General Chemistry Reactions

There are an incalculable number of chemical reactions that produce a dizzying array of different compounds. We don’t need to know every single reaction under the sun and all that memorization is unlikely to help us on the MCAT. Thankfully all of those chemical reactions are easily distilled into five major categories. Furthermore being able to balance and utilize these chemical equations is foundational on the MCAT and we will perpetually revisit this idea as we look into more specific reaction types in later lessons.

Types of Reactions

Synthesis reactions brings together two or more different reactants into one product. These reactions are exceedingly common and typically occur between diatomic gases although solids and liquids also undergo synthesis reactions.

[latexpage]
\[
2\; H_2(g) + O_2(g) \to 2\; H_2O(g)
\]

\[
3\; H_2(g) + N_2(g) \to 2\; NH_3(g)
\]

\[
4\; Al(s) + 3\; O_2(g) \to 2\; Al_2O_3(s)
\]

Decomposition reactions are the reverse of synthesis reactions and break apart larger molecules into their components. Typically, this occurs due to some type of energy input such as excessive heat, radiation, or electrolysis.

[latexpage]
\[
2\; H_2O_2(l) \to 2\; H_2O (l) + O_2(g)
\]

\[
H_2CO_3(l) \to H_2O(l) + CO_2(g)
\]

\[
2\; H_2O(l) \to 2\; H_2(g) + O_2(g)
\]

Combustion reactions occur when a hydrocarbon such as methane is heated in the presence of oxygen. This results in the production of carbon dioxide gas and water gas.

[latexpage]
\[
C_{10}H_8(g) + 12\; O_2(g) \to 10\; CO_2(g) + 4\; H_2O(g)
\]

\[
2\;C_4H_{10}(g) +13\;O_2(g) \to 8\;CO_2(g) +10\;H_2O(g)
\]

\[
2\;CH_3OH(g) + 3\;O2(g) \to 2\;CO_2(g) + 4\;H_2O(g)
\]

In single replacement reactions, one of the elements in a compound switches places with another element or ion. Most commonly these are oxidation-reduction reaction where electrons shifts as elements switch places.

[latexpage]
\[
Zn+ 2\; HCl \to ZnCl_2 + H_2
\]

\[
K + NaCl \to KCl + Na
\]

\[
Cl_2 + 2\; NaBr \to 2\; NaCl + Br_2
\]

In a double displacement reaction, the elements in each of the molecules switch places to form two new compounds. On the MCAT these are most commonly neutralization reactions or oxidation reactions. In neutralization reactions, acids and bases react to form water and salt. Thus the acid and base are neutralized.

[latexpage]
\[
HCl(aq)+NaOH(aq)\to H_2O(l)+NaCl(aq)
\]