Chemical reactions Study Guide

Electrolysis is the process of using electricity to split compounds. During electrolysis, various substances can be produced at the electrodes, depending on the electrolyte and the reactivity of the ions present. The cathode is the negative electrode, and the anode is the positive electrode. At the cathode, positively charged ions (cations) are attracted. These include metal ions or hydrogen ions (H+). If the metal is more reactive than hydrogen, then hydrogen gas will be produced at the cathode. If the metal is less reactive than hydrogen, then the metal will be deposited at the cathode. Essentially, metals or hydrogen are formed at the cathode. At the anode, negatively charged ions (anions) are attracted. These are typically non-metal ions like halide ions (Cl-, Br-, I-) or hydroxide ions (OH-) or oxide ions. Non-metals, such as chlorine gas or oxygen gas, are formed at the anode. It is important that the electrodes used are inert, meaning they do not react with the products or electrolyte, to ensure only products from the electrolyte are collected. Graphite or platinum are common examples of inert electrodes used in electrolysis.

The pH scale is a measure of the acidity or alkalinity of a solution. It is defined by the concentration of hydrogen ions (H⁺) present in the solution. You need to understand that the pH scale is logarithmic, meaning each whole number change in pH represents a tenfold change in hydrogen ion concentration. Specifically, as the concentration of hydrogen ions increases by a factor of ten, the pH value of the solution decreases by one. Conversely, if the hydrogen ion concentration decreases by a factor of ten, the pH value increases by one. This inverse relationship is crucial for understanding how solution acidity changes. For instance, a solution with a pH of 3 has ten times the hydrogen ion concentration of a solution with a pH of 4, and one hundred times the hydrogen ion concentration of a solution with a pH of 5. This exponential relationship means that even small changes in pH can correspond to very significant changes in the chemical properties and reactivity of a solution, which is particularly important in biological systems and industrial processes. Pure water, which is neutral, has an equal concentration of hydrogen and hydroxide ions and a pH of 7. Acids have a pH less than 7 due to a higher concentration of hydrogen ions, while alkalis have a pH greater than 7 due to a lower concentration of hydrogen ions relative to hydroxide ions.