges interact, the more stable the system.

First Law of ThermodynamicsThe system is the specific part of the

universe that we are “studying”. The surroundings are the rest of the universe outside the system.

system + surroundings = universe∆E = Ef – Ei

∆Esys + ∆Esurr = 0

Or ∆Esys = -∆Esurr

Examples of SystemsOpen IsolatedClosed

heat heat

Water vapor

More on EnergyTotal change in energy (∆E)

aka. First Law of Thermodynamics

∆E = q + w (heat + work)

q = heat absorbed by/lost from the system

w = work done on/by a system on its surroundings

Work and HeatSign Conventions for Work and Heat

For q

+ means system gains heat

- Means system loses heat

For w

+ means work done on system

- Means work done by the system

For ∆E

+ means net gain of energy by system

- Means net loss of energy by system

Example ProblemExample 1. There is a flow of 31.2kJ of heat

to a system. At the same time, the system does 2.5kJ of work on the surroundings. Calculate the change in energy.

A reaction is exothermic if it gives off heatenergy flows from the system to the

surroundings (q is negative)

A reaction is endothermic if it absorbs/takes in heat

energy flows from the surroundings into the system

(q is positive)

Calorimetry• Is the laboratory technique used to

measure the heat released or absorbed during a chemical or physical change