Reading 6.1 – Chemical Reactions

What Is a Chemical Reaction?

Imagine you are baking a cake. First you combine flour,

sugar, eggs, butter, and maybe some chocolate. You

pour the batter into a cake pan and put it in the oven.

When you take it out, the liquid batter has turned into a

solid, fluffy cake. Do you think this process represents a chemical change or a physical

change? In the space below, explain why you think baking a cake is a physical or a chemical

change.

We can also call chemical changes ‘Chemical Reactions.’ A chemical reaction occurs when

two or more substances interact to break and/or form chemical bonds to produce a new

substance. As we study chemical reactions you will learn different ways in which

substances can interact, and how to tell if a change like baking a cake is truly a chemical

reaction.

Signs of a Chemical ReactionScientists observing chemical interactions look for several telltale signs that a reaction has

occurred. There are 4 main signs that a chemical reaction may have occurred:

An unexplained color change – Think about when you added Iodine

to powder C. The iodine was brown, the powder was white, but the resulting

substance was dark purple/black. This color change can’t be explained by the

colors of the reactants, so it indicates that a chemical change has likely taken

place.

An unexplained temperature change – Because of the

breaking/formation of chemical bonds, chemical reactions often cause a

temperature change. Reactions that give off energy (temperature increase) are

called exothermic. Reactions that absorb energy from their surroundings

(temperature decrease) are called endothermic.

Production of a gas (aka bubbles) - one of the new substances

produced by a chemical reaction might be a gas. This will show up as fizzing

or bubbling. A classic example of this is vinegar and baking soda, which react

to form Carbon Dioxide gas.

Formation of a Precipitate – This is when a new solid substance is

produced by a chemical reaction. Of all the signs of a reaction, this is the one

you’re least likely to have seen in your everyday life. Keep an eye out for

Precipitates as we continue our unit!

In the space below, describe the signs of a reaction you observed during our calcium

chloride experiment:

Changes in PropertiesAlthough these signs of a chemical reaction are helpful, they are not always reliable.

Opening a can of soda produces bubbles, but it’s not a chemical reaction. Heating

something up causes a temperature change, but it’s not a chemical reaction. The best way to

know if a reaction has occurred or not is to test whether a new and different substance has

been created. And recall that we use properties of matter to tell the difference between two

substances. Remember, a reactant is an ingredient that goes INTO a chemical reaction. A

product is what comes out of a chemical reaction. You can test the properties of the

reactants and products in a chemical reaction, just as we tested the properties of our

mystery powders. If the products have different properties from the reactants, a

chemical reaction has occurred. If you look at the data table you made during our

calcium chloride experiment, you will find conclusive evidence that a new substance was

created when a chemical reaction occurred.

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What Happens to the Molecules?All of the information about chemical reactions that we have discussed so far is information

you can observe in the lab. If you use signs and properties to determine whether a chemical

reaction has occurred, you don’t have to know anything about atoms or molecules. But to

truly understand a chemical reaction you need to know what’s going on at the molecular

level.

When a chemical reaction occurs, chemical bonds between atoms are broken and/or

formed. This is what makes a chemical reaction different from a mixture or a phase

change. If you mix kool-aid powder into water to make a tasty drink, the sugar molecules

and the water molecules don’t break apart, they just hang out next to each other. If you

separated them again, they would still be the same substances you started with. If you heat

up water until it enters the gas phase, the water molecules become farther apart, and they

gain energy. But the hydrogen atoms and oxygen atoms stay bonded together.

Boiling water and water electrolysis both produce bubbles of clear gas. But if you could see

what was happening at the molecular level, you would know that one was a chemical

reaction and one was not. Look at the diagram below for a visual representation of both.

Now that you have some ideas about how to tell if a chemical reaction has occurred, fill in

the box below, answering our original question. Is baking a cake a chemical reaction?

Give two reasons why or why not.

When water is heated, the molecules move away from each other, but the chemical bonds stay intact

Electrolysis breaks the bonds between the hydrogen and oxygen atoms. The oxygen atoms then bond with one another to create O2 molecules, and the Hydrogen atoms bond to create H2 molecules.

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Reading 6.2 Writing Chemical EquationsWhen you write about a reaction, you are writing a chemical equation. A chemical equation is a way to represent chemical reactions. The equations include plus (+) signs and an arrow. Notice that the reactants side of this chemical reaction has a plus (+) sign. In a chemical equation, the plus sign means that the reactants are interacting. The arrow could be though of as meaning “changes to”. This chemical equation tells more about the atoms and molecules of each substance. You can see the number of atoms and the types of atoms. You can see the number of molecules and the types of molecules. You can see which atoms and molecules react and what they form. A chemical equation can help you understand what is happening to the old substances and new substances in a chemical reaction. The following is the chemical equation for burning magnesium in sparkler:

2 Mg + O2 2MgO

Notice that there are the same number of magnesium atoms on the reactant side and the product side (2 magnesium atoms on each side). And there are 2 oxygen atoms on each side. That is because a chemical reaction never destroys or creates new matter, it just rearranges the atoms in a new way.CHEMICAL EQUATION PRACTICE In the space below each chemical equation, write how many of each substance there are. 1. Chemical equation for rust (Helpful Info: Fe=iro O2=oxygen FeO3=Iron Oxide (rust))

2Fe + 3O2 2Fe2O3

2 atoms ofthe element magnesium

1 molecule of oxygen

2 molecules of magnesium oxide

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2) Chemical equation for burning methane gas (Helpful Info: CH4=methane)

CH4 + O2 CO2 + 2H2O

3) Химическое уравнение для лаборатории хлористого кальция, которую мы сделали в классе. (Полезная информация: NaHCO3 = пищевая сода CaCl2 = хлорид кальция NaCl = поваренная соль CaCO3 = мел)

2NaHCO3 + CaCl2 2NaCl + CaCO3 + CO2 +

H2O

3) This is the chemical equation for photosynthesis. (Helpful info: C6H12O6 is glucose, the sugar plants use for food)

6CO2 + 6H2O C6H12O6 + 6O2 Use the words to explain what the equation is showing.

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Representing Chemical Equations

You can describe what’s happening in a chemical equation 3 ways. The first is in words. This is what you practiced on the previous page. The second is in chemical formula, and the third is visually. It can be helpful to use a visual of what’s happening in the relationship because it allows you to visualize what is happening with each of the molecules as the reaction proceeds. Let’s practice writing the reaction that took place in your copper in all three ways.

Word Formula

Chemical Equation

Visual Model

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Reading 7.1- Reaction Types Synthesis Reactions - Chemists divide all of the different ways that chemicals react into three main categories, based on what the atoms do during the reaction. The first type of chemical reaction we will study is called a Synthesis Reaction. Synthesis comes from the greek word meaning ‘to put together’ or ‘to combine.’ This tells you that a synthesis reaction means when two molecules are put together/chemically bonded into a new molecule. In a synthesis reaction, the product is always a larger, more complex molecule than either of the reactants. Because chemical bonds are formed in this reaction, which releases energy, synthesis reactions are often EXOTHERMIC.

You encounter synthesis reactions on a daily basis. Rust is a synthesis reaction where iron combines with oxygen atoms from the air to form Iron Oxide. We have discussed the formation of salt from Sodium and Chlorine, this is a synthesis reaction too.

Word

FormulaHydrogen Gas + Oxygen Gas Water

Chemical

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Equation2H2 + O2 2 H2O

Visual Model

Decomposition Reactions - Hydrogen peroxide is a widely used disinfectant for minor cuts and scrapes. You may have a bottle of hydrogen peroxide in your medicine cabinet at home. Did you ever notice that hydrogen peroxide is always kept in a dark brown bottle? Do you know why? The reason is that hydrogen peroxide needs to be protected from light. If it isn’t, it will gradually break down to water and oxygen. This is a decomposition reaction. 

You can think of decomposition reactions as the opposite of synthesis reactions. Decomposition means to break down, and that’s what a decomposition reaction does. The products of a decomposition reaction are always less complex than the reactants because decomposition reactions break chemical bonds to create two or more substances from one original substance. Because chemical bonds are broken in this reaction, and that takes energy, decomposition reactions are often ENDOTHERMIC, meaning they absorb heat from their surroundings in order to proceed.

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Word

Formula

Silver Chloride Silver Metal + Chlorine

Gas

Chemical

Equation2AgCl2 Ag2 + 2Cl2

Visual Model

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Replacement Reactions - In some reactions, portions of the reactants and the products switch places. This involves breaking molecules down and putting them back together a different way, and these types of reactions are called replacement reactions.

Single ReplacementIf only one element switches place in the reaction, we call it a ‘single replacement reaction.’ A single replacement reaction might look like this The number of reactants is the same as the number of products, but the atoms have been rearranged.

An example of a real-world single replacement reaction is Hydrochloric acid and zinc, which react to form zinc chloride and hydrogen gas, as shown in the table . The chlorine changes

partners, breaking apart from the hydrogen and bonding with the zinc.

Double ReplacementIf more than one element switches place, we call the reaction a double-displacement reaction. Again, we usually have the same number of products and reactants, but multiple parts of the molecules have been switched around.

A real world example of a double replacement reaction is Sodium Hydroxide and Hydrochloric Acid

Word formula 1 Sodium Hydroxide + 1 Hydrochloric Acid 1 water +1 salt

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Word Formula

Hydrochloric Acid + Zinc Metal Zinc Chloride + Hydrogen Gas

Chemical

Equation2HCl + Zn ZnCl2 + H2

Visual Model

Chemical Equation

NaOH + HCl H2O + NaCl

Visual Model

Reading 8.1 - The Law of Conservation of MassCan Matter Disappear? In the box below, record your thinking. When a puddle dries up, where does

the water go?

Matter may change in many ways. A solid may melt into a liquid. Elements may react together to form compounds. What happens to the mass

of matter in a bowl of water when it is left to stand in the hot sun? What happens to the mass of matter in a piece of paper when it is burned? Sometimes in situations like this it seems as if matter is disappearing. But the disappearance of matter is an illusion. 

Matter may change from one form into another. For example, when the water in the bowl absorbs energy from the sun and evaporates, it becomes water vapor in the atmosphere. The piece of paper gives off heat and light energy as it burns, and the matter in it is converted into carbon dioxide, water vapor, and other gases that escape into the atmosphere. Some of the mass will remain behind as ash. In both cases, the matter changes its form, but its total mass stays the same. The same mass of each element is present before and after the change. Matter is neither created nor destroyed during these changes.How do we know that matter can’t be created or destroyed?

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It took early scientists hundreds of years of scientific study before the law of conservation of mass became accepted. For a long time, scientists had suspected that matter could not be created or destroyed, but nobody had performed an experiment that proved it.During the late 18th century, French chemist Antoine Lavoisier and his wife Marie-Anne conducted several experiments that demonstrated the conservation of mass. Antoine was famous for his accurate observations and insistence on careful measurements. He used accurate balances that could measure very small changes in mass during his experiments. Many of the Lavoisiers' experiments were conducted in sealed glass containers from which matter could not escape or enter. For example, in one experiment, Antoine put fruit into a sealed container, measured its mass, and then left it in a warm place for a few days. The fruit rotted and changed into a putrid mess. Gas was released from the decomposing fruit and droplets of water formed on the glass, but nothing escaped from the container. Lots of changes had taken place, but the mass of the sealed container and the rotten fruit was equal to the mass measured at the beginning of the experiment.

In other experiments, Antoine heated elements in enclosed containers with air inside them. He discovered that new substances were formed but that the container and its contents had the same mass as they did before heating. When he measured the mass of the new solid substances he had made, he discovered that they were heavier than the original elements he heated. In this way, he determined that they must have gained their mass from the air. On the basis of these experiments, he also concluded that air contained several gases, one of which reacted with the elements in the experiment. He called this gas oxygen (which had previously been discovered and described—but not named—by Carl Wilhelm Scheele and by Joseph Priestley).

In 1789, Antoine wrote the best textbook on chemistry the world had seen. In it, he introduced a new scientific law that he called the law of conservation of mass. This law stated that in any closed system (as small as a sealed container or as big as the whole universe!) the total mass remains the same, regardless of what changes take place inside. In other words, matter can not be created or destroyed, only

changed .

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Conservation of Mass and Balancing EquationsThe law of conservation of mass influences our knowledge of our entire universe. It also has to influence the way that we write equations to represent chemical reactions. You may have noticed that sometimes numbers get added when we transition from word formulas to chemical formulas. For example, ‘Hydrogen Peroxide Water + Oxygen gas’ becomes 2H2O2 2H2O +O2. Where did those two’s come from? Why did we end up with 2 water molecules but only one oxygen molecule?

When we write a reaction, we must write it so that it follows the law of conservation of mass. That means that however many atoms we start with on the reactants side, we must end with that number on the products side. Otherwise it might seem like we’ve created or destroyed atoms, which Antoine Lavoisier proved does not happen. When a reaction is written with an equal number of atoms on the right as on the left, we say that it is ‘balanced.’ Look at the reactions below. Count the atoms on either side. Are they balanced or not balanced? 1. 2Na + I2 2NaI Balanced? ___________________

2. KClO3 KCl + O2 Balanced? __________________

3. Zn + HCl ZnCl + H2 Balanced? _________________

Once you know an equation is not balanced, THINK – what numbers could you add to

balance it? YOU CAN ONLY ADD COEFFICIENTS, NEVER SUBSCRIPTS.

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