Current flow in a voltaic cell.
Current flow in a voltaic cell

Electrochemistry and Battery Charger Chemistry

by Lewis Loflin


In the discussion of electric and electronic circuits, current flow (in the real world) is negative to positive. The negative polarity is the source of electrons or cathode. Electrons flow to the anode. In electrochemistry, labeling can confuse some people. We addressed that problem with GROK the X AI while I filled in the blanks and edited the text.

Lewis: does the anode and cathode designation change? This question was regarding vacuum tubes, Crook's tubes, etc. In vacuum tubes, the cathode is the source of electrons.

This question came about due to labeling anode and cathode with Crook's tubes and vacuum tubes in general. In chemistry and electrochemistry based on oxidation-reduction (redox) reactions.

In electrochemical cells, oxidation (loss of electrons) happens in the anode or negative terminal. It is the electrode from which electrons leave the cell to the external circuit. The cathode is typically considered the positive electrode in a galvanic or voltaic cell).

The cathode is where reduction (gain of electrons) occurs. It is the electrode that receives electrons from the external circuit. The cathode is typically the positive electrode in a galvanic cell.

So, in a voltaic cell or battery, electrons flow from the anode (negative electrode) to the cathode (positive electrode) through the external circuit. This flow of electrons is what generates electric current. So, electrons go from the anode (negative) to the cathode (positive).


Current Flow External Circuit

Lewis to GROK: In the external circuit electrons move from the negative terminal to positive terminal?

Answer: Yes, that's correct in the context of how we typically discuss the flow of electricity:

In a voltaic cell (also known as a galvanic cell or battery), electrons move through the external circuit from the anode (which is the negative terminal in this context) to the cathode (which is the positive terminal).

This movement of electrons from negative to positive is what we measure as electric current. However, it's worth noting that:

In conventional current flow, which was established before the discovery of electrons, current is considered to flow from the positive terminal to the negative terminal. This is based on the direction of positive charge flow.

Electron flow, which reflects the actual movement of electrons, is from the negative terminal (anode in a voltaic cell) to the positive terminal (cathode in a voltaic cell).

So when talking about electron flow specifically in an external circuit of a battery or voltaic cell:

Electrons do indeed move from the negative terminal (where they are generated by oxidation) to the positive terminal (where they are consumed by reduction).

Redox reaction during charging and discharging rechargeable voltaic cell.
Redox reaction during charging and discharging rechargeable voltaic cell.

Charge Movement in a Voltaic Cell

Lewis: What charges move within a voltaic cell?

Answer In a voltaic cell, also known as a galvanic cell, the movement of charge involves:

Electrons (e⁻): Electrons are negatively charged particles that move through the external circuit from the anode (-) to the (+) cathode, creating an electric current. This flow of electrons is what we measure as electricity.

Cations (positively charged ions): These move through the electrolyte towards the cathode to balance the charge as electrons add to the cathode during the reduction reaction.

Anions (negatively charged ions): These move through the electrolyte towards the anode to balance the charge as electrons leave the anode during the oxidation reaction.

Salt Bridge or Porous Barrier: A salt bridge allows for the movement of ions between the two half-cells to maintain electrical neutrality. The salt bridge or a porous barrier facilitates ions' flow but not solutions' mixing.

At the anode, metal atoms or other substances oxidize, releasing electrons into the external circuit. This action creates positive ions (cations) in the solution, which can then move towards the cathode.

At the cathode, the anode's electrons that flow through the external circuit are used in the reduction process to convert ions into neutral metal atoms or molecules. The reduction process consumes positive ions, leaving excess negative ions (anions) in the solution around the cathode, which will move toward the anode.

In the electrolyte, to keep the cell electrically neutral, ions move in the opposite direction of the electron flow in the external circuit. Cations (+) move towards the cathode, and anions (-) move towards the anode.

The movement of these charges - electrons through the external circuit and ions within the cell or through the salt bridge - allows for the continuous operation of the voltaic cell, providing the electrical energy we harness from the chemical reactions within the cell.


To summarize electrochemical cells (like batteries or voltaic/galvanic cells):

Anode: Where oxidation (loss of electrons) occurs, produces electrons, and it's considered the negative terminal in the context of electron flow. Electrons leave this electrode and move through the external circuit.

Cathode: Where reduction (gain of electrons) takes place, consuming electrons, and this is the positive terminal. Electrons flow to this electrode in the external circuit.

Man with army jacket working as chemist.


What are Endothermic and Exothermic Reactions?

Exothermic Reactions: These are reactions where energy, often in the form of heat, is released to the surroundings. The term "exothermic" comes from the Greek words "exo" meaning "outside" and "therm" meaning "heat."

Characteristics: The system loses heat, meaning ΔH (change in enthalpy) is negative; The products have less energy than the reactants.

Examples include:

Combustion reactions like burning natural gas, where methane (CH4) reacts with oxygen (O2) to produce carbon dioxide (CO2), water (H 2O), and heat.

Neutralization reactions between acids and bases, e.g., hydrochloric acid (HCl) and sodium hydroxide (NaOH) producing water and sodium chloride (NaCl).

Temperature: The temperature of the surroundings increases as heat is given off.

Endothermic Reactions

Definition: These reactions absorb energy from the surroundings, making "endo" meaning "inside" and "therm" relevant here as the heat goes into the reaction system.

Characteristics: The system gains heat, so ΔH is positive. The products have more energy than the reactants.

Photosynthesis, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen and the thermal decomposition of calcium carbonate (CaCO₃) to produce calcium oxide (CaO) and carbon dioxide (CO₂). The surrounding temperature become cooler because the reaction absorbs heat.

This stored energy is released during combustion of wood or fossil fuels, which then becomes an exothermic reaction.

The Direction of Heat Flow: Exothermic reactions release heat to the environment; endothermic reactions take in heat from the environment.

In exothermic reactions, energy is released, lowering the energy of the system. In endothermic reactions, energy is absorbed, increasing the system's energy. As seen from the system:

Enthalpy Change (ΔH): Exothermic: ΔH < 0, Endothermic: ΔH > 0.

Understanding these reactions is crucial in fields like chemistry, biology, and environmental science, as they dictate how energy is managed in chemical processes.

Is dissolving salt in water endothermic?

This is both endothermic and exothermic, with endothermic overall greater.

When dissolving sodium chloride (NaCl), commonly known as table salt, in water is actually an endothermic process. Here's why:

Breaking Bonds: The process starts with the hydration of Na⁺ and Cl⁻ ions. This involves breaking the ionic bonds within the salt crystal (sodium chloride), which requires energy because ionic bonds are strong. Overcoming the attraction between water molecules (hydrogen bonding) to make room for ions, which also uses energy.

Hydration Energy: After the bonds are broken, water molecules surround the sodium and chloride ions in a process known as solvation or hydration. This step releases energy as the water molecules form new, albeit weaker, interactions with the ions.

Net Energy Change:

Although hydration releases energy, the initial step of breaking the ionic bonds in the salt lattice requires more energy than is given off when the hydration occurs. Therefore, the overall process of dissolving NaCl in water absorbs heat from its surroundings, making it endothermic. When adding salt to water, the temperature of the solution might decrease slightly. This temperature drop indicates that heat is being absorbed from the water, confirming the endothermic nature of the dissolution process.

However, the magnitude of temperature change might be small and not always easily perceptible without precise measurement.

This process used in cheap a hand-cranked ice cream makers. When salt (typically rock salt or sodium chloride) is added to ice, it lowers the freezing point of the ice-water mixture. This phenomenon is known as freezing point depression.

Thus salt water can be cooled below 32 Deg. F to 28.4 Deg. F. This cold water can be used to make ice, or ice cream.


The above material credit GROK the X AI with additions by Lewis Loflin.

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I've been a part-time adjunct professor at a local community college teaching electricity and electronics.

Today I do this for the shear love of electronics.

I have 45 years experience in electronics, from vacuum tubes to modern solid state and industrial controls. I tend to teach from a general science viewpoint.

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