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Moiz_6047
#1 Posted : Tuesday, June 01, 2021 3:24:36 AM
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1) In a calorimetry problem, if it says that the water heated up can we assume that the rxn is exothermic. Any other tricks to remember the sign of the reaction?

2) What is the difference between Ecell (naught) and Ecell? Similarly, what is the difference between G (naught) and G?

3) If we are given an E cell for a question will they usually tell us if it is for reduction or oxidation?

4) Generally speaking the more positive the E cell the more likely it is able to undergo reduction and more spontaneous it is?
INSTR_Katrina_128
#2 Posted : Tuesday, June 01, 2021 8:10:39 PM
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1. Very clever! I like that.

If you blow something up (i.e. combustion), that's an exothermic reaction. Booms are exothermic. 😉

When a phase change occurs:

- melting sucks in energy ---> solid to liquid is ENDOthermic
- evaporation sucks in energy ---> liquid to vapour is ENDOthermic
- sublimation sucks in energy ---> solid to vapour is ENDOthermic

(Notice a pattern?)

- freezing releases energy ---> liquid to solid is EXOthermic
- condensation releases energy ---> vapour to liquid is EXOthermic
- deposition releases energy ---> vapour to solid is EXOthermic

(Notice another pattern?)

2. Anything that has a "naught" associated with it signifies that the reaction is being done under standard conditions.

So, ΔG° is called the STANDARD Gibbs free energy and occurs under standard conditions, while ΔG is just the Gibbs free energy and can occur under any conditions.

Standard conditions means that the reaction must be carried out at 25°C and at 1 atm (usually). Materials are assumed to behave in an ideal manner, and exist in their standard states under these conditions. For example, under these conditions, oxygen exists as O2(g) while mercury exists as Hg(l).

You can find tabulated data for thermodynamic properties under standard conditions! To get non-standard data, you need to use maaaath. 😁

3. If you are given Ecell for a question, then the question is GIVING you Ecell = cathode - anode. Remember: for a galvanic (i.e., voltaic) cell, Ecell = + value for a spontaneous reaction, and for an electrolytic cell, Ecell = - value for a non-spontaneous reaction (because external energy drives the reaction).

4. 😀 It depends on what type of cell you have.

If voltaic, yes. The more +, the more spontaneous because the more negative the ΔG term.
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