How Chemicals React

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Chapter 20

Chemical Reaction

atoms rearrange to create one or more new compounds.

Chemical Equation

shows the reacting substances (reactants) to the left of an arrow that points to newly formed substances (products).

How are reactants and products typically shown?

By elemental or chemical formulas, molecular models, or simply names. Phases are often shown: solid (s), liquid (l), gas (g), compounds dissolved in water (aq). Also, coefficients are shown to indicate the ratio in which they are either combined or formed.

Coeffiecient

represent numbers of individual atoms and molecules.

Law of Mass Conservation

states that matter is neither created nor destroyed during a chemical reaction. NO atoms are lost or gains during any reaction.

Balanced Equation

each atom must appear on both sides of the arrow the same number of times.

What does the speed of the reaction depend on?

concentration and temperature of the reacting molecules.

Reaction Rate

how quickly the concentration of products increases or how quickly the concentration of reactants decreases.

Reactant molecules must ________ in order for a reaction to occur.

Collide

How can the rate of reaction be increased?

By increasing the number of collisions.

What is an effective way to increase the number of collisions?

Increase the concentration of reactants. The more more molecules in a given volume makes collisions between molecules more probable.

Temperature of molecules makes them...

move faster or slower. The faster they move the more forceful collisions are.

Higher temperature _______ reaction rates.

creates faster

Where does the energy required to break bonds come from?

absorption of electromagnetic radiation. As the radiation is absorbed by reactant molecules, the atoms in the molecules may start to vibrate so rapidly that the bonds between them are easily broken.

True or false: The direct absorption of electromagnetic radiation is sufficient to break chemical bonds and initiate a chemical reaction.

True!

True or False: All collisions between reactant molecules lead to products.

False, the molecules must collide in a certain orientation in order to react. Another reason they might not react is because there needs to be enough kinetic energy to break bonds.

What is required for a reaction to occur?

Certain collision orientation and enough Kinetic energy.

What is a necessary first step in most chemical reactions?

Broken bonds, as a result of collisions, the absorption of electromagnetic radiation, or both.

Energy Barrier

The energy required for initial breaking of bonds

Activation Energy

The minimum energy required to overcome the energy barrier.

True or false: Reactant molecules can transform to product molecules only if the reactant molecules possess an amount of energy equal to or greater than the activation energy.

True!

At any given temperature, there is a wide distribution of...

Kinetic energies in reactant molecules. Some are moving slowly and others are moving quickly.

When the temperature of the reactants is increased...

the number of reactant molecules possessing sufficient energy to pass over the barrier also increases.

When the temperature of the reactants is decreased...

there are fewer molecules having sufficient energy to pass over the barrier which is why reactions are slower at lower temperatures.

Most chemical reactions are influenced by temperature including those occurring in....

living bodies.

The body temperature of animals that regulate their internal temperature is

fairly constant.

Third way to increase the speed of a reaction is to add a...?

Catalyst

Catalyst

any substance that lowers a reaction's activation energy.

Stratosphere

part of the atmosphere where the Sun's ultraviolet rays cause oxygen molecules to transform into ozone molecules. (3O2 (g) + UV --> 2O3 (g)

The ozone, once it is formed, is able to transform ultraviolet light into _____.

heat

Ozone reactions block about ___% of the ultraviolet rays that comes to our planet from the sun.

95

In the 1970s, scientists began to recognize what man made substance posing a significant threat to the stratospheric ozone?

Chlorofluorocarbons, CFCs.

CFCs do what to ozone molecules?

Break them up. They drift up to the stratosphere where the intense UV rays break CFC's apart producing chlorine atoms which speed up the rate at which ozone atoms get destroyed.

True of false: Catalysts are destroyed in the reaction that it catalyzes.

False, it survives so that it can speed up the reaction repeatedly.

Chlorine Monoxide is...?

molecule that forms during the chlorine-catalyzed destruction of the ozone.

What do catalytic converters do for automobiles?

metal catalysts in a converter speed up reactions that convert exhaust pollutants to less toxic substances.

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