U4.7 · Types of Chemical ReactionsPractice · 化学反应的类型

Two substances combine into one: a synthesis reaction. Check oxidation numbers: Ca stays +2, C stays +4, and O stays −2. Nothing changes, so it isn’t a redox reaction. A synthesis reaction isn’t automatically redox (compare $\ce{2Mg + O2 -> 2MgO}$).

In a precipitation reaction, two soluble ionic compounds swap ions in solution and form an insoluble solid: $\ce{BaSO4}$ and $\ce{Fe(OH)3}$ are both insoluble. Zn and $\ce{CuSO4}$ do form solid Cu, but Zn is oxidized and $\ce{Cu^2+}$ is reduced, so that is redox. $\ce{HNO3}$ and KOH is a neutralization, and $\ce{K2CO3}$ with hydrochloric acid is an acid–base reaction that forms a gas, not a precipitate.

In $\ce{C2H6O}$, H is +1 and O is −2, so C is −2; in $\ce{CO2}$, C is +4. C’s oxidation number rises: it is oxidized. O goes from 0 in $\ce{O2}$ to −2: it is reduced. H stays +1. Every combustion is a redox reaction, with or without ions.

Activity series: K, Ca, Na, Mg, Al, Zn, Fe, Sn, Pb, (H), Cu, Hg, Ag, … A metal can displace any metal listed after it from a solution of its salt, and only metals above H displace hydrogen from acids. Fe is above Cu: $\ce{Fe(s) +}$ $\ce{CuSO4(aq) ->}$ $\ce{FeSO4(aq) +}$ $\ce{Cu(s)}$. Cu is less active than Zn and Fe, and Ag is below H, so those don’t react.

Na goes from 0 to +1 in NaOH, losing electrons: it is oxidized. Some of the H in water goes from +1 to 0 in $\ce{H2}$, gaining electrons: it is reduced. Two Na atoms lose 2 electrons in all, and forming one $\ce{H2}$ gains 2. A base forms, but there is no $\ce{H+}$ transfer between an acid and a base.