Chemical Equations and

Reactions

Student Success Statements

× 8.5E investigate how evidence of chemical reactions indicates that new substances with different properties are formed and how that relates to the law of conservation of mass.

× Students will be able to: × Determine if a chemical change has occurred by looking for one

or more of the evidences of a chemical reaction× Identify that a new substance exists after a chemical reaction

has occurred × Understand that the law of conservation of mass is

demonstrated in a chemical reaction and that this can be shown in a chemical reaction equation

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Chemical Formulas – Reminders…

× A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of the substance

× For example, we don’t usually write the formula for water as 32H2O no matter how many molecules there are, but instead use the smallest amount to represent it on the page, which is simply H2O

× Therefor, the formulas show which elements, the number of elements and the number of atoms

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What they don’t show:

× If we look at NH3, it does show how many atoms and elements and the ratio that make up that molecule.

× What a formula doesn’t show is the structure or shape of the substance

So why do we use the formula?

× There are many reasons, but the best reason is to demonstrate on paper, chemical reactions!

× Chemical reactions are shown with chemical equations.

× Learning how to “read” an equation is very important in chemistry.

How chemical reactions are shown

× Chemical reactions are shown by scientists as equations× Here is one you are familiar with:

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Chemical Equations

× Example 1: Rust is the result of a chemical reaction between iron and oxygen, otherwise known as iron oxide

× Equation:

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Chemical Equations

× Example 2: burning wood -in wood combustion we observe solid wood disappear and be converted to gas products, leaving only some ashes

× C6H10O5 + 6O2 → 6CO2 + 5H2O

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What happens during a chemical reaction?

× Notice in these examples, that a new substance is made after the reaction!

× First, bonds in molecules of the reactants are broken.

× Then, new bonds are made and this results in new substances, we call these products

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A closer look at chemical equations

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Chemical Equations

× A chemical equation is a symbolic representation of a chemical reaction

Elemental

Symbol –

tells what

elements

Subscript and

coefficients tell the

number of them

× Each side of an equation represents a combination of chemicals

× The combination is written as a set of chemical formulas, separated by + symbols to show that they are chemically combined.

CH4 + 2 O2 → CO2 + 2 H2O

Chemical Equations

× The two sides of the equation are separated by an arrow× On the left side – is the combination

of chemicals before the reaction× On the right side - the combination of

chemicals after the reaction

Chemical Equations

Language of Chemical Equations

× In this reaction, sodium (Na) and oxygen (O2) react to produce a single molecule, Na2O (this together is known as “sodium oxide”

4Na + O2 → 2Na2O

Reactants Products

Yields

Reactants

Arrow

(yields)

Products

Law of Conservation of Mass

× In this case the word “conservation” doesn’t have anything to do with the environment

× Rather, it means to “conserve” or keep something

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Law of Conservation of Mass

× Math equations – 6 - 2 = 4× See how it is equal or balanced?× The same is true for chemical equations× 4Na + O2 → 2Na2O × The arrow can be seen as the same as

the equals sign in math equation

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Law of Conservation of Mass

× Why are chemical equations written this way?

× It is simply a way of demonstrating a law of nature – the law of conservation of mass

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Law of Conservation - Explained

× This law states that during a chemical reaction, matter cannot be created nor destroyed.

× The balancing of a chemical reaction shows the atoms that are rearranged or recombined, making new substances that were not previously there until these particular atoms interacted.

× Balancing equations just manipulates the coefficients in front of the formulas, until the numbers of atoms on both sides are equal.

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Conservation of Mass and/or Matter

What this means:× 1. Matter is conserved → type of atoms does

not change (in the entire equation)× Nothing is created or destroyed

× 2. Mass is conserved → amount of atoms cannot change (in the entire equation)× Nothing is created or destroyed

Important –

highlight this in

your notes!

Important –

highlight this in

your notes!

Law of Conservation in “Action”

× Here is an equation:× 2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

× Just FYI – this is potassium nitrate combined with carbonic acid to make potassium carbonate and nitric acid

× Matter is conserved from the left to the right side – how do I know? Look at the elements! Are they different elements or the same ones?

Law of Conservation in “Action”

× Again the reaction: × 2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

× Mass is conserved from the left to the right side – how do I know?

Law of Conservation in “Action”

× 2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

× What elements are on the left side ofthe equation?

× Now look at the right side…

Law of Conservation in “Action”

× 2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

× Now count the number of atoms per element

Law of Conservation in “Action”

2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

2K 2H 2K 2H2N 1C 1C 2N6O 3O 3O 6O

× The elements, the number of each element and the overall number of atoms is the same on each side of the equation –this demonstrates the law of conservation!

Check your chart!

2 KNO3 + H2CO3 → K2CO3 + 2 HNO3

2K 2H 2K 2H

2N 1C 1C 2N

6O 3O 3O 6O

Conservation of MATTER

× Again the law states that matter is neither lost or gained in chemical reactions – it just changes form

× This can be measured by determining the mass in a reaction.

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Determining mass

× 88g27

Write in the table

on your page!

Finding the missing amount:

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To Summarize:

▪ Elements can combine to form new substances

▪ Substances can be broken down into simpler substances (down to the elemental level)

▪ Atoms are re-arranged within the equation; NOT created or destroyed

Law of Conservation of Mass

× Video link: https://www.youtube.com/watch?v=2S6e11NBwiw

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How do we know that a chemical reaction

has caused a chemical change?

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Evidences of a Chemical Reaction

× Gas Production – gas is given off as part of a chemical reaction

× Can be seen as bubbles if itoccurs in a liquid

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Evidences of a Chemical Reaction

× Change in Temperature

× Temperature changes can be either an increase or a decrease in temperature

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Exothermic and Endothermic Reactions

× An exothermic reaction is one which produces heat as chemical bond energy is converted into thermal energy (heat).

× An endothermic reaction absorbs heat from its environment. A hallmark of this type of reaction is that it feels cold.

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Energy Changes in Reactions Handout:

× You are expected to read this and know it!

In graph form:

× A few symbols to understand 1. H = change in heat

× 2. KJ = kilojoules

(Time that reaction occurs)

In graph form:

Evidences of a Chemical Reaction

× Light is made during or after a reaction

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Evidences of a Chemical Reaction

× Precipitate forms

× A precipitate iswhen a solid iscreated from acombination ofliquids.

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Evidences of a Chemical Reaction

× Unexpected change in color

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Evidences of a Chemical Reaction

× Smells or odors can be given off when a chemical reaction forms vapor

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Evidences of a Chemical Reaction

× Sound (not as commonly used as an evidence)

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Chemical Reactions Video

× Link: https://www.youtube.com/watch?v=cZMkqagL8Ps

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Stations work

× Chemical reactions in action!× Each station may show a chemical reaction (4 stations)× Warning – you are working with potentially dangerous

chemicals× Follow the instructions at the station, and record your

observations on your page× Answer any questions as well – wait for the timer to

move to the next station44