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Chemistry Review: Chemical Change

Powder particulars

A glass of carbonated soda
  • If you look at baking soda and baking powder, they look so similar that it’s hard to tell them apart. If you add them to vinegar they both form bubbles in a chemical reaction, but the bubbling looks different.
  • How can chemical reactions help you tell the difference between similar looking substances?

Testing baking soda and baking powder

This video shows how you can differentiate between baking soda and baking powder by combing each with water.

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  • One way to tell similar looking substances apart is by the way they react chemically with another substance.
  • For example, if you test baking soda and baking powder with water and they each react differently, then you know they are different substances.

Chemical reaction

  • When vinegar (liquid) was added to both powders (solid), a gas was produced. Since something new was made, a chemical reaction occurred.
  • A chemical reaction happens when one or more substances change into entirely new substances with different properties.
  • How do we know if a chemical reaction has happened?
  • There are different clues that a chemical reaction has occurred: (1) a change in color, (2) a gas is produced, (3) a change in temperature, (4) a precipitate is formed.

Ionic bonding

An atom of sodium transfers its valence electron to an atom of chlorine.  The resulting ions attract one another.  This is ionic bonding.
  • In a chemical reaction, bonds between atoms are broken, the atoms rearrange, and then form new bonds.
  • But how are atoms bonded together in the first place?
  • Atoms can bond in different ways. Bonds can be either ionic or covalent. In either type of bond, the electrons (negative charge) and protons (positive charge) from each atom are attracted to the oppositely charged protons and electrons in the other atom.
  • If electrons in an atom feel a stronger attraction toward another atom, one or more electrons may leave that atom and join the other.
  • The atom that lost an electron becomes a positively charged ion and the atom that gained an electron becomes a negatively charged ion.
  • The oppositely charged ions attract each other and form an ionic.

Covalent bonding

Two atoms of hydrogen share an electron between them.  This is a covalent bond.
  • A covalent bond between two atoms forms when one or more electrons from each atom end up moving around the nuclei of both atoms. The atoms share these electrons.
  • For example, when two hydrogen atoms get close enough together, the attraction is balanced in both directions and they share the electrons between them. A covalent bond is made and hydrogen gas (H2) is formed.
  • The shaded area around each hydrogen atom indicates where its electron is most likely to be found. In the hydrogen molecule (H2) the darker area between the two nuclei shows where the two electrons, which are now shared, are most likely to be.

Chemical reactions

This animation shows one type of chemical reaction—the combustion of a molecule called methane. In combustion, one molecule is combined with oxygen to form carbon dioxide and water. The chemical species on the left side of the reaction arrow are called the reactants, while the chemicals on the right side of the reaction arrow are called the products.

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  • In a chemical reaction, bonds in the reacting molecules are broken and new bonds are formed to make the products.
  • The atoms that make up the reactants are not destroyed. They rearrange and bond in new ways to form the new products.

Baking soda and vinegar

This video shows an endothermic reaction. When baking soda and vinegar are combined, more energy is required to break the bonds in the reactants than is released when the products are formed. To drive the reaction, then, energy must be absorbed from the surroundings. This energy is absorbed in the form of heat, which we can observe a drop in temperature as the reaction withdraws heat from its surroundings. Because this reaction is endothermic and absorbs heat, the temperature goes down.

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  • When you mix baking soda and vinegar, a gas is produced. This is one clue of a chemical reaction.
  • A change in temperature is another piece of evidence that a chemical reaction has taken place.
  • A baking soda and vinegar reaction causes the temperature to decrease. It is an endothermic reaction.

Baking soda with calcium chloride

This video shows an exothermic reaction. When baking soda solution is added to calcium chloride, the formation of the resulting chemical products release more energy than was needed to break the bonds in the reactants. This energy is released in the form of heat, which we can observe as a change in temperature. Because this reaction is exothermic and releases heat, the temperature goes up.

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  • A reaction with different substances causes a different temperature change.
  • Combining a baking soda solution with calcium chloride results in a chemical reaction.
  • The reaction produces a gas but causes the temperature to increase.
  • This is an exothermic reaction.

Why does the temperature change in a chemical reaction?

This animation demonstrates why the combustion of methane is an exothermic reaction. In simple terms, more energy is released when the products of the chemical reaction are formed than was required to break the bonds in the reactants. The reaction thus releases energy, and is exothermic.

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  • In a chemical reaction, it takes energy to break the bonds in the reactants and energy is released when bonds form to make the products.
  • If more energy is required to break the bonds of the reactants than is released when the new bonds are formed in the products, the reaction is endothermic, and the temperature decreases.
  • An example of this type of reaction is the reaction between baking soda and vinegar.
  • If less energy is required to break the bonds of the reactants than the amount released when new bonds are formed, the reaction is exothermic and the temperature increases.
  • An example of this type of reaction is the reaction between baking soda solution and calcium chloride.

Baking soda and vinegar: Endothermic

  • In the chemical reaction with baking soda and vinegar, breaking bonds between the atoms in acetic acid (vinegar) requires energy. It also takes energy to break the bonds between the atoms in sodium bicarbonate (baking soda).
  • Forming new bonds from these atoms to make carbon dioxide and the other products of the reaction releases energy.
  • Since more energy was used to break the bonds of the reactants than was released in forming the products, this reaction is endothermic and the temperature goes down.

Calcium chloride and baking soda solution: Exothermic

  • In the chemical reaction with baking soda solution and calcium chloride, breaking bonds between the atoms of sodium bicarbonate (baking soda) requires energy. It also takes energy to break the bonds between the ions in calcium chloride.
  • Forming new bonds to make carbon dioxide and the other products of the reaction releases energy.
  • Since more energy was released in forming the products than was used to break the bonds of the reactants, this reaction is exothermic and the temperature goes up.

Chemical reactions and color changes

This animation depicts the the process of proton transfer between two water molecules. One water molecule approaches another, and when they get close enough the oxygen atom of the first water molecule develops an attraction for a hydrogen atom of the second water molecule. Eventually, the hydrogen atom of the second water molecule is transferred to the first water molecule. This forms two charged species, H3O+ and and OH-.

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  • To understand acids and bases, we first need to look at water.
  • Water is H2O, and reacts with itself. When two water molecules react, the ions H3O+ and OH- are produced.
  • In pure water, the concentrations of H3O+ and OH- are equal.
  • If the concentration of H3O+ in a solution is greater than it is in pure water, the solution is an acid.
  • If the concentration of H3O+ in a solution is less than it is in pure water, the solution is a base This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Chemical change and neutralizing

Two atoms of hydrogen share an electron between them.  This is a covalent bond. Two atoms of hydrogen share an electron between them.  This is a covalent bond.

Making acidic and basic solutions:

  • The concentration of H3O+ determines whether the solution turns pink or green.
  • Adding cream of tartar increases the concentration of H3O+ and the solution turns pink.
  • Adding laundry detergent decreases the concentration of H3O+ and the solution turns green.

Neutralizing acidic and basic solutions:

  • Adding laundry detergent to the pink cream of tartar solution can make the pink color go back to blue.
  • Adding cream of tartar to the green laundry detergent solution can make the green color go back to blue.
  • Both examples combine excess H3O+ and OH- to produce water. This process is called neutralizing.

Strength and concentration

  • Acids and bases can be strong, weak, or somewhere in-between.
  • The factor that determines the strength of an acid is its ability to produce H3O+ in water. A strong acid produces a lot of H3O+. A weak acid produces a smaller amount of H3O+ in water.
  • The factor that determines the concentration is the amount of acid itself that is added to water.
  • For example, if you add the same amount of a strong acid and a weak acid in the same amount of water in separate containers, the solutions have the same concentration. But because one acid is stronger than the other, the solution with the stronger acid is more acidic.

How to make an easy red cabbage indicator

This is how you can make red cabbage indicator.

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  1. Shred a few leaves of purple cabbage.
  2. Place the cabbage into a zip closing bag.
  3. Add water.
  4. Push out the air from the bag, and then zip it closed.
  5. Mix with your hands until the water turns medium to dark blue.
  6. The indicator is now ready to use. No need to boil, blend, or freeze!

Precipitates

Two atoms of hydrogen share an electron between them.  This is a covalent bond.

The soap solution dissolves easily in regular water (left), but forms a precipitate when added to “hard” water(right)—water that has a high concentration if dissolved Group II ions.

  • We have seen that a chemical reaction can result in the production of a gas, a change in temperature, and a color change.
  • Chemical reactions can also form a precipitate. A precipitate is a solid formed in a solution as the result of a chemical reaction.
  • The reaction with baking soda solution and calcium chloride produced a white precipitate (calcium carbonate).
  • Another common precipitate is soap scum. You can add a soap solution to an Epsom salt solution to produce a soap scum precipitate.

Summary

  • A chemical reaction happens when one or more substances change into entirely new substances with different properties.
  • The clues of a chemical reaction are production of a gas, change in temperature, color change, production of a precipitate.
  • In a chemical reaction, it takes energy to break the bonds in the reactants and energy is released when bonds are formed in the products.
  • If more energy is required to break the bonds than is released when new bonds are formed, the reaction is endothermic.
  • If less energy is required to break the bonds than is released when new bonds are formed, the reaction is exothermic.
  • Water reacts with itself to produce H3O+ and OH-.
  • An acid has more H3O+ than pure water. A base has less H3O+ than pure water.
  • An acid and a base can neutralize each other.
  • By testing different powders and comparing production of a gas, change in temperature, color change, or formation of a precipitate, you can tell the difference between similar-looking powders.

Molecular Animation Downloads

The animations featured in the slideshow above are available for download by clicking on the "Download this animation" link below each file.

Each video is offered for download in both Quicktime Movie (.mov) and Windows Media Video (.wmv) format.

To play these videos, you will need either Quicktime or Windows Media Player.