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Physical Science: GED Test Prep (page 3)

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Updated on Mar 9, 2011

Writing Chemical Reactions

A chemical reaction can be represented by a chemical equation, where the reactants are written on the left side and the products on the right side of an arrow that indicates the direction in which the reaction proceeds. The following chemical equation represents the reaction of glucose (C6H12O6) with oxygen (O2) to form carbon dioxide (CO2) and water (H2O). Your body runs this reaction all the time to obtain energy.

    (C6H12O6) + 6 (O2) → 6 (CO2) + 6 (H2O)

The numbers in front of the molecular formulas indicate the proportion in which the molecules react. No number in front of the molecule means that one molecule of that substance is reacting. In the previous reaction, one molecule of glucose is reacting with six molecules of oxygen to form six molecules of carbon dioxide and six molecules of water. In reality, there are many molecules of each of the substances and the reaction tells you in what proportion the molecules react. So if you had ten molecules of glucose react with 60 molecules of oxygen, you would obtain 60 molecules of carbon dioxide and 60 molecules of water. In many ways, chemical equations are like food recipes.

    2 Bread + 1 Cheese + 2 Tomato → Sandwich

With two slices of bread, one slice of cheese, and two slices of tomato, you can make one sandwich. If you had six slices of bread, three slices of cheese, and six slices of tomato, you could make three sandwiches. The same principles of proportion apply in chemical reactions.

Types of Chemical Reactions

Similar reactions can be classified and categorized into specific types of reactions. For example, chemical reactions can be classified as synthesis reactions, decomposition reactions, single-replacement reactions, and double-replacement reactions. Each of these reactions proceeds as you may expect by its name.

Synthesis Reaction

A + B → AB

Decomposition Reaction

AB → A + B

Single-Replacement Reaction

C + AB → CB + A

Double-Replacement Reaction

AB + CD → AD + CB

Just as in the sandwich equation previously described, the reactants will always combine in specific ratios to form the product. If two slices of bread are on the left side of the equation, then the sandwich formed on the right side will always have two slices, never one or three. If fours slices are on the left side, then you will end up with four slices on the right.

Look at the following synthesis reaction:

There are two nitrogen atoms on both sides of the equation. Also, there are six hydrogen atoms on each side of the equation. Matter is conserved.

Now look at this synthesis reaction involving ions:

    2F+ Ca2+ → CaF2

In addition to showing the conservation of matter, this example shows the conservation of charge. The two fluoride ions, each with a charge of –1 combine with a calcium ion which has a charge of +2. The product formed is neutral—the two –1 charges and the one +2 charge cancel each other out—charge is conserved.

In fact, all chemical reactions must conserve:

  • matter (mass)
  • energy
  • electric charge

Heat of Reaction (Enthalpy)

The breaking of molecular bonds releases energy stored in those bonds. The energy is released in the form of heat. Similarly, the formation of new bonds requires an input of energy. Therefore, a chemical reaction will either absorb or give off heat, depending on how many and what kind of bonds are broken and made as a result of that reaction. A reaction that absorbs energy is called endothermic. A container in which an endothermic reaction takes place gets cold, because the heat of the container is absorbed by the reaction. A reaction that gives off energy is called exothermic. Burning gasoline is a reaction that is exothermic—it gives off energy.

Increase in Disorder (Entropy)

Disorder, or entropy, is the lack of regularity in a system. The more disordered a system, the larger its entropy. Disorder is much easier to come by than order. Imagine that you have 100 blue beads in one hand and 100 red beads in the other. Now place all of them in a cup and shake. What are the chances that you can pick out 100 beads in each hand so that they are separated by color, without looking? Not very likely! Entropy and chaos win. There is only one arrangement that leads to the ordered separation of beads (100 blue in one hand, 100 red in the other), and many arrangements that lead to mixed-up beads (33 blue, 67 red in one hand, 33 red and 67 blue in the other; 40 blue, 60 red in one hand, 60 blue, 40 red in the other…). The same is true of atoms. Sometimes arrangement and order can be achieved. Atoms and molecules in solids, such as snowflakes, have very regular, ordered arrangements. But given enough time (and temperature), the snow melts, forming less ordered liquid water. So, although reactions that lead to a more ordered state are possible, the reactions that lead to disorder are more likely. The overall effect is that the disorder in the universe keeps increasing.

Catalysts

Often, a reaction needs help getting started. Such help can come from a catalyst. A catalyst is a substance or form of energy that gets a reaction going, without itself being changed or used up in the reaction. A catalyst acts by lowering what is called the activation energy of a reaction. The activation energy is often illustrated as a hill separating two valleys that needs to be crossed in order to get from one valley to the other (one valley representing the reactants, and the other the products). The catalyst acts by making the hill lower.

Physical Science

Light is a catalyst for the photosynthesis reaction. In living systems reactions are catalyzed by special protein molecules called enzymes.

Reversible and Irreversible Reactions

Some reactions can proceed in both directions—reactants can form products, which can turn back into reactants. These reactions are called reversible. Other reactions are irreversible, meaning that reactants can form products, but once the products form, they cannot be turned back into reactants. While wood can burn (react with oxygen) to produce heat, water, and carbon dioxide, these products are unable to react to form wood. You can understand reversibility better if you look at the activation energy diagram in the previous section. The hill that needs to be crossed by reactants to form products is much lower than the hill that needs to be crossed by products to form reactants. Most likely, such a reaction will be irreversible. Now look at the following diagram. The hill that needs to be crossed is almost the same for reactants and for products, so the crossing could take place from both sides—the reaction would be reversible.

Physical Science

Motions, Forces, and Conservation of Energy

A force is a push or a pull. Objects move in response to forces acting on them. When you kick a ball it rolls. A force is also required to stop motion. The ball stops rolling because of the frictional force. What happens here? First your body breaks the chemical bonds in the food you have eaten. This supplies your body with energy. You use up some of that energy to kick the ball. You apply a force, and as a result the ball moves, carrying the energy your foot supplied it with. But some of that energy is transferred from the ball to the ground it rolls on in the form of heat, due to the frictional force it encounters on the surface of the ground. As energy is lost this way, the ball slows down. When all of the energy is used up through friction, the ball stops moving. This example illustrates the concept of conservation of energy, as well as Newton's first law—the Law of Inertia.

What is the difference between speed and velocity? A speed, such as "30 miles per hour," has magnitude. A velocity has magnitude and direction (30 miles per hour, north). A similar distinction can be made in considering the difference in the terms distance and displacement. If you walk 20 feet to your mailbox and 20 feet back, the distance you traveled is 40 feet. Your displacement is zero, because displacement compares your ending point to the starting point.

Velocity is defined as the displacement divided by elapsed time. When you look at the change in velocity divided by the elapsed time, you are looking at acceleration. An acceleration that is negative (due to an ending velocity that is less than the starting velocity) is called a deceleration. For velocity of motion to change, either the speed and/or the direction must change and a net or unbalanced force must be applied. To summarize, an object at rest (whose speed is zero) remains at rest, unless some force acts on it—a person pushes it, the wind blows it away, gravity pulls it down… An object that is moving continues to move at the same speed in the same direction, unless some force is applied to it to slow it down, to speed it up, or to change its direction. The amount of acceleration or deceleration is directly proportional to the force applied. The harder you kick the ball, the faster it will move. The mass of the ball will also determine how much it will accelerate. Kick a soccer ball. Now kick a giant ball made of lead with the same force (watch your foot!). Which ball moves faster as a result of an equal kick? These observations constitute Newton's second law—the Law of Acceleration.

A good way to learn about the laws of motion is to shoot pool. What happens to billiard balls if you miss and fail to hit any of them? Nothing. They stay at rest. What happens when you hit the cue ball with the cue? It moves in the direction you hit it in. The harder you hit it, the faster it moves. Now, what happens when the cue ball collides with another ball? The other ball starts moving. The cue ball slows down. The energy is transferred from the cue ball to the ball it collided with. When an object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object. This is Newton's third law—the Law of Interaction.

Types of Forces

Newton's laws do not depend on the type of force that is applied. Some types of forces include gravitational, electromagnetic, contact, and nuclear.

Gravitational Force

Gravitation is an attractive force that each object with mass exerts on any other object with mass. The strength of the gravitational force depends on the masses of the objects and on the distance between them. When we think of gravity, we usually think of Earth's gravity, which prevents us from jumping infinitely high, keeps our homes stuck to the ground, and makes things thrown upward fall down. We, too, exert a gravitational force on the Earth, and we exert forces on one another, but this is not very noticeable because our masses are very small in comparison with the mass of our planet. The greater the masses involved, the greater the gravitational force between them. The Sun exerts a force on the Earth, and the Earth exerts a force on the Sun. The moon exerts a force on the Earth, and the Earth on the moon. The gravitational force of the moon is the reason there are tides. The moon's gravity pulls the water on Earth. The Sun also exerts a force on our water, but this is not as apparent because the Sun, although more massive than the moon, is very far away. As the distance between two objects doubles, the gravitational force between them decreases four times.

What is the difference between weight and mass?

On Earth, the acceleration due to gravity, g, is –9.8 m/s2.Your weight (w) is really a force. The formula F = ma becomes w = mg. Since the acceleration, g, is –9.8 m/s2, the overall force (w) is negative, which just means that its pull is in the downward direction: The Earth is pulling you towards its center. You have probably heard somebody say: "You weigh less on the moon!" This is true because the gravitational force on the moon is less than the Earth's gravitational force. Your mass, however, would still be the same, because mass is just a measure of how dense you are and the volume you take up.

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