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Chemical Reactions

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Chemistry

Have you ever tried to make soap at home? The art of soap-making is a type of chemical reaction! First, look at the structure of a soap molecule.

Chemical reactions Structure of Soap StudySmarterSoap molecule structure, Isadora Santos - StudySmarter

Soap is a type of salt made up of fatty acids. The structure of soap consists of a nonpolar hydrocarbon chain containing a carboxylic acid group. The carboxylic acid group forms an ionic bond with a sodium or potassium ion. So, soap has a hydrophobic end that does not dissolve in water (hydrocarbon chain) and a hydrophilic end (ionic salt). When soap comes into contact with grease and water, the hydrophobic end of the soap interacts with the nonpolar molecules of grease and suspends the grease/oil, allowing it to be washed away by water!

Soap forms by the reaction between the triglycerides in fat/oil and an alkali such as aqueous sodium hydroxide or potassium hydroxide.

The chemical reaction for the making of soap is:

Chemical reactions saponification chemical rections examples StudySmarterThe saponification process, Isadora Santos - StudySmarter

  • First, we will talk about chemical reactions and how to balance them.
  • Then, we will look at the different types of chemical reactions and give some examples.
  • We will also learn about oxidation numbers and how to calculate them!

Definition of a Chemical Reaction

During a chemical reaction, different substances combine to form a new substance. When this happens, we can see changes in color, gas formation, temperature changes, or even the formation of a precipitate. Or, in some cases, the result looks the same as before.

A chemical reaction is a process that results in the interconversion of chemical species.

We can use chemical equations to show what happens during a chemical reaction. Chemicals equations are divided into two sides: the reactant side and the product side.

Reactants are the chemicals on the left side of the equation. These are the chemicals that will react with each other to yield the product.

Products are the chemicals on the right side of the equation. Products are formed by the reaction that occurs between the reactants.

For example, take photosynthesis, which is the process plants, algae, and some bacteria use solar energy to make chemical energy and store it in molecules. Six moles of carbon dioxide and six moles of water (reactants) react to make one mole of glucose and six moles of oxygen as products. Sunlight is what provides the energy for plant chloroplasts to make the simple sugar, glucose (food).

The chemical equation showing what occurs during photosynthesis is:

Let's look at an example!

In a chemical reaction, carbon monoxide (CO) and oxygen (O2) react together to make Carbon dioxide (CO2). Which substances are the reactants, and which are considered the products? And why is this a chemical reaction?

If CO and O2 are the starting materials, meaning that they are used to form CO2, then we can say that they are both reactants while CO2 is the product. This is a chemical reaction and not a physical one because we create a new type of molecule ( CO2).

Balancing Chemical Equations

When dealing with chemical reactions, there is a skill that you need to learn. That skill is knowing how to balance chemical equations. When balancing chemical equations, it is essential to follow the law of conservation of mass and the law of conservation of charge.

  • The law of conservation of mass states that the mass of products is equal to the mass of the reactants.
  • The law of conservation of charges states that the charge of products is equal to the charge of reactants.

A balanced chemical equation is an equation where the number of moles of each element and the charges on the reactant side are the same as the number of moles and charges on the product side.

The ability to balance equations comes with patience and a lot of practice! Lucky for us, there are some steps that we can follow to tackle different problems involving balancing equations! Let's pretend that we need to balance an equation involving the reaction between nitrogen (N2) and hydrogen (H2) to yield ammonia (NH3).

Steps to Balance Chemical Equations

Here are the steps to balance chemical equations.

Step 1: Write down the unbalanced chemical equation.

N2 and H2 are the reactants in this chemical reaction, and NH3 is the product. So, the unbalanced chemical equation would be:

Step 2: Count the number of moles of each element present on both sides of the equation.

To find the number of moles, we must multiply the coefficient by the number of elements given by the subscript. If no coefficient is shown, then assume the coefficient is 1. We usually don't write the coefficient unless the coefficient is greater than 2, but to make it simpler to understand, let's add it for now.

Coefficients are the numbers that are written before the chemical symbol.

On the reactant side, we have 2 moles of nitrogen and 2 moles of hydrogen:

On the product side, we have 1 mole of nitrogen and 3 moles of hydrogen:

Step 3: Look for multiples of the coefficient to balance the chemical equation.

So, we need to find a coefficient that will make the number of moles of each element to be equal on both sides.

If I add a coefficient of 2 on the product side, then that would change the number of moles on the product side to 2 moles of nitrogen and 6 moles of hydrogen! However, the reactant side would still have 2 moles of nitrogen and 2 moles of hydrogen.

If we add a coefficient of 3 to the hydrogen on the reactant side, that would change the number of moles on the product side to 2 moles of nitrogen and 6 moles of hydrogen!

So, this would make the number of moles of each element on both sides equal, so our reaction is balanced!

Reactant side:

Product side:

Types of Chemical Reactions and Examples

Now that you know the definition of a chemical reaction and how to balance chemical equations, let's dive into the different types of chemical reactions that you might encounter in your chemistry journey!

The essential types of chemical reactions include:

  • Synthesis reactions (also called combination reactions)
  • Decomposition reactions
  • Single and double replacement reactions
  • Combustion reactions

  • Redox reactions
  • Acid-base reactions (also called neutralization reactions)
  • Precipitation reactions

We will talk about the general difference between these types of reactions. However, for your AP exam, you should focus more on the last three types of reaction: redox, acid-base, and precipitation.

Synthesis Reactions

Synthesis reactions are chemical reactions that occur when two or more reactants produce a single product.

The general formula for synthesis reactions is: A + B → AB.

Decomposition Reactions

Decomposition reactions are the opposite of synthesis reactions.

So, instead of two substances reacting to form one product, decomposition reactions are chemical reactions where one reactant is broken down to form many products. In this type of reaction, the general formula is: AB → A + B.

Replacement Reactions

Single replacement reactions are chemical reactions where the element by itself replaces one of the elements in the compound.

The general formula for single replacement reactions is: A + BC → AC + B.

Double replacement reactions are chemical reactions that occur in an aqueous solution.

In this type of reaction, the cation (positive ions) and anion (negative ions) of one compound switch places with the cation and anion of the other compound to yield two different products. The general formula for double replacement reactions is:

AB + CD → AC + BD

Combustion Reactions

In a combustion reaction, oxygen (O2) will react with any chemical to form carbon dioxide (CO2) and water (H2O). The general formula is A + O2 → CO2 + H2O in combustion reactions.

Up until now, the types of chemical reactions that we described were simple. Now, let's look at the types of chemical reactions that are a little more complex and commonly appear in the AP exam!

Oxidation-Reduction (Redox) Reactions

In redox reactions, there is a transfer of electrons that happens between the reactants involved in the reaction. Combustion reactions are actually a type of oxidation-reduction reaction!

There is a pretty cool mnemonic that can help you remember what occurs in redox reactions!

OIL RIGOxidation Is Loss, Reduction Is Gain

This means that the element that loses electrons will be oxidized, while the element that gains electrons will be reduced. Elements that undergo oxidation will increase their oxidation number and become more positively charged. Elements that are reduced will decrease their oxidation number and become more negatively charged.

Oxidation numbers can tell us which chemicals have lost or gained electrons in a chemical reaction. The oxidation number of an atom shows you what would be the charge of an atom in a special case. We give both electrons of an electron pair (covalent bond) to the more electron negative atom.

For example, when we look at the structure of CO2, we notice that both oxygen atoms have double bonds with carbon. So, this means that both oxygen atoms will have two extra electrons than what you would expect them to have. On the other hand, carbon has four electrons less, giving it a +4 oxidation state.

Chemical reactions CO2 structure StudySmarterStructure of Carbon Dioxide, Isadora Santos - StudySmarter

Calculating Oxidation Numbers

To find the oxidation number, we can follow some general rules:

  1. First, notice whether the ion or compound has a charge.

  2. Then, look for atoms with set oxidation numbers. If any, write these oxidation numbers above the atoms.

  3. Finally, calculate the oxidation state for the remaining atom. The sum of all oxidation numbers should be equal to the charge of the ion or compound. If a compound is neutral, then it should be equal to zero.

Assigning Oxidation Numbers

When assigning fixed oxidation numbers, there are some common rules we need to follow.

  • Metals in group I have an oxidation number of +1
  • Metals in group II have an oxidation number of +2
  • Al has an oxidation number of +3
  • F has an oxidation number of +1
  • With the except of metal hydrides, H has an oxidation number of +1
  • Cl usually has a -1 oxidation number, except when present in compounds containing O or F.
  • O has an oxidation number of -2, except in peroxides and compounds containing F.

Let's solve an example!

Calculate the oxidation number of chromium (Cr) in the dichromate ion (Cr2O72-).

Does the ion have a charge? Yes, it has a charge of -2. So, keep in mind that the sum of all oxidation numbers will be equal to -2.

Do any of these atoms have a fixed oxidation number? Yes. In this case, oxygen has an oxidation number of -2.

Now, solve for the oxidation number of Chromium:

Electronegativity

Electronegativity increases from left to right in the periodic table and decreases down a group. Notice that noble gases don't even appear in the image below. They do not bond, so they do not have electronegativity values. Electronegativity also increases with the increase in effective nuclear charge.

Electronegativity is the ability of an atom to attract a shared pair of electrons.

Acid-base Reactions

In an acid-base reaction (also known as a neutralization reaction), protons transfer between the chemical species in the chemical reaction. When an acid and a base react together, they form salt and water as products: acid + base → water + salt. Acid-base reactions are considered a type of double displacement reaction!

When weak acids and bases react with each other, equilibrium is reached. At this point, the reactants and products are reacting at the same rate, and the reaction becomes reversible.

When a strong acid and a strong base reaction occurs, the reaction is not considered reversible because the acids dissociate completely during the chemical reaction.

If you want to learn more about acid-base reactions, check out the article "Acid-Base Reactions."

What happens when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH)?

HCl is a strong acid, and NaOH is a weak acid. When these two reactants are allowed to react, they dissociate entirely and form sodium chloride (NaCl) and water (H2O).

Precipitation Reactions

Last but not least, let's talk about precipitation reactions. As the name suggests, precipitation reactions form a precipitate (solid) as the product. A precipitate is defined as an insoluble substance that separates itself from the solution.

What happens when you add silver nitrate (AgNO3) to aqueous sodium chloride (NaCl)?

When AgNO3 reacts with aqueous NaCl, the products formed are AgCl (a white precipitate) and aqueous NaNO3

Chemical reactions happen all around us and now that you know about them, next time you wash your hands or stare at some wood burning in a fireplace, think about the cool types of chemical reactions happening there!

Chemical Reactions - Key takeaways

  • A chemical reaction is when different chemicals are combined to produce a new substance.
  • Chemical reactions have a product and a reactant side.
  • When balancing chemical equations, make sure that the number of moles on each element on the reactant side is the same as the number of moles on the product side.
  • The three types of chemical reactions that you should be familiar with are redox reactions, acid-base reactions, and precipitation reactions.

References

  1. Arbuckle, D., & Albert.io., The Ultimate Study Guide to AP® Chemistry, 1 March 2022
  2. Moore, J. T., & Langley, R. (2021). McGraw Hill: AP Chemistry, 2022
  3. Malone, L. J., Dolter, T. O., & Gentemann, S., Basic concepts of Chemistry (8th ed.), 2013
  4. Swanson, J. W., Everything you need to Ace Chemistry in one big fat notebook, 2020

Frequently Asked Questions about Chemical Reactions

During a chemical reaction, different substances combine to form a new substance. When this happens, we see changes in color, gas formation, temperature changes, or even the formation of a precipitate.

Increasing temperature, concentration, and surface area will increase the rate of a chemical reaction. 

A chemical reaction is when different chemicals are combined to produce a new substance.

Enzymes are catalysts that work by increasing the rate of a reaction. They achieve this by lowering the activation energy needed to start a reaction.

In a chemical reaction, reactants are the chemicals on the left side of the equation. Reactants will react with each other to form the product.

Final Chemical Reactions Quiz

Question

A _______ is a process where different chemicals are combined to produce a new substance.

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Answer

Chemical reaction

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Question

True or false: chemical reactions consist of reactants and products.

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Answer

True

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Question

____ are the chemicals on the left side of the chemical equation.

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Answer

Reactants

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____ are the chemicals on the right side of the equation.

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Answer

Products

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Question

True or false: The law of conservation of mass states that the mass of products is inversely proportional to the mass of the reactants.

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Answer

False

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Question

A ________ is an equation where the number of moles of each element on the reactant side is the same as the number of moles on the product side.

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Answer

Balanced chemical equation

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Question

The steps for balancing chemical equations include the all of the following except:

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Answer

Write down the unbalanced chemical equation.

Show question

Question

When balancing a chemical equation, how can we find the number of moles of each element?

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Answer

You multiply the coefficient by the number of elements given by the subscript.

Show question

Question

True or false:  When the number of moles of each element on both sides is equal, then the reaction is balanced.

Show answer

Answer

True

Show question

Question

Types of Chemical Reactions

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Answer

  • Synthesis reactions (also called combination reactions)
  • Decomposition reactions
  • Single and double replacement reactions
  • Combustion reactions 

  • Redox reactions
  • Acid-base reactions (also called neutralization reactions)
  • Precipitation reactions

Show question

Question

Chemical reactions where two or more reactants produce a single product are called ______ reactions.

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Answer

Synthesis

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Question

______ reactions are chemical reactions where one reactant is broken down to form many products.

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Answer

Decomposition

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Question

______ are chemical reactions where the element by itself replaces one of the elements in the compound

Show answer

Answer

Single replacement reactions

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Question

_______ reactions are chemical reactions that occur in an aqueous solution. In this type of reaction, the cation (positive ions) and anion (negative ions) of one compound switch places with the cation and anion of the other compound to yield two different products.

Show answer

Answer

Double replacement

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Question

Which type of reaction has the general formula:  A + O2 → CO2 + H2O

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Answer

Combustion reaction

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True or false: combustion reactions are considered a type of redox reaction.

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Answer

True

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Question

In ______ reactions, there is a transfer of electrons that happens between the reactants involved in the reaction.

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Answer

 Redox

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OIL RIG means:

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Answer

 Oxidation Is Loss, Reduction Is Gain

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An element that loses electrons will be _____ , while an element that gains electrons will be _____.

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Answer

Oxidized; Reduced

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Elements that are oxidized will have their oxidation numbers _____. 

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Answer

increased

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Elements that are reduced will have their oxidation numbers _____.

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Answer

decreased

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What are oxidation numbers?

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Answer

The specific charges on an element that tells whether an element is able to gain, lose or share electrons.

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The ______ is the element is reduced. The ______ is the element that is oxidized.

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Answer

Oxidizing agent; Reducing agent

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True or false: When an acid and a base react together, they form salt and water as products: acid + base → water + salt.

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Answer

True

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Precipitation reactions form a _____ as the product.

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Answer

Precipitate (solid)

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A _____ is defined as an insoluble substance that separates itself from the solution.

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Precipitate

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What type of ion do acids produce?

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Answer

Hydronium, H+

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What type of ion is produced by alkalis?

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Answer

Hydroxide ion, OH-

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​​​​​What colour does litmus turn upon contact with an acid?


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Answer

red

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What are the products of a neutralisation reaction?

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Answer

Salt and Water

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What type of oxides are basic oxides and why?

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Answer

Metal oxides are basic, as they produce hydroxide ions when dissolved.

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What type of ion is produced when non-metal oxides are dissolved?

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Answer

 H+ ions

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At what pH is a solution acidic?

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Answer

below 7

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What is the pH of water?

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Answer

pH = 7

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If a pH meter gives a reading of 10, is the solution alkaline or acidic?

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Acidic

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What is the formula for neutralisation reactions?

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Answer

alkali + acid => salt + water

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What is the formula for the reaction of acids with carbonates?

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Answer

acid + carbonate => salt + water + CO2

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How can you determine the concentration of an acid if you have a known concentration of an alkali?

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Answer

Titrate the alkali into the acid and stop at the neutralisation point. 

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Bases and alkalis are an interchangable term. 

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Answer

True

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What salt would be produced in the reaction of NaOH with HCl?

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Answer

NaOH

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Would water be created in the reaction of KOH with HCl?

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Answer

Yes! 

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What is the definition of oxidation in terms of electrons? 

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Answer

Loss of electrons

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What is the definition of reduction in terms of electrons? 

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Gain of electrons

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What is the definition of oxidation in terms of oxygen? 


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Answer

Gain of oxygen

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What is the definition of reduction in terms of oxygen? 


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Answer

Loss of oxygen

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What happens to the oxidation state during oxidation?

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Answer

Oxidation state decreases

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What happens to the oxidation state during oxidation?

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Answer

Oxidation state decreases

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Question

Which one of the following is a reducing agent?

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Answer

dichromate

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If you want a specific element to lose electrons, would you use an oxidising agent or reducing agent?

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Answer

Oxidising agent?

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Question

What will happen to an element if you add a reducing agent to it?

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Answer

It will either lose oxygen or gain electrons. 

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