lecture b. day 4 activities: exothermic or endothermic? objectives –be able to identify exothermic...

Lecture B

Day 4

• Activities: Exothermic or endothermic?

• Objectives– Be able to identify exothermic and endothermic

reactions based on temperature change.

Exothermic Reactions

• Heat is released from the chemical chemicals and into the surroundings.

• When you feel this, it is warm. • Ex- out• Examples: burning wood, heating pack,

Combustion of Natural Gas, Neutralization of HCl with NaOH

• Most spontaneous chemical reactions are this type.

Endothermic Reactions • Heat is absorbed by the chemicals and

stored.• When you feel this, it feels cold. • En = enter• Most endothermic reactions are non-

spontaneous and require energy input.• Example: Photosynthesis, instant ice pack,

breaking up of water into hydrogen and oxygen.

Day 5• Activities:

– Introduction: Enthalpy notation– Worksheet: Enthalpy intro

• Objectives:– Write chemical reactions with proper enthalpy

notation.– Use notation to determine endothermic or exothermic

Enthalpy

• Enthalpy in chemistry is often talked about as stored ‘heat content’ of a chemical.

• So we use the symbol ‘H’ when talking about it.• Energy is stored in the bonds of chemical

compounds.• When bonds break, energy is released.• When bonds form, energy is absorbed.

Enthalpy• So, in the process of a chemical reaction:

– 1) Energy is released when reactants break bonds– 2) Energy is absorbed when products form bonds.

• The difference between energy released and energy absorbed is what determines whether it is an exothermic or endothermic reaction.

• If absorbed > released, then it will feel _____.• That’s endothermic.• If released < absorbed, then it will feel _____.• That’s exothermic.

Enthalpy• H (delta H) means change in enthalpy.• The enthalpy of a reaction is calculated by

comparing the enthalpy of the products to the enthalpy of the reactants.

• Hreaction = Hproducts – Hreactants

• Therefore, if Hproducts > Hreactants , the reaction is endothermic and H = (+)

• If Hproducts < Hreactants , the reaction is exothermic and H = (-)

H2O(s) H2O(l)

Graphing Enthalpy Change

CH4 + 2O2 CO2 + 2H2O

Writing Enthalpy as Part of a Chemical Equation

• Once we know what the enthalpy change of a particular reaction is, it is good to include it as part of the equation.

• We can do this in one of two ways.– 1) Include it within the reaction itself– 2) Separate it from the equation as a ΔH notation.

Writing Enthalpy as Part of a Chemical Equation v.1

• If we include the enthalpy change within the reaction– Exothermic has heat in the products– Endothermic has heat in the reactants

• CH4 + 2O2 CO2 + H2O + 890 kJ

• 2H2O + 286 kJ 2H2 + O2

Writing Enthalpy as Part of a Chemical Equation v.2

• The more common way of including enthalpy is to write it in ΔH notation.

• This separates the energy from that matter.• In this format:

– Exothermic gives ΔH = (-)– Endothermic gives ΔH = (+)

• CH4 + 2O2 CO2 + H2O ΔH = -890 kJ

• 2H2O 2H2 + O2 ΔH = +286 kJ

Calculating Enthalpy

• Using a balanced equation, enthalpy can be calculated using stoichiometric ratios, just like we can calculate any other component in a reaction.

Law of Conservation of Energy• Energy can not be destroyed or created but

it is changed from one form to another.• When you start a car, electrical energy is

converted into mechanical energy.• Kinetic Energy- Energy in Motion• Potential Energy- Energy at Rest