Three conditions for the formation of primary batteries: "three looks".

Look at the electrodes first: the two poles are conductors with different reactivity;

Look at the solution again: the two poles are inserted into the electrolyte solution;

Third, look at the loop: form a closed loop or bipolar contact.

Principle 3: (1) Using a relatively active metal as the negative electrode, electrons are lost and oxidation occurs.

(2) use a relatively inactive metal (or carbon) as an anode to obtain electrons and generate a reduction reaction.

(3) A current flows through the wire (contact), so that chemical energy can be converted into electrical energy.

Comparison between primary battery and electrolytic cell

(1) defines the forming conditions of (2)

Device for converting chemical energy of primary battery into electric energy; Suitable electrode, suitable electrolyte solution and loop formation.

The electrode, electrolyte solution (or molten electrolyte) and external electric energy are used to convert the electric energy of the electrolytic cell into chemical energy.

Source, forming a loop

(3) electrode name (4) reaction type (5) electron flow direction of external circuit

Anodizing, anode outflow and anode inflow

Anode reduction

Anodize ...

Cathode reduction anode outflow, cathode inflow

I. Problems related to primary batteries

1. Whether it is a primary battery or not.

(1) First, analyze whether there is an external power supply. If there is an external power supply, it is an electrolytic cell. If there is no external power supply, it may be a primary battery. Then, according to the analysis and judgment of the formation conditions of the primary battery, it is mainly "four looks": look at the electrodes-the two poles are conductors and have different activities (the electrodes of fuel cells are generally inert electrodes), look at the solution-the two poles are inserted into the solution, look at the loop-to form a closed loop or direct contact between the two poles, and look at the essence-whether there is a redox reaction.

(2) When multiple batteries are connected, but there is no external power supply, the battery with the largest difference in activity between the two electrodes is the primary battery, and other batteries can be regarded as electrolytic cells.

2. Determination of anode and cathode of primary battery

(1) is determined by the relative activity of the two poles: the metal with strong relative activity is the negative electrode, and the metal or conductive nonmetal with poor relative activity is the positive electrode. The negative electrode material should generally be able to react with electrolyte solution, such as: in a primary battery composed of Mg-Al-HCl solution, the negative electrode is Mg; But in the primary battery composed of Mg-Al-NaOH solution, the negative electrode is Al.

(2) According to the rise and fall of the valence of the substances reacting on the two electrodes. For example, methanol fuel cells, as the name implies, generally produce carbon dioxide when methanol burns, so the valence state of carbon rises and electrons are lost. Therefore, the electrode through which methanol is introduced is a negative electrode.

(3) It is determined by the change of the electrode: if an electrode is continuously dissolved or its mass is continuously reduced, and the electrode undergoes an oxidation reaction, the electrode is a negative electrode; If gas is generated on the electrode, the quality of the electrode is constantly increasing or unchanged, and the electrode undergoes a reduction reaction, then the electrode is the positive electrode, except for the fuel cell. For example, in a galvanic cell composed of Zn-C-CuSO4 solution, there will be purple-red solid substances deposited on the C electrode, so C is the positive electrode of the galvanic cell.

(4) According to some color phenomena: generally, the reaction, valence fluctuation, oxidation or reduction reaction, H+, OH- or I- discharge of the electrode can be analyzed and inferred according to the changes of color indicators (litmus, phenolphthalein, wet starch, potassium permanganate solution, etc.). ) close to the electrode, so as to determine the positive and negative electrodes.

(5) According to the movement direction of free electrons in the external circuit, the electrode where electrons flow out is called the negative electrode, and the electrode where electrons flow in is called the positive electrode.

(6) According to the movement direction of free ions in the inner loop, the electrode to which cations move in the inner loop is called the negative electrode, and the electrode to which anions move is called the positive electrode.

3. Electrode reactive writing

(1) If the topic is graphic device, analyze the positive and negative electrodes first, and then write the electrode reaction formula according to the reaction law of the positive and negative electrodes.

(2) If the topic is the total reaction formula, we can analyze the oxidation reaction or reduction reaction in this reaction (that is, analyze the valence change of related elements), and recommend choosing a simple change to write the electrode reaction formula. The electrode reaction of the other electrode can be written directly or each reaction can be regarded as a mathematical algebraic expression, and the written electrode reaction can be subtracted from the total reaction (addition and subtraction).

Inducing battery reaction by addition and subtraction;

Total battery reaction: reductant+oxidant+medium = oxidation product+reduction product+others.

Negative electrode: reductant -NE-+ medium = oxidation product (reductant loses electrons and undergoes oxidation reaction).

Positive electrode: oxidant +NE-+ medium = reduction product (oxidant gains electrons and undergoes reduction reaction).

The medium is usually conductive ions, such as H+, OH-, H2O, etc. And its orientation moves to the vicinity of the electrode. If the oxidation (or reduction) product can have a metathesis reaction with it, it is generally written in the electrode equation.

The total reaction ionic equation of the battery is obtained by adding the reaction formulas of the two electrodes, and the reaction formula of the simpler electrode is obtained by subtracting the reaction formula of the simpler electrode from the total reaction ionic equation of the battery. The galvanic cell reaction can generally be written by addition and subtraction.

When writing, both the total reaction formula and the electrode reaction formula should satisfy the electron transfer, charge and mass conservation.

4. Design method of primary battery

On the basis of redox reaction, we must first determine the positive and negative electrodes, electrolyte and electrode reactions of the primary battery; Then write the electrode reaction formula with basic knowledge and solve the problem with reference to Zn-Cu-H2SO4 primary battery model. For example, according to the reaction: Cu+2fecl3 = 2cucl2+2fecl2, a primary battery is designed with Cu as the negative electrode and C (or Pt) as the positive electrode. FeCl3 _ 3 is an electrolyte solution, the negative reaction is Cu-2e-= Cu2+, and the positive and negative reaction should be 2E3++2e-= 2Fe2+.

Second, the related problems of electrolytic cell

1. Determination of electrode and reaction type

(1) Look at the positive and negative poles of the power supply. If it is connected to the negative pole of the power supply, it is the cathode.

(2) Look at the rise and fall of valence states of elements around the electrode. If the valence state rises, it is the anode. Note: The determination of valence state of elements is often based on various experimental phenomena (such as the description of gas components) to infer the reaction products.

(3) Look at the flow direction of electrons. If it is an electrode where electrons flow, it is only a cathode. Note: it must not be judged according to the relative size of bipolar activity.

(4) Anode: If it is an active electrode, the electrode itself loses electrons and undergoes an oxidation reaction. Cathode: The electrode itself does not react (whether it is inert or active). Cations in the solution gain electrons at the cathode and undergo a reduction reaction.

2. Judgment of electrode products in electrolysis process

The anode product of (1) is first judged by the electrode. If the active electrode (before Ag in the metal active sequence table) is used as the anode, the electrode material loses electrons and the electrode dissolves. If it is an inert electrode (Pt, Au, graphite), we only look at the electron-losing ability of ions in the solution, and the order of anion discharge is: S2->; I-> br-& gt； cl->； Oh-(water)

(2) The judgment of cathode products is directly based on the cation discharge sequence (as follows):

ag+& gt； Hg2+>； Fe3+(→Fe2+)& gt； Cu2+>； h+& gt； Pb2+>； sn2+& gt； Fe2+>； Zn2+>； (H+) (water)

Remember a few words:

Primary battery: negative oxygen loss (electron loss, valence increase, oxidation, negative electrode)

Change from low to positive (get electrons, reduce valence, be reduced, be positive)

Electrons flow from the cathode to the anode through an external circuit.

Electrolytic cell: oxygen returns to negative (electron loss, oxidation, anode; Get electrons, be reduced, become a cathode)

Due to the external electric field, the cations in the electrolytic cell move directionally, and the anode moves to the cathode to generate current.