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Transcript of Chapter 8
Ability to Change
Forming New Substances
There are different forms of chemical changes:
One compound is decomposed into its elements
Decomposition reaction - water is broken down into hydrogen gas and oxygen gas when electricity is added.
Two elements join to form one compound
Synthesis reaction - Salt (NaCl) is composed to two very different elements Na and Cl. When mixed together they are very reactive and create NaCl or salt.
All chemical changes produce substances that are different from the starting substances.
You use properties of matter to help identify objects and substances. What you are use are usually physical properties: color, luster, etc.
However, you can use both chemical and physical properties to help identify substances. A chemical property is the ability or inability of a substance to combine with or change into one or more new substances.
Chapter 8 - Chemical Reactions
To observe a chemical property, the substance undergoing a change must be converted into one or more different substances. It could also describe what the substance won't do (non-corrosive.)
Wood burning, Milk souring, Apple slice turning brown, Rusting
How would you describe yourself? Tall/Short, Brown/Blue eyed, Brown/Blonde hair. All of these are physical properties.
Remember, physical properties include:
conductivity, ductility, malleability, color/luster, melting/boiling, hardness/brittleness, state of matter
Chemical and Physical Properties
A chemical change is the change of one or more substances into other substances.
This requires atoms to rearrange and form one or more new substances. They do this by breaking bonds and forming new ones.
When paper burns, there is no way to make the ash and smoke back into paper.
When iron rusts, there is no way to remove the oxide from the iron to make the iron shiny again.
All chemical changes can't be reversed.
Chemical Changes - Not Reversible
Physical change is a change in which the properties of a substance change but the identity of the substance stays the same.
A change is state from solid to liquid to gas - the substance stays the same but the molecules just move farther apart and move more quickly.
Most physical changes are reversible, very different than chemical changes.
Dissolving is a process in which substances mix evenly with one another. If sugar or salt is dissolved in water, can you get it back?
Physical Changes & Dissolving
When a substances changes, its properties change. When ice melts, the solid water becomes liquid water. This is a physical change.
When iron rusts, it is no longer iron. It is a new substance iron oxide, which has different properties as iron.
Lesson 2 : Chemical Equations
In chemical reactions, substances might seem to appear or disappear, but if you look carefully you will observe that same amount of matter is there before and after a reaction.
During a chemical reaction, bonds of the starting substances break and reform into new bonds and new compounds as the final substances.
Antoine Lavoisier (1743-1794) was given credit for showing that mass is conserved in chemical reactions. He created the first balance...this allowed him to make more precise measurements than had been possible before. He would perform experiment after experiment and measure the before mass and compare it to the after mass. He found over and over and over again that the mass before was the same as the after mass.
Using a large lens and sunlight he heated a container of tin. The tin changed in color and texture - creating tin oxide. The before and after substance had the same mass.
Law of Conservation of Mass
Chemical reaction is another name for chemical change. This is where one or more substances combine to create something totally different from either of the starting substances. The important thing is the even though it changes the amount of matter does not. This is called the conservation of mass : the total mass before a chemical reaction is the same as the total mass after the reaction.
Writing Chemical Equations
A chemical equation is a convenient way to describe what happens in a chemical reaction.
In chemical equations, the
are written on the left side of an arrow points to the right. If there is more than one, they are separated by a plus sign.
The plus sign is read as "
The arrow is read as "
The products are written on the right side of the arrow. The
are the new substances that are formed. Again, if there is more than, they are separated by a plus sign.
You can write the chemical equation using chemical symbols and formulas or writing the word equation:
tin + oxygen gas --> tin oxide
reactants produce product
carbon + oxygen --> carbon dioxide
Word equations have limitations. They can be long and they do not show that mass is conserved. You have no idea of how many of each molecule or element is needed to observe the law of conservation of mass.
Elements, Compounds, Molecules
Instead of writing long word equations, chemists use symbols to represent elements.
: Formulas are used to represent molecules and ionic compounds. A molecules is a neutral particle in which atoms share electrons. Molecules may be elements or compounds.
Oxygen gas and Hydrogen gas are often times bonded to itself. As in oxygen gas has two oxygen atoms and hydrogen gas has two hydrogen atoms. These are called
: Molecules composed of two or more different atoms are compounds. A formula unit is defined as the smallest whole-number ratio of the element in an ionic compound. Common compounds would be water and salt.
Examples of Formulas & Molecules
Lavoisier showed experimentally that in chemical reactions, mass i neither gained or lost (just rearranged!) The same is true for chemical equations. Balancing an equation means showing the same number of the same kind of atom on both sides of the equation.
Steps to balancing equations:
There is a list of some of the common chemical reactions on pages 354-355 in your book!
Lesson 3: Energy and Chemical Change
What happens to your body when you run fast? Get hot or cold? Sweat? This is a sign of a chemical reaction happening in your body. Your cells are generating energy by breaking bond in molecules. How does a chemical reaction make energy or heat?
Remember, a chemical reaction breaks bonds in the reactants to rearrange atoms to make new molecules in the products. The "rearranging" involves so sort of energy! Usually, it's heat energy!
Fireworks produce energy in the form of sound, light, heat, and the movement of particles.
Light from Chemical Reactions
Conservation of Energy
Some reactions give off light with almost no thermal energy, or heat. A glow stick is a plastic tube with a glass vial inside. When you break the glass vial by bending the outer plastic, you see it "glow." The chemicals contained in the glass vial mix with the chemicals in the plastic tube. This is cold light. Cold light occurs at room temperature, and under the temperature of an incandescent light. Many organisms are able to produce cold light to find food or prey, use for defense, or communication.
The law of conservation of energy states that energy is neither created nor destroyed in chemical reactions. Energy is stored in the reacting molecules or the bonds that hold them together. The energy just changes into other forms, such as light, heat, and sound. Energy stored in gasoline burns and changed into usable energy (engine) and heat.
Your body needs energy to function properly. Your body breaks the bonds of foods containing proteins, fats, and carbohydrates to keep you moving and function. It also keeps your body function at the proper temperature 98.6 is due to the heat being released when the bonds of food molecules are broken in your body.
Release of Energy
Breaking bonds requires energy and forming new bonds released energy. There usually is a net change in energy in any chemical reaction. This is either a release of energy or an absorption of energy (one or the other). If less energy is required to break the original bonds than is given off when new bonds form, there is a new release of energy, an exothermic reaction. However, sometimes the energy required to break bonds is greater than the energy released when new bonds form, making an absorption of energy, or an endothermic reaction.
Endothermic vs. Exothermic
Endothermic: the reactants are lower in energy than the products, meaning that the reactants are more stable than the products
Common examples: melting ice, cooking an egg, photosynthesis
Exothermic: the reactants have higher energy than the products, meaning that energy is released along the way
Common examples: making ice cubes, a candle flame, combustion of fuel