Methylamine (CH3NH2) forms hydroxide ions in aqueous solution. Why is methylamine a Brønsted-Lowry base but not an Arrhenius base?
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To understand why methylamine (CH3NH2) is a Brønsted-Lowry base but not an Arrhenius base, let's first define what each concept means.
1. Arrhenius base: According to the Arrhenius definition, a base is a substance that releases hydroxide ions (OH-) when dissolved in water. In other words, an Arrhenius base is defined based on its behavior in aqueous solutions. For example, sodium hydroxide (NaOH) is an Arrhenius base because it dissociates in water to produce hydroxide ions (NaOH → Na+ + OH-).
2. Brønsted-Lowry base: According to the Brønsted-Lowry definition, a base is a substance that accepts a proton (H+ ion) from an acid. In this definition, the focus is on the ability of a substance to act as a proton acceptor, regardless of whether it is in an aqueous solution or not.
Now, let's apply these concepts to methylamine:
1. Arrhenius base: Methylamine does not fit the Arrhenius definition of a base because it does not release hydroxide ions when dissolved in water. Instead, it acts as a weak base by accepting a proton from water molecules to form the methylammonium ion (CH3NH3+). The reaction can be written as:
CH3NH2 + H2O ⇌ CH3NH3+ + OH-
2. Brønsted-Lowry base: Methylamine can be classified as a Brønsted-Lowry base because it can accept a proton (H+) from an acid. In this case, it accepts a proton from water to form the methylammonium ion. Therefore, when considering its ability to accept a proton, methylamine can be classified as a base according to the Brønsted-Lowry definition.
In summary, methylamine is a Brønsted-Lowry base because it can accept a proton, but it is not an Arrhenius base because it does not produce hydroxide ions in aqueous solution. Remember, the Arrhenius definition is based on behavior in water, while the Brønsted-Lowry definition focuses on the ability to accept or donate protons.