GED Science Practice Test: Introduction To Chemical Reactions

A chemical reaction is a process that leads to the transformation of one set of substances to another. This transformation is accomplished through the formation and/or breaking of chemical bonds between atoms. The substances that start a chemical reaction are called the reactants, while the substances that result from a chemical reaction are called the products.

There are certain observable signs that a chemical reaction has occurred, such as a color change, formation of a gas (bubbles), heat change, light, and/or the formation of a precipitate. For example, when baking soda and vinegar are added together, a chemical reaction has occurred, and we know this because bubbles are formed.

However, it can sometimes be tricky to distinguish between chemical reactions and physical changes.  For example, when you boil water, bubbles form in the bottom of the pot.  However, the change of water from a liquid to a gas is a physical change:  the substance before and after boiling is still the same H2O. Another interesting example is dissolving sugar in water.  Though this physical change does not exhibit any of the five characteristics of a chemical reaction, it seems like a brand new substance is formed, and that the sugar “disappears.”  However, the sugar can be easily gathered from sugar water by physical means; you simply allow the water to evaporate out of the solution, leaving crystallized sugar behind.

Enthalpy and entropy are two important terms that are often used to characterize both physical changes and chemical reactions.  Entropy deals with the disorder of particles and substances.  Entropy is similar to a messy bedroom-without energy put in to organize and clean, a room tends to get messy and disorganized.  Enthalpy is a measure of the amount of heat in a system, and the change in the enthalpy of a system is represented by ΔH. In short, enthalpy looks at the energy of the particles, whereas entropy looks at the organization of the particles themselves.

Many chemical reactions release energy in the form of heat, light, or sound. These are called exothermic reactions. In an exothermic reaction, the change in enthalpy is negative (ΔH < 0), because heat is released, and the entropy (disorder) of the system is increased. Some exothermic reactions may happen spontaneously, as they release, rather than use or require, energy. The following graph shows the energy changes in an exothermic reaction.  Notice that the energy of the products is less than the energy of the reactants.  This means that heat is released, and this is what characterizes an exothermic reaction:


Other chemical reactions must absorb energy in order to proceed, and thus, cannot occur spontaneously. These are called endothermic reactions. In an endothermic reaction, the change in enthalpy is positive (ΔH > 0), because heat is absorbed, and the entropy (disorder) of the system is decreased. Remember the messy room analogy—in order for a room to become more ordered (less entropy), an input of energy is required.  In an endothermic reaction, energy must be put in, or absorbed, in order for the entropy of the products to be lower than the reactants. The following graph shows the energy changes in an endothermic reaction.  Notice that the energy of the products is more than the energy of the reactants.  This means that heat is absorbed, and this is what characterizes an endothermic reaction:


You notice the phrase activation energy on the graphs above. Activation energy can be defined as the minimum energy required to start a chemical reaction. As the diagram above shows, more activation energy is needed to start a non-spontaneous endothermic reaction than is needed to start a spontaneous exothermic reaction. Activation energy can be thought of as the hurdle that must be overcome for a chemical reaction to start. For a chemical reaction to proceed at a reasonable rate, there should be a large number of reactant molecules with energy equal to or greater than the activation energy.

Catalysts are substances that lower the activation energy required for a given reaction.  Thus, catalyzed reactions result in a higher reaction rate at the same temperature. Unlike other reactants in the chemical reaction, a catalyst is not consumed by the reaction. Protein-based biological catalysts, called enzymes, play a critical role in increasing the rates of biochemical reactions so that they can support life. The effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons (which reduce the activity of the catalyst) or promoters (which increase the activity). The following graph shows how a catalyst can affect the activation energy, which is like a hurdle the reactants must overcome:

As previously mentioned, exothermic reactions tend to happen more quickly than endothermic reactions.  Additionally, catalysts and inhibitors can increase and decrease, respectively the rates of a chemical reaction.  Reactions tend happen at a characteristic reaction rate, which is also influenced by temperature, chemical concentration, and/or the surface area of the reactants; in general, the higher the temperature, chemical concentration and/or surface area, the faster the reaction will occur up to a point. Reactions can be slowed down or stopped if one or more reactants is available in only limited quantities or not at all; such reactants are called limiting reactants.

Many of the things that affect the rate of reaction, do so because they allow more of the reactants to come into contact with each other.  Consider surface area for a moment.  A higher surface area of one reactant (let’s call it reactant A) allows more of reactant A to come into contact with the other reactant (reactant B).  The following diagram shows the effect of surface area on rate of reaction:


The above image shows acid particles trying to react with magnesium atoms.  In the low surface area picture, the acid particles can only eat away at the magnesium atoms on the outside of the block of magnesium atoms.  In the high surface picture, the more of the magnesium atoms are exposed to the acid particles.


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