Chapter 2: Chemical Reactions

A chemical reaction is a process in which substances undergo physical and chemical changes that result in the formation of a new substance with new properties.   All chemical reactions result in the formation of a new substance!

A substance that enters into a chemical reaction is called the reactant. These are the starting materials! The new substances produced by a chemical reaction are called products. These have different properties than the reactants!

A general equation for chemical reactions can be drawn as: Reactants  Products   All chemical reactions involve a change in energy. This energy is either absorbed or released during a chemical reaction.

There are 3 possible events for a reaction: 1.    atoms combine to form molecules H2 + O2  H2O 2.    molecules can break apart to form atoms CO2  C + O2 3. molecules can react with other molecules H2O + CO2  C6H12O6 + O2

A chemical equation uses symbols and formulas to represent a chemical reaction. It shows both the reactants and products for a chemical reaction. An arrow shows the direction of the chemical change. The arrow takes the place of an equal sign (=).

Law of Conservation of Mass: Atoms can neither be created nor destroyed in a chemical reaction. The number of atoms before and after a reaction is always the same. The changes involved in a reaction only rearrange the atoms, it cannot create or destroy atoms.

The number of atoms of each element must be the same on both sides of the equation! This is called a balanced chemical equation Examples: Unbalanced: Mg + O2  MgO Balanced: 2Mg + O2  2MgO

Steps in Balancing a Chemical Reaction: 1 Steps in Balancing a Chemical Reaction: 1.    write the chemical equation with correct symbols and formulas. Mg + O2  MgO 2. Count the number of atoms of each element on each side of the arrow. 1 Mg and 2 O on left, 1 Mg and 1 O on right

3. Balance the atoms by using coefficients 2 Mg + O2  2 MgO 4. Check your work by counting atoms of each element. The equation is balanced when the numbers are equal on both sides of the equation! H2 + Cl2   -->   2 HCl 2 H2 + O2   -->   2 H2O

There are 4 types of chemical reactions: 1.    synthesis 2.    decomposition 3.    single-replacement 4. double-replacement

Synthesis Reaction 2 or more simple substances combine to form a new, more complex substance. A + B  AB 2Na + Cl2  2NaCl

Decomposition Reaction A complex substance breaks down into 2 or more simpler substances. These are the reverse of synthesis reactions! AB  A + B 2H2O  2H2 + O2

Single-Replacement Reaction An element replaces another element that is part of a compound. A + BC  AC + B 2Na + 2H2O  2NaOH + H2

Double-Replacement Reaction Atoms from two different compounds replace each other. Two compounds react to form two new compounds. AX + BY  AY + BX MgCO3 + 2HCl  MgCl2 + H2CO3

All chemical reactions involve a change in energy All chemical reactions involve a change in energy. We have already learned that energy can be either absorbed or released. Based on the type of energy change involved, chemical reactions can be classified as either exothermic or endothermic.

Exothermic Reactions A chemical reaction where energy is released is called an exothermic reaction. This energy is commonly released in the form of heat or light. Example: the burning of a match Energy Diagram:

Endothermic Reaction A chemical reaction where energy is absorbed is called an endothermic reaction. This energy is generally in the form of heat or light. Because energy is being absorbed, the energy of the products will be greater than the energy of the reactants. Energy Diagram:

The energy needed to “jumpstart” a chemical reaction is called activation energy. All chemical reactions require activation energy.   Kinetics is the study of reaction rates. The rate of a reaction is a measure of how fast the reactants turn into the products.

According to the collision theory, the rate of a chemical reaction depends on four factors: concentration surface area temperature catalysts

Concentration is a measure of the amount of a substance in a given volume. An increase in the concentration of the reactants will speed up a reaction. A decrease in the concentration of the reactants will slow down a reaction.

Surface area is how much material is exposed Surface area is how much material is exposed. The greater the surface area, the faster a reaction will proceed.

An increase in temperature will generally increase the rate of a reaction. The higher the temperature, the more energy is present.

Substances that increase the rate of a reaction are called catalysts Substances that increase the rate of a reaction are called catalysts. Catalyst generally lower the activation energy needed for a reaction. Therefore, the reaction can proceed more easily. Example: enzymes in the body