Energy changes and rates of reactions (Chapter 5)

Thermochemistry

the study heat change in chemical reactions

potential energy

stored energy that results from the position or shape of an object

kinetic energy

the energy an object has due to its motion

thermal energy

The total amount of potential and kinetic energy (how fast particles inside an atom are moving)

chemical energy

Energy stored in chemical bonds

nuclear energy

Energy stored in the nucleus (protons and neutrons) of an atom

electric energy

energy associated with the flow of electrons

heat

The transfer of thermal energy between objects that are at different temperatures

Temperature

measurement of thermal energy

Law of Conservation of Energy

Energy cannot be created or destroyed

system

the specific part of the universe that contains the reaction or process you wish to study

open system

A system in which matter can enter from or escape to the surroundings.

closed system

A system in which no matter is allowed to enter or leave but energy can be transferred

isolated system

A system that can exchange neither energy nor matter with its surroundings.

What type of energy does nuclear energy release?

Always exothermic

What are the two types of nuclear reactions

fission and fusion

What is nuclear fusion?

nuclei of small atomic mass combine to form a larger molecule

What is nuclear fission?

large nuclei with high atomic mass combine to form a larger molecule

Calorimetry

The precise measurement of heat flow into or out of a system for chemical and physical processes

energy released by a system =

energy absorbed by surrounding

laws of thermodynamics

First law: the total energy of the universe is constant
Second law: When two objects in thermal contact, transfer happens from the hot object to the cooler one until they are the same temperature

Calorimeter

an insulated device used to measure the absorption or release of heat in chemical or physical processes

Calorimetry equation

q=mcΔT

Calorimetry assumptions

• no loss of energy or matter between calorimeter and outside (closed system)
• any heat released or absorbed by the calorimeter is negligible
• dilute aqueous solution have the same density and (c) of water

What is the density of water:

1 g/mL

Specific heat capacity of water

4.184 J/gC

Relationship between q (surrounding) and q (system)

q (system) = - q surrounding

Enthalpy (H)

total amount of energy in a substance

Enthalpy change

the amount of energy absorbed or lost by a system as heat during a process at constant pressure

thermochemical equation

an equation that includes the quantity of energy released or absorbed as heat during the reaction

Endothermic

• positive enthalpy change
• absorbs heat

Exothermic

• negative enthalpy change
• releases hear

ways of showing energy change

1. including it as a term in chemical equation
2. stating the enthalpy change
3. stating molar enthalpy of a specific reaction
4. drawing a potential energy diagram

Enthalpy change formula

∆H = n∆Hr°

difference between delta H and delta H nought

∆H - change in enthalpy with no given conditions
∆H° - change in enthalpy at standard state

Average bond energy / bond dissociation energy

amount of energy required to break 1 mole of bonds in the gaseous state

breaking bonds of reactants is

endothermic ( takes energy)

making bonds as a product

exothermic (releases energy)

Factors that affect bond length

1. shorter - requires more energy to break
2. multiple bonds - require more energy to break
3. bond polarity - greater attraction = stronger bond + higher bond energy

Hess' Law of Heat Summation

When reactants are converted to products, the change in enthalpy is the same, no matter how many steps it takes

Steps to solve Hess' law problems

1. rearrange to produce overall equation
2. add reactions and enthalpy terms

Standard enthalpy of formation (∆Hfº)

enthalpy change that results in 1 mol of a compound, formed at SATP

STAP

Standard Ambient Temperature and Pressure

Standard Ambient Temperature and Pressure

Pressure = 100kPa
Temperature = 25°C

How to calculate molar heat capacity

Molar c = c (M)

As the number of bonds increases, the bond length _

shortens