Unit 11: Energy in Chemical Reactions

The Universe

Is made up of the system and the surroundings

Energy can be transferred between the system and the surroundings

Open, closed and isolated system

Chemical Reaction

• A chemical reaction is a chemical change.• What you start with is different than what you end

up with.• They have different properties• Most reactions happen in aqueous or gas state

because it is easy for the particles to touch.

In order to have a chemical reactionInvolves breaking of existing bonds (requires energy) and formation of new bonds (releases energy)

Evidence of a chemical reaction

In a physical change:• No new substances are

formed• No bonds are broken or

formed

Intensive and Extensive propertiesPhysical properties such as:

IntensiveColorOdorLusterMalleabilityDuctilityConductivityHardnessBoiling Point/Melting PointDensityTemperature

ExtensiveMass WeightVolume Length

• Watch this (3:51)

Stop and Think

Watch this (1:35 BN)

Name some intensive properties of matter that are different after a chemical change.

Chemical Change or Physical Change?

parts of a chemical equation

Reactants Products

state of matter

A chemical equation represents a chemical reaction.

Write an equation to represent this chemical change

NitrogenChlorine

Collision Theory

“Atoms, ions, or molecules must collide in order to react”

1. Reactant particles must collide.2. Reactant particles must have the correct orientation

when they hit.3. Reacting substances must collide with enough energy

to break bonds.

collision orientation

Every collision does not result in a chemical reaction.

Watch This (0:54)

A potential energy diagram plots the change in potential energy that occurs during a chemical reaction.

Watch This (1:16)

Activation Energy- Energy required to start a chemical reaction. Both endothermic and

exothermic reactions need activation energy.

– Measured from the starting point of the reaction to the highest point on the curve

Endothermic Reaction

Exothermic Reaction

What do exothermic and endothermic reactions look like?

Watch this (4:35)

Enthalpy (H)• A measure of heat content of a system. • The heat lost or gained in a reaction.• An extensive property• Measured in units of Joules/mole (J/mol) or kJ/mol

• H = change in heat• H = ∆Hf° (products) – ∆Hf° (reactants)

This equation can be found on the bottom of your STAAR chart.“ ° “ refers to STP conditions

How to determine H • Method 1: By using the ∆Hf° values for each

reactant and product• Method 2: By analyzing a graph• Method 3: By looking at a chemical equation that

includes the energy term• Method 4: By making observations of a chemical

reaction• Method 5: Calorimetry• Watch This L stop at 4:46

Method 1: Enthalpy calculations

Use the table of ∆Hf° values and the equation

H = ∆Hf° (products) – ∆Hf° (reactants) to determine the enthalpy of the reaction. • - H indicates that a reaction is exothermic• +H indicates that a reaction is endothermic

Using enthalpies of formation, calculate the standard change in enthalpy for the thermite reaction: Fe2O3(s)+2Al(s) Al2O3(s)+2Fe(s)ΔHf Fe2O3 = (-826 kJ/mol) ΔHf Al2O3 = (-1676 kJ/mol)

Hints: i. all elements have a ΔHf = 0. ii. if the substance is multiplied by a coefficient, multiply ΔHºf by the same coefficient.

Practice problem

Is the reaction described by the following equation an endothermic or exothermic reaction? What is the H value?

• CH4(g) + O2(g) CO2(g) + 2H2O(l)

• Substance Hf (kJ/mol)

CH4 -74.8

CO2(g) -393.5 H2O(l) -285.8

Method 2: Analyzing Graphs

• Energy of reactants is bigger than energy of products

• Excess energy leaves to the surroundings.• Exothermic (0:56)

Method 2: Analyzing Graphs

• The energy of the products is bigger than the energy of the reactants

• Energy is needed from the surroundings• Endothermic (5:45)

Method 3: chemical equation

EXOTHERMIC

• 4Fe + 3O2 2Fe2O3 + 1625 kJ

or 4Fe + 3O2 2Fe2O3 + energy

• Heat is written on the product side because it exits the system.

Method 3: chemical equation

ENDOTHERMIC

• 27 kJ + NH4NO3 NH4+ + NO3

-

or energy + NH4NO3 NH4+ + NO3

-

• Heat is written on the reactant side because it enters the system.

Method 4: observations of chemical reaction

• Feels hot -----EXOTHERMIC • Feels cold -----ENDOTHERMIC

Method 5: Calorimetry (10:44)• Calorimetry - the act of

measuring the heat of chemical reactions or physical changes, or the science of making such measurements

• Calorimeter – the instrument used in calorimetry

• Q = mΔCT• Q is the same as ΔH

Reaction Rate• Rate at which reactant is converted to product• depends on:

– Concentration (more particles … more collisions… more reactions)– Temperature (particles moving faster.. more collisions.. more

reactions)– Presence of a catalyst (lowers the activation energy, speeds up

reaction rate) or inhibitor (raises the activation energy, slows down the reaction rate)

– Nature of reactants (think back to “reactivity trends” for metals and nonmetals…K is more reactive than Li so K will react faster than Li)

– Watch This (6:24)– Watch This (1:20)

Catalyst

They increase the frequency of collisions or change the orientation of the molecules so more collisions are effective

Inhibitor

Pathway with inhibitor

Inhibitors raise the activation energy. Food preservatives are inhibitors.

Equilibrium• When a reaction starts, the reactants are used up and

products are made. Reactants Products

• After awhile, the products re-form to make reactants. Reactants Products

• Processes that proceed in both the forward and reverse direction are said to be reversible.

Reactants Products

Watch This (1:47)

Equilibrium

• All reactions are reversible if they are in a closed system. That means there is a top on the container.

• Dynamic equilibrium is the condition wherein the rates of the forward and reverse reactions are equal.

• Once the reaction reaches equilibrium, the concentrations of all the chemicals remain constant because the chemicals are being consumed and made at the same rate.

• Watch This (4:18)