Chemistry 12 - Energy Changes and Reaction Rate

Energy

a property matter describing the ability to do work.

Work

Is done when an object is moved through a distance by force acting on the object.

Kinetic Energy

Associated with the motion of an object.

Potential Energy

Stored energy due to an object’s position.

Internal Energy

Sum of the kinetic and potential energies of the particles in a substance.

1st Law of Thermodynamics

The law of conservation of energy. Energy can be transformed from one form to another but it cannot be created or destroyed.

Units of Energy

Joule (kg/m²/s²)

1 calorie =

4.184J

Heat energy (q)

is the actual energy transferred between substances.

Temperature

A measure of the average kinetic energy of the molecules in a substance.

Thermal energy

is the total of the kinetic energy of the molecules and the potential energy.

Systems

Substances in which a change (physical or chemical) occurs. (ex. reactants and products, represented by a chemical equation)

Surroundings

The rest of the universe!

Endothermic

process where heat is absorbed by the system from the surroundings.

Exothermic

process where heat is released by the system from the surroundings.

Open System

Both matter and energy can flow freely.

Closed System

Energy can flow freely but not matter.

Isolated System

Neither matter nor energy can flow freely (ideal but impossible).

Heat capacity

Amount of heat energy required to raise the temperature of a substance. (C)

Specific Heat Capacity

Amount of heat energy required to raise the temperature of 1g of a substance by 1℃ or 1K. (c)

Molar Heat Capacity

Amount of heat energy per mole required to raise the temperature of 6.02×10²³ molecules of a substance.

Factors which affect heat capacity

1. Mass

2. Temperature Change

3. Type of Substance

Mass (Heat Capacity Factor)

number of molecules⬆, heat required⬆

Temperature Change

temperature change ⬆, amount of heat required⬆

Type of Substance

each substance has a different density and a different ability to absorb heat.

Chemical systems

kinetic and potential energy

Kinetic Energies

Involved with the motion of particles

Potential Energies

Involved with particles’ positions within an attractive or repulsive force field.

Enthalpy Change (△H)

The difference in enthalpies of reactants and products during a change.

Exothermic Reactions

• Reactants = Less energy is required to break bonds.

• Products = less stored potential energy than the reactants.

Endothermic Reactions

• Reactants = more energy required to break bonds

• Products = more stored potential energy than the reactants had

▵H

“state dependent’ property. it is affected by temperature and pressure.

▵H°

indicates that the values is the “standard enthalpy of reaction.”

Calorimetry

the science of measuring the change in heat of chemical reactions or physical changes.

Calorimeter

an insulated reaction vessel in which a reaction can occur and where the change in temperature of the system can be measured.

Calorimetry (Purpose)

measures changes in temperature of a system being studied in order to “track'‘ heat change.

Calorimeter (Purpose)

isolates the system from its surroundings.

Calorimetry Assumptions (1)

• No heat is transferred between the calorimeter and the outside environment.

Calorimetry Assumptions (2)

Any heat absorbed or released by the calorimeter itself is insignificant.

Coffee-cup calorimeter

• Cannot be used for reactions involving gases.

• Cannot be used for high temperature reactions.

Bomb Calorimeter

• Reaction takes places in a sealed metal container.

• Temperature difference of the water is measured

• Calculations are more complex because they must take into account heat flow through the metal container.

How do chemists deal with chemical systems that cannot be analyzed using Hess’s Law?

• Slow reactions

• Small changes in enthalpy

Hess’s Law of Summation

For any reaction that can be written in a series, the standard heat of reaction is the same as the sum of the standard heats of reaction for the steps.

Hess’s Law of Summation and Enthalpy Changes (Correlation)

If a set of reactions occurs in different steps but the initial reactants and final products are the same, the overall enthalpy change is the same.

Formation Reactants

Reactions in which compounds are formed from their elements (in their standard states).

Rates of Reactions

The speed at which a chemical change occurs, generally expressed as change in concentration per unit time.

Collision Theory

states that in order for reactions to occur molecules must collide.

Chemical kinetics

the area of chemistry that deals with rates of reactions.

Reactions that produce Gas (Measurement)

Faster reaction = more gas in less time

Reactions that involve Ions (Measurement)

more ions = higher conductivity

Reactions that Change Colour (Measurement)

Spectrophotometer is used to measure

Activation Energy Recall

for a reaction to occur:

• particles must collide in a specific orientation

• particles must collide with sufficient kinetic energy

Activation energy

the minimum collision energy required for a successful reaction to occur.

Maxwell Baltzmann Distribution

plots of kinetic energy vs. number of particles

More particles will have sufficient energy required to react because..

increasing temperature = increasing kinetic energy

When molecules collide..

kinetic energy of the particles is converted to potential energy.

Transition State

point when reactants are converted to product.

Activated complex

chemical species that exist at the transition state.

Catalyst

a substance that increases the rate of a chemical reaction without being consumed in the reaction.

Catalyst works by

lowering the activation energy of a reaction so that a larger number of reactants have sufficient energy to react.

Homogenous catalysts

• exist in the same phase as the reactants

• most often catalyze gaseous & aqueous rxns

Heterogenous Catalysts

• exists in a different phase than reactants.

• without a catalyst this type is very slow.

Enzymes

• organic catalyst used in biological reactions

Substrate (enzymatic reaction)

reactant molecules

Active site (enzymatic reaction)

the portion of the enzyme where the substrate binds to the enzyme.

Reaction Mechanism

a series of elementary steps that add to the overall reaction.

Elementary step

Each individual step in a multistep reaction

Elementary reaction

Each individual step of reaction mechanism

Reaction Intermediates

molecules that form in one step and are consumed in the next

Elementary Step Classification

• Unimolecular

• Bimolecular

• Termolecular

Unimolecular (Classification)

elementary reaction with one particle

Bimolecular (Classification)

elementary reaction with two particles

Termolecular (Classification)

elementary reaction with three particles (rare)

Elementary reaction

the exponents in the rate law equation are the same as the stoichiometric coefficients.

Hess’s Law

The total enthalpy change of a chemical reaction is the same, no matter how many steps the reaction takes.