Chem in a ZipLoc Post-Lab Questions

Question #1 PHYSICAL CHANGES CHEMICAL CHANGES Color Change Gas production Temperature change (either hot or cold) New odor Dissolving

Gas is trapped in the bag Vs. candle demo Question #2 Imagine that you had measured the total mass of the bag (reactants and contents) prior to mixing. Would you expect the overall mass to increase, decrease, or remain the same after the reaction is complete? STAY THE SAME! Gas is trapped in the bag Vs. candle demo

Question #3 Cl2, H2, O2, CO2 Will go out DEADLY (mustard gas) explode What gas do you think was being produced inside the bag? The reactants were Baking Soda (NaHCO3) and calcium chloride (CaCl2). Can you think of a diagnostic test that could be used to determine the identity of the gas? Cl2, H2, O2, CO2 Will go out DEADLY (mustard gas) explode burn brighter and larger

Question #4 Baking soda → COLD Calcium Chloride → HOT EXO → outside Did either reaction feel hot or cold? Which one? What does the prefix exo and endo mean? Baking soda → COLD Calcium Chloride → HOT EXO → outside ENDO → inside

Thermochem Notes #2: Exo/Endo & Entropy WCHS Chemistry

System vs. Surroundings System - The chemicals or reaction Surroundings - the container and everything else outside of the system. ***** Be careful in your explanations!! → What you state as system vs. surroundings affects your answer!

Reaction or Process Exothermic Releases energy in the form of heat Be careful! May not always feel hot! Examples: Fire

Exothermic Reaction or Process LOSS OF ENERGY Reactants have more energy than products (lose energy)

Reaction or Process Endothermic Absorbs energy from the surroundings Be careful! May not always feel “cold”! Examples: Baking soda in water Ice pack

Endothermic Reaction or Process ADDITION OF ENERGY Products have more energy than reactants (gain energy)

Entropy “Disorder” Measure of the randomness/disorder of a system The more disordered, the greater the entropy Symbol for Entropy = S Ex → If your room is very messy = very disordered = large amount of entropy

Entropy Levels CRYSTAL SOLID LIQUID AQUEOUS SOLUTION GAS LOW ENTROPY HIGH ENTROPY Nature tends to favor reactions/processes that have MORE ENTROPY! (=are more disordered) “Spontaneous” → A process that occurs without any ongoing outside intervention.

Examples of Spontaneous Processes Burning a Candle Energy is released so… EXOTHERMIC INCREASING ENTROPY AND becoming more disordered, so… CRYSTAL SOLID LIQUID AQUEOUS SOLUTION GAS Ice cube melting Energy is absorbed → ENDOTHERMIC INCREASING ENTROPY BUT → becoming more disordered so

INCREASE IN ENTROPY (more disordered) Spontaneity Two factors that contribute to spontaneity: EXOTHERMIC PROCESS INCREASE IN ENTROPY (more disordered) Sometimes they work together, but sometimes they don’t!

Heat Pack Example https://www.youtube.com/watch?v=KcRX8oGFIf0 Why is the process that the heat pack (NaC2H3O2) goes through considered spontaneous? NaC2H3O2(l) → NaC2H3O2(s) Energy is released, so… EXOTHERMIC DECREASING ENTROPY Becoming less disordered, so… CRYSTAL SOLID LIQUID AQUEOUS SOLUTION GAS

Ammonium Nitrate Example We put solid Ammonium Nitrate (NH4NO3) in water, and it dissolved (= spontaneous) NH4NO3(s) → NH4NO3(aq) Energy is absorbed so… ENDOTHERMIC INCREASING ENTROPY Becoming more disordered so… CRYSTAL SOLID LIQUID AQUEOUS SOLUTION GAS

Freezing Water How about putting liquid water in the freezer? H2O(l) → H2O(s) Becoming less disordered so… DECREASING ENTROPY Energy is released, so… EXOTHERMIC