lecture b. day 4 activities: exothermic or endothermic? objectives –be able to identify exothermic...
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Lecture B
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Day 4
• Activities: Exothermic or endothermic?
• Objectives– Be able to identify exothermic and endothermic
reactions based on temperature change.
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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.
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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.
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Day 5• Activities:
– Introduction: Enthalpy notation– Worksheet: Enthalpy intro
• Objectives:– Write chemical reactions with proper enthalpy
notation.– Use notation to determine endothermic or exothermic
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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.
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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.
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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 = (-)
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H2O(s) H2O(l)
Graphing Enthalpy Change
CH4 + 2O2 CO2 + 2H2O
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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.
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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
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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
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Calculating Enthalpy
• Using a balanced equation, enthalpy can be calculated using stoichiometric ratios, just like we can calculate any other component in a reaction.
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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