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4.1 Writing and Balancing Chemical Equations

Learning objectives.

By the end of this section, you will be able to:

• Derive chemical equations from narrative descriptions of chemical reactions.
• Write and balance chemical equations in molecular, total ionic, and net ionic formats.

An earlier chapter of this text introduced the use of element symbols to represent individual atoms. When atoms gain or lose electrons to yield ions, or combine with other atoms to form molecules, their symbols are modified or combined to generate chemical formulas that appropriately represent these species. Extending this symbolism to represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change involves writing and balancing a chemical equation . Consider as an example the reaction between one methane molecule (CH 4 ) and two diatomic oxygen molecules (O 2 ) to produce one carbon dioxide molecule (CO 2 ) and two water molecules (H 2 O). The chemical equation representing this process is provided in the upper half of Figure 4.2 , with space-filling molecular models shown in the lower half of the figure.

This example illustrates the fundamental aspects of any chemical equation:

• The substances undergoing reaction are called reactants , and their formulas are placed on the left side of the equation.
• The substances generated by the reaction are called products , and their formulas are placed on the right side of the equation.
• Plus signs (+) separate individual reactant and product formulas, and an arrow (⟶) (⟶) separates the reactant and product (left and right) sides of the equation.
• The relative numbers of reactant and product species are represented by coefficients (numbers placed immediately to the left of each formula). A coefficient of 1 is typically omitted.

It is common practice to use the smallest possible whole-number coefficients in a chemical equation, as is done in this example. Realize, however, that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted as ratios. Methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio. This ratio is satisfied if the numbers of these molecules are, respectively, 1-2-1-2, or 2-4-2-4, or 3-6-3-6, and so on ( Figure 4.3 ). Likewise, these coefficients may be interpreted with regard to any amount (number) unit, and so this equation may be correctly read in many ways, including:

• One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules.
• One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules.
• One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and 2 moles of water molecules.

Balancing Equations

The chemical equation described in section 4.1 is balanced , meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal. Note that the number of atoms for a given element is calculated by multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together. For example, both product species in the example reaction, CO 2 and H 2 O, contain the element oxygen, and so the number of oxygen atoms on the product side of the equation is

The equation for the reaction between methane and oxygen to yield carbon dioxide and water is confirmed to be balanced per this approach, as shown here:

A balanced chemical equation often may be derived from a qualitative description of some chemical reaction by a fairly simple approach known as balancing by inspection. Consider as an example the decomposition of water to yield molecular hydrogen and oxygen. This process is represented qualitatively by an unbalanced chemical equation:

Comparing the number of H and O atoms on either side of this equation confirms its imbalance:

The numbers of H atoms on the reactant and product sides of the equation are equal, but the numbers of O atoms are not. To achieve balance, the coefficients of the equation may be changed as needed. Keep in mind, of course, that the formula subscripts define, in part, the identity of the substance, and so these cannot be changed without altering the qualitative meaning of the equation. For example, changing the reactant formula from H 2 O to H 2 O 2 would yield balance in the number of atoms, but doing so also changes the reactant’s identity (it’s now hydrogen peroxide and not water). The O atom balance may be achieved by changing the coefficient for H 2 O to 2.

The H atom balance was upset by this change, but it is easily reestablished by changing the coefficient for the H 2 product to 2.

These coefficients yield equal numbers of both H and O atoms on the reactant and product sides, and the balanced equation is, therefore:

Example 4.1

Balancing chemical equations.

Next, count the number of each type of atom present in the unbalanced equation.

Though nitrogen is balanced, changes in coefficients are needed to balance the number of oxygen atoms. To balance the number of oxygen atoms, a reasonable first attempt would be to change the coefficients for the O 2 and N 2 O 5 to integers that will yield 10 O atoms (the least common multiple for the O atom subscripts in these two formulas).

The N atom balance has been upset by this change; it is restored by changing the coefficient for the reactant N 2 to 2.

The numbers of N and O atoms on either side of the equation are now equal, and so the equation is balanced.

Check Your Learning

It is sometimes convenient to use fractions instead of integers as intermediate coefficients in the process of balancing a chemical equation. When balance is achieved, all the equation’s coefficients may then be multiplied by a whole number to convert the fractional coefficients to integers without upsetting the atom balance. For example, consider the reaction of ethane (C 2 H 6 ) with oxygen to yield H 2 O and CO 2 , represented by the unbalanced equation:

Following the usual inspection approach, one might first balance C and H atoms by changing the coefficients for the two product species, as shown:

This results in seven O atoms on the product side of the equation, an odd number—no integer coefficient can be used with the O 2 reactant to yield an odd number, so a fractional coefficient, 7 2 , 7 2 , is used instead to yield a provisional balanced equation:

A conventional balanced equation with integer-only coefficients is derived by multiplying each coefficient by 2:

Finally with regard to balanced equations, recall that convention dictates use of the smallest whole-number coefficients . Although the equation for the reaction between molecular nitrogen and molecular hydrogen to produce ammonia is, indeed, balanced,

the coefficients are not the smallest possible integers representing the relative numbers of reactant and product molecules. Dividing each coefficient by the greatest common factor, 3, gives the preferred equation:

Link to Learning

Use this interactive tutorial for additional practice balancing equations.

Additional Information in Chemical Equations

The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water ( aqueous solutions , as introduced in the preceding chapter). These notations are illustrated in the example equation here:

This equation represents the reaction that takes place when sodium metal is placed in water. The solid sodium reacts with liquid water to produce molecular hydrogen gas and the ionic compound sodium hydroxide (a solid in pure form, but readily dissolved in water).

Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the equation’s arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta (Δ) over the arrow.

Other examples of these special conditions will be encountered in more depth in later chapters.

Equations for Ionic Reactions

Given the abundance of water on earth, it stands to reason that a great many chemical reactions take place in aqueous media. When ions are involved in these reactions, the chemical equations may be written with various levels of detail appropriate to their intended use. To illustrate this, consider a reaction between ionic compounds taking place in an aqueous solution. When aqueous solutions of CaCl 2 and AgNO 3 are mixed, a reaction takes place producing aqueous Ca(NO 3 ) 2 and solid AgCl:

This balanced equation, derived in the usual fashion, is called a molecular equation because it doesn’t explicitly represent the ionic species that are present in solution. When ionic compounds dissolve in water, they may dissociate into their constituent ions, which are subsequently dispersed homogenously throughout the resulting solution (a thorough discussion of this important process is provided in the chapter on solutions). Ionic compounds dissolved in water are, therefore, more realistically represented as dissociated ions, in this case:

Unlike these three ionic compounds, AgCl does not dissolve in water to a significant extent, as signified by its physical state notation, s .

Explicitly representing all dissolved ions results in a complete ionic equation . In this particular case, the formulas for the dissolved ionic compounds are replaced by formulas for their dissociated ions:

Examining this equation shows that two chemical species are present in identical form on both sides of the arrow, Ca 2+ ( aq ) and NO 3 − ( a q ) . NO 3 − ( a q ) . These spectator ions —ions whose presence is required to maintain charge neutrality—are neither chemically nor physically changed by the process, and so they may be eliminated from the equation to yield a more succinct representation called a net ionic equation :

Following the convention of using the smallest possible integers as coefficients, this equation is then written:

This net ionic equation indicates that solid silver chloride may be produced from dissolved chloride and silver(I) ions, regardless of the source of these ions. These molecular and complete ionic equations provide additional information, namely, the ionic compounds used as sources of Cl − and Ag + .

Example 4.2

Ionic and molecular equations.

Balance is achieved easily in this case by changing the coefficient for NaOH to 2, resulting in the molecular equation for this reaction:

The two dissolved ionic compounds, NaOH and Na 2 CO 3 , can be represented as dissociated ions to yield the complete ionic equation:

Finally, identify the spectator ion(s), in this case Na + ( aq ), and remove it from each side of the equation to generate the net ionic equation:

Write balanced molecular, complete ionic, and net ionic equations for this process.

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Home > Science Worksheets > Chemical Reactions

Reaction can take place quickly or slowly depending on the nature of the substances involved. If you tried to form rust from some iron, it can take an average of two weeks. On the other hand, if you put hydrochloric acid in the presence of sodium hydroxide you will get a reaction that produces salt and water in seconds. These worksheets will look at all the different types of chemical reaction you will see in any science curriculum. This group of sheets really focuses on understanding how a reaction can be understood from the molecular masses all the way through to diagramming the reaction that takes place.

Get Free Worksheets In Your Inbox!

Print chemical reactions worksheets, click the buttons to print each worksheet and associated answer key., balancing reactions.

Provide the coefficients to balance the chemical equations shown below.

Gram Formula Mass

What is the mass of one mole of each of these substances?

Percentage Mass

The proportion of the total mass of a compound that is due to each element can be calculated and is called the mass percent.

Binary Formulas

Provide the formulas for the compounds formed from combining the listed ions.

Name Binary Compounds

Provide the names for the ionic compounds shown.

Naming Binary Covalent Compounds

Name the compounds that share electrons.

Determining Formulas of Polyatomic Ions

What compound forms? For extra credit name the compound in words.

Name Non-Binary Compounds

These are a little more difficult. Make sure to read everything throughly.

Name Mixed Compounds

Write the name of each compound shown here.

Writing Formulas Based on Names

Write the equation for each formula that you come across.

Balance Chemical Equations

Take another crack at stoichiometry with this worksheet.

Name-Based Chemical Equations

Write out what happens completely here with these.

Reaction Types

Classify each chemical reaction as either a decomposition, single replacement, double replacement, or synthesis.

Name Organic Compounds

Make sure to count your units of oxygen and hydrogen.

Draw Structural Formulas

You will be given an organic compound and asked to structural draw it.

Isomers of Organic Molecules

Structural isomers are molecules that have the same chemical formula, but different structural formulas.

Organic Chemistry Crossword

Find all the organic chem. terms in that puzzle.

What Are Reactions?

Reactions rates can be sped up with the application of energy like heat, sunlight, or electricity or by increasing the concentration or pressure of the reactants.

What Are They? Questions

Alternatively, the addition of a third substance called an inhibitor can close down a reaction.

Synthesis Reactions

In a synthesis reaction, two substances combine to form a new substance.

QUESTIONS: Synthesis Reactions

Are the reactants in a synthesis reaction typically individual elements or compounds?

Decomposition Reactions

In a decomposition reaction one more complex substance breaks down to form two separate, simpler substances.

QUESTIONS: Decomposition Reactions

Most decomposition reactions are endothermic. In an endothermic reaction, energy is applied in the form of heat, electricity, or sunlight in order to break down the bonds of a more complex molecule.

Combustion was the first chemical change discovered by man. The ability to use and control fire helped early humans to survive and revolutionized the way that they lived.

QUESTIONS: Combustion

Once started, how long will a combustion reaction continue?

Single and Double Displacement Reactions

A single displacement reaction is also known as a single replacement reaction or a substitute reaction.

QUESTIONS: Single and Double Displacement Reactions

In a double displacement reaction, also known as a metathesis reaction, the compounds on the left side of the equation switch substances.

Photochemical Reactions

A photochemical reaction is a chemical reaction in which the heat needed to induce a chemical reaction comes from molecules of light called photons.

QUESTIONS: Photochemical Reactions

What is the name of the structures plants use to capture sunlight?

Catalysts and Inhibitors

A common way to add a catalyst to a chemical reaction is to add energy in the form of heat, sunlight, or electricity.

QUESTIONS: Catalysts and Inhibitors

An inhibitor is the opposite of a catalyst. Adding an inhibitor increases the amount of activation energy that is needed in order for a chemical reaction to occur.

Reactants, Reagents, and Products

A reactant is a substance that is present at the beginning of a chemical reaction and that is changed by the reaction to create a new substance called the product.

QUESTIONS: Reactants, Reagents, and Products

How and where do you note if a reaction has taken place under certain circumstances?

Acid-Base Reactions

In chemistry, an acid is a substance that can release a proton, and a base is a substance that can receive a proton.

QUESTIONS: Acid-Base Reactions

The most often performed school science experiment is an acid-base reaction: the volcano experiment in which you add vinegar to baking soda.

Oxidation-Reduction Reactions

A redox reaction always take place in two parts: a reduced half and an oxidized half. These two parts always occur together.

QUESTIONS: Oxidation-Reduction Reactions

What kind of reactions are typically (but not always) redox reactions with oxygen being oxidized?

What Causes Chemical Reactions?

Every day, different chemical reactions happen around us – cooking eggs, lighting a fire, and more. Each of them is a unique set of circumstances, but they have a general cause.

Chemical reactions are caused by the breaking of bonds in reactants to form new bonds of products. Energy is needed to form bonds but is released when bonds are broken. In a reaction, the amount of matter is preserved regardless of changes in structure.

Keep reading to learn more about chemical reactions, such as different types and physical indicators.

What Is a Chemical Reaction?

Chemical reactions vary a lot. Some occur quickly, while others need a catalyst. Some have apparent changes, while others don’t. But they are all processes that lead to products. 

A chemical reaction is the conversion of substances, called reactants, into new compounds, called products. These products have new structures that are caused by breaking and forming bonds. There are different types of chemical reactions.

Reactions can either be reversible or not. In reversible forms, products can reform into the original reactants, given the right conditions. Thus, it can move in two directions: forward into products or backward into reactants.

Contrary to reversible forms, irreversible ones can only move forward. Thus, products cannot form back into reactants.

Chemical reactions are happening all the time around use and we just don’t realize it. Ever clean your kitchen with something other than water? Ever rinse with mouth wash? Have you seen the glorious baking soda and vinegar volcano that is so popular at science fairs? They happen when molecules from one substance break apart and combine, to some degree, with another. This usually is irreversible and forms a new substance all together. Some of them give off heat, like hand warmers. Some take in heat (get colder). Instant ice packs are an example of that. Others give off brilliant colors. Where do you think firework reactions come from?

What Are Physical Indicators of a Chemical Reaction?

While not applicable to all reactions, many processes display physical indicators of a chemical reaction.

These are the most common indicators:

• Change in color
• Change in temperature
• Production of light
• Production of gas
• Formation of precipitate

These indicators are the result of changes in chemical structure. 

For instance, exothermic reactions , which involve breaking bonds, are accompanied by signs of increased temperature because energy was released. 

On the other hand, endothermic types have reduced temperatures because they absorb energy instead of releasing them.

What Are the Basic Types of Chemical Reactions?

Thousands of them occur every day. While they may be all unique in one way or another, they can still be categorized into general types.

The following are general descriptions of the transformation that takes place. Every reaction can be categorized into a basic type.

These are the basic types these of chemical reactions:

• Combination/Synthesis Two or more reactants combine to form one new product. General Equation: A + B → AB
• Decomposition A reactant separates into two or more substances. It requires energy. General Equation: AB → A + B
• Single-Replacement One element reactant replaces a similar element in a compound. General Equation: A + BC → AB + C
• Double-Replacement Ions in a compound exchange places to form new ones. General Equation: AB + CD → AD + CB
• Combustion Compounds react with oxygen and produce gas and light. General Equation: CwHx + O 2 (g) → yH 2 O (l) + zCO 2 (g)

What Are Other Types of Chemical Reactions?

Other forms, such as acid-base reactions, are more specific regarding the components, mechanisms, and products.

These are a few examples of other types of chemical reactions:

• Acid-base These are common and essential reactions. It involves the exchange of ions or electrons to produce new compounds. It may also be a form of neutralization reactions (i.e., bases neutralize acids to form salt and water). 
• Precipitation Dissolved reactants form a solid product in precipitation reactions. A precipitation reaction is a kind of double-replacement reaction . Precipitates will develop depending on the solubility properties of reactants. 
• Redox Redox reactions, or oxidation-reduction reactions , involve the transfer of electrons that lead to ionic products. A combustion reaction is an example of a redox reaction. 
• Hydrolysis Hydrolysis reactions use water as one of the reactants. Smaller products are formed from these reactions. 
• Condensation Contrary to hydrolysis, water is a product of condensation. Two substances combine to create a larger compound with water as a byproduct. 

Final Thoughts

Chemical reactions occur because of changes in bond structure – either new ones are formed, old ones are broken, or both. In the process, energy may be released or used. 

There are signs of such reactions, like color or temperature changes, but not all of them have apparent indicators. Each reaction can be categorized into basic or more specific types. 

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Chemistry library

Welcome to the chemistry library, unit 1: atoms, compounds, and ions, unit 2: mass spectrometry, unit 3: chemical reactions and stoichiometry, unit 4: more about chemical reactions, unit 5: electronic structure of atoms, unit 6: periodic table, unit 7: chemical bonds, unit 8: gases and kinetic molecular theory, unit 9: states of matter and intermolecular forces, unit 10: chemical equilibrium, unit 11: acids and bases, unit 12: buffers, titrations, and solubility equilibria, unit 13: thermodynamics, unit 14: redox reactions and electrochemistry, unit 15: kinetics, unit 16: studying for the ap chemistry exam, unit 17: meet the chemistry professional.

Unit 7 - Chemical Reactions

• Due: March 26/27
• Partial Key - make sure you check your answers! 
• Intervention Review Sessions:  TBA
• Assignment #1 - % composition , Empirical vs. molecular formula , Calculations for EF and MF
• Assign #2 -  Oxidation/ Reduction Reactions

Worksheets & Notes

• Unit 7 PowerPoint Slides
• Language of Chemistry Lab
• Chemical vs. Physical Reactions
• Naming and Formula Writing
• Identifying 5 Types of Rxn + Activity
• Mixed Naming & Formula Writing + Sweet 16 Tournament
• Balancing Chemical Reactions (Online Exploration + Practice WS)
• Writing & Predicting Products of a Reaction
• Unit 7 Study Guide     (key)

Online Practice Tests

Naming & Formulas ( online practice ) (choose scatter/ test/ or space race)

       a)   Writing compound formulas  (in real life, you don't leave charges on the ions)        b)   Practice with Polyatomic Ions        c)   Practice with Colvalent Molecules

Phet Online- Balancing Equations ( click here  for site)

Balancing Chemical Equations (Online practice)

Identifying the reaction type (online practice)

Online practice test  (Entire Unit...skip volume of gas questions)

Interesting Links

• Rising Acidity of the Ocean

Day 1: March 6/9

• Finish  Language of Chemistry Lab
• Review Test and clear ESA's
• Worksheet:  Chemical vs. Physical changes
• Finish wkst: Chemical vs. Physical Changes

Day 2: March 10/11

• Notes: Naming and formula writing 
• Wkst:  Naming and Formula writing
• Online Practice game
• Finish Wkst:   Naming and Formula Writing

Day 3: March 12/13

• Notes: 5 types of Chemical reactions
• Wkst:  Identifying 5 Types of Chemical Reactions
• Activity:  Reaction Types
• Finish Wkst:  Identifying 5 Types of Chemical Reactions

Day 4: March 16/17

• Finish Types of Reactions Activity
• Mixed Naming & Formula Writing
• Naming Relay (if time)
• Sweet 16 Tournament

Day 5: March 18/19

• Online Exploration: Balancing Chemical Equations
• U se Phet online balancing   ( click here  for site)...Use HTML 5 for phones/ chromebooks
• Notes: Balancing
• Practice:  Balancing & Types of Reactions WS
• Intro to Activity Series & Solubility Rules
• Handout  Study Guide
• Finish  Balancing & Types of Reactions WS  (back)
• Study guide Due Tues/ Weds 3/24 & 3/25

Day 6: March 20/23

• Review types of rxn activities 
• Notes/Discussion: Predicting Products
• WS-   Writing, Balancing, & Identifying Chemical Reactions (front)
• Single & Double Replacement Activity 
• Honors lab 
• Unit 7 Study Guide     (Key)

Day 7: March 24/25

• Review study guide  Key
• Unit 7 HW packet

Day 8: March 26/27

• Turn in Homework Packet
• Unit 7 Multiple Choice Test

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1.5: Chemical Reactions - Examples of Chemical Change

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Chemical reactions are the processes by which chemicals interact to form new chemicals with different compositions. Simply stated, a chemical reaction is the process where reactants are transformed into products. How chemicals react is dictated by the chemical properties of the element or compound- the ways in which a compound or element undergoes changes in composition.

Chemical reactions are constantly occurring in the world around us; everything from the rusting of an iron fence to the metabolic pathways of a human cell are all examples of chemical reactions. Chemistry is an attempt to classify and better understand these reactions. One key reaction in modern civilization is combustion

A chemical reaction is typically represented by a chemical equation , which represents the change from reactants to products. The left hand side of the equation represents the reactants , while the right hand side represents the products . A typical chemical reaction is written with stoichiometric coefficients, which show the relative amounts of products and reactants involved in the reaction. Each compound is followed by a parenthetical note of the compound’s physical state: \((l)\) for liquid, \((s)\) for solid, \((g)\) for gas. The symbol \((aq)\) is also commonly used in order to represent an aqueous solution, in which compounds are dissolved in water.

Butane is a gas at room temperature and atmospheric pressure and is highly flammable, colorless, easily liquefied gas (under light pressure). Butane can be used for gasoline blending, as a fuel gas, fragrance extraction solvent, either alone or in a mixture with propane, and as a feedstock for the manufacture of ethylene and butadiene, a key ingredient of synthetic rubber. The chemical formula of butane if \(\ce{C4H10}\). When oxygen is plentiful, butane burns to form carbon dioxide and water vapor as observed in modern lighters (Figure \(\PageIndex{1}\)).

This reaction in words is:

\[\text{butane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}\]

and the corresponding chemical equation for this reaction is:

\[\ce{2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g)}\]

In the above chemical equation, \(\ce{C4H10}\) and \(\ce{O2}\) are the reactants that reacted to form the products : \(\ce{CO2}\) and \(\ce{H2O}\),

Writing Chemical Equations

To write an accurate chemical equation, two things must occur:

• Each product and reactant must be written using its chemical formula
• Coefficients are used in front of the chemical formulas to reflect the ratio species (discussed further in a later chapter)
• Adding the phase of each chemical in parentheses (although this is often dropped for convenience)

Key Takeaways

• Chemical reactions are written to represent chemical changes.
• Chemical formulas are used to represent the reactants (starting substance) and the products (the new substances).

Chemical Communications

Synthesis of benzooxepane-fused cyclobutene derivatives via pd-catalyzed cascade reactions of haloarenes and diynylic ethers.

A tandem palladium-catalyzed Sonogashira coupling, propargyl-allenyl isomerization, and [2+2] cycloaddition sequence between electron-deficient haloarenes and 1,8-diynylic ethers is developed. The reaction shows good functional tolerance and proceeds under mild conditions to provide a new profile of benzooxepane-fused cyclobutene derivatives in moderate to high yields with high selectivity. The reaction mechanism is validated both by experimental studies and DFT calculations.

Supplementary files

• Supplementary information PDF (9948K)
• Crystal structure data CIF (830K)

Article information

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S. Zhu, W. Huang, S. Liu, R. Yu, Y. Ma, H. Wang, R. Zhang, B. Liu, Y. Lan and R. Shen, Chem. Commun. , 2024, Accepted Manuscript , DOI: 10.1039/D4CC00999A

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