Endothermic, Exothermic and

Enthalpy

Making Connections between Micro and Macro Levels

Micro Macro REACTIONS

A + B C +D Delta H = #

Needs some conceptual(Formative )Questions & Intro

What is …? (in notebook or on paper)

(2) Pressure?Macro – Force/unit areaMicro - ?

(1) Temperature?Macro – What we measure with

a thermometerMicro - ? What would stay the same if we doubled the number of molecules?

(3) Enthalpy (H) - The sum of all potential and kinetic energies of a system (most related to which graph?)∆H - Changes of Energy level in the SYSTEM, not the surrounding environment, for a Reaction.

Making connections between Macro and Micro Using Simulation

If a reaction is to be carried out so the temperature is returned to ‘normal’ or the temperature we started at, what do we have to do in terms of adding or removing energy for an endothermic vs. exothermic reaction?

Before going on to lab you and partner need to[1] Explain in your own words why the ∆H for an exothermic reaction is negative and why ∆H is positive for an endothermic (we are part of the ‘environment’ … what does the system experience [not us])&[2] Make predictions based on the enthalpies for the reactions we are to do, which might feel the coolest … warmest? Rank them from expected coldest to warmest and then compare to what you actually experienced.

?Why negative?

?Why positive?

[2] Table listing reaction, predicted order in terms of coldest to warmest (1, 2,3, ..etc or -4,-3,) , & actual results ? May use a simple -3 to 0 to +3 to for “sensation scale”[High (3), Medium (2), and Low(1) Cold (-) or Hotness (+)] … (or possible temperature differences)

[1] Explanation of Negative/Positive Enthalpies

In (Lab) notebook.

Reaction Predicted Rank

Observed Other observations

Explanations or Conjectures

A + B C +DDelta H = #

-3 -1 Precipitate ;; tiny amount

Not fair comparison bec. Small amounts

1

2

A system that releases heat to the surroundings, an exothermic reaction, creates a negative ΔH because the enthalpy of the products is lower than the enthalpy of the reactants of the system (Figure 1).

A system that absorbs heat from the surroundings, an endothermic reaction, creates a positive ΔH because the enthalpy of the products is higher than the enthalpy of the reactants of the system.

Share explanations & Results.

Need to revisit conceptual questions and summary …