Recall that entropy measures the number of microstates in a substance, which is highest in gases, followed by liquids and solids. Hence, entropy increases if the products have more gaseous molecules than the reactants.
In reaction one, chlorine gas is converted into solid NaCl, so entropy decreaes.
In reaction two, four moles of gas are converted into two moles of gas, so entropy decreases.
In reaction three, ice is melted to form water, which has higher entropy. Thus, entropy increases.
In reaction four, a liquid is freezed into its solid phase, leading to a decrease in entropy.
In reaction five, oxygen gas is converted into solid MgO, so entropy decreases.
The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit. The external battery supplies the electrons. They enter through the cathode and come out through the anode
Acids react with calcium carbonate and more specifically carbonate to form carbon dioxide. An acid will give protons to the carbonate anion to produce carbonic acid which then decomposes into carbon dioxide and water. I hope this helps. Let me know if anything is unclear.
