SCEN102: Chapter 17

Catalyst

Any substance that increases the rate of a chemical reaction by lowering its activation energy.

Chemical equation

A representation of a chemical reaction in which reactants are drawn before an arrow that points to the products.

How can you make a chemical reaction happen faster?

Because reactant molecules must collide in order for a reaction to occur, the rate of a reaction can be increased by increasing the number of collisions. An effective way to increase the number of collisions is to increases the concentration of the reactants. Not all collisions between reactant molecules lead to products. The higher the temperature of a material, the faster its molecules move and the more forceful the collisions between them. The energy required to break bonds can also come from the absorption of electromagnetic radiation. As 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. The energy required for this initial breaking of bonds can be viewed as an energy barrier.The minimum energy required to overcome this energy barrier is known as the activation energy (Ea). A third way to increase the rate of reaction (in addition to increasing concentration or temperature) is to add a catalyst, which is any substance that increases the rate of a chemical reaction by lowering its activation energy.

What happens during a chemical reaction?

During a chemical reaction, chemical bonds are broken and atoms rearrange to form new chemical bonds. Such breaking and forming of chemical bonds involves changes in energy. To pull bonded atoms apart requires energy input. When atoms combine, there is an energy output, usually in the form of faster-moving atoms and molecules, electromagnetic radiation, or both. The amount of energy required to pull two bonded atoms apart is the same as the amount released when they are brought together.

Reaction rate

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

Exothermic reactions

For any exothermic reaction, energy can be considered a product and is thus sometimes included after the arrow of the chemical equation. In an exothermic reaction, the potential energy of atoms in the product molecules is lower than their potential energy in the reactant molecules. The amount of energy released in an exothermic reaction depends on the amounts of the reactants.

Relationship between dispersion and concentration of energy and their favorableness

Processes that result in the dispersion of energy tend to occur on their own—they are favored. Processes that result in the concentration of energy tend to not occur—they are not favored.

Endothermic

Reactions in which there is a net absorption of energy.

Exothermic

Reactions in which there is a net release of energy.

Law of mass conservation

States that matter is neither created or destroyed during a chemical reaction. No atoms are lost or gained during any reaction.

Entropy

Term we use to describe this natural spreading of energy. Exothermic reactions spread energy out to the surroundings. This is an increase in entropy. Exothermic reactions are favored to occur. An endothermic reaction requires that the reactants absorb energy from the surroundings. This is a concentration of energy. Endothermic reactions can be sustained only with continual input of some external source of energy.

Bond energy

The amount of energy that is either absorbed as a chemical bond breaks or is released as a chemical bond forms. • A positive bond energy represents the amount of energy absorbed as a bond breaks and a negative bond energy represents the amount of energy released as a bond forms.

Molar mass

The mass of 1 mole of the substance. Thus, the units of molar mass are grams per mole.

Products

The new materials formed in a chemical reaction.

Avogadro's number

The number of particles contained in one mole of anything (6.02x10^23).

What determines the rate of a chemical reaction?

The rate of collisions.

Reactants

The reacting substances in a chemical reaction.

Formula mass

The sum of the atomic masses of the elements in its chemical formula.

Endothermic reactions

When the amount of energy released in product formation is less than the amount of energy absorbed when reactant bonds break, the reaction is endothermic. In an endothermic reaction, the potential energy of atoms in the product molecules is higher than their potential energy in the reactant molecules. Raising the potential energy of the atoms in the product molecule requires a net input energy, which must come from some external source, such as electromagnetic radiation, electricity, or heat.