Energy changes in chemical & nuclear reactions

Energy

• Defined as the ability to do “work", which is to move an object against an opposing force

• Can be classified as Kinetic or potential

• It comes in many forms such as heat, electricity, light, sound and chemical energy

Heat

• Energy transfer that occurs as a result of a temperature difference

• Produces an increase in disorder in how the particles behave

• increases the average kinetic energy(temperature) of the molecules

Chemistry Universe

• Made up of two parts:Chemical system and surroundings

• Chemical system- area of interest/ made up of reactants and products which is classified as open or closed

• Surroundings is everything else in the system

Open vs Closed System

Open- can exchange matter and energy with its surroundings

closed- can only exchange energy and matter cannot go in and out e.g glow sticks

Law of conservation of Energy

• Energy cannot be created or destroyed but only converted from one form to another

• So energy can be exchanged between system and surroundings, but total energy has to stay the same during the process

• Energy from the system is gained by surroundings and vice versa

• e.g BBQ(Chemical to thermal and light)

Enthalpy

Total Internal Heat content content at constant pressure( sum of kinetic and potential energy)

Examples of energies of enthalpy

This includes the energies of:

• moving electrons within atoms

• vibration of atoms connected by chemical bonds

• rotation and translation of molecules

• nuclear potential energy of protons and neutrons in atomic nuclei

• electronic potential energy of atoms connected by chemical bonds

IT IS IMPOSSIBLE TO MEASURE ENTHALPY OF A CHEMICAL SYSTEM

Enthalpy Change (△H)

• CAN be measured in kJ/mol

• measure of the amount of heat energy contained in a substance/system.

• result from chemical bonds and intermolecular forces in the system being broken and then formed

• during reactions, the difference in enthalpy (△H) between reactants and products results in heat change that can be measured

Endothermic Reaction

• net absorption of energy from surroundings (energy absorbed to break bonds is GREATER than energy released when products form)

• △H is positive

• temperature of surroundings decreases

• Products have a higher enthalpy than reactants

• products are less stable than reactants

Exothermic Reaction

• net release of energy to surroundings (energy absorbed to break bonds is LESS than energy released when products form)

• △H is negative

• temperature of surroundings increases

What is the heat change calculation formula and what do each variable mean

increases in temperature when an object is heated is

dependent on three things:

1. mass of the object (g)

2. the heat added (K)

3. nature of the substance/specific heat capacity (heat needed to increase the temperature of a unit mass by 1 K. Depends on the number of particles present in a sample. Units: J/gK)

q= mc△T

Relating System and surroundings

when a change occurs in a system (△H), the chemical potential energy change is NUMERICALLY EQUAL to the heat (q) transferred to the surroundings.

Mathematically, this can be shown by:

ΔHsystem = -q surroundings

Standard Enthalpy Changes (△Ho)

the enthalpy changes for a reaction depends on the conditions under which the reaction occurs

Standard enthalpy changes are measured under the following conditions:

pressure= 100 kPa

concentration= 1.0 moldm-3 for all solutions

all substances in their standard states (pure

form of the substance at 298 K and 100 kPa)

REPRESENTING ENTHALPY CHANGE

• can use a thermochemical equation (a balanced chemical equation which incorporates the energy change for the system)

potential energy diagram

• shows the relative change in energy of reactants/products

molar enthalpy change

• energy change that occurs when 1 mol of a substance undergoes a physical, chemical, or nuclear change

• represented by ∆Hr

• units are J/mol

• *the molecule you are getting the ΔHr MUST have a coefficient of 1

Calorimetry

• process of measuring energy changes during a chemical or physical change

• use a device called a calorimeter

• we assume the calorimeter is a closed system

• surroundings is water