A chemical reaction is a process where substances transform into new substances with different properties. During a chemical reaction, atoms rearrange themselves, chemical bonds break and form, and this results in the creation of new substances. Chemical reactions often involve the release or absorption of energy. The original substances are called reactants, and the new substances formed are called products.
Chemical reactions can be classified into several types based on how atoms rearrange. Synthesis reactions occur when two or more substances combine to form a more complex product. For example, hydrogen atoms can combine to form hydrogen gas. Decomposition reactions are the opposite, where a complex substance breaks down into simpler ones. Single replacement reactions involve one element replacing another in a compound. Double replacement reactions involve the exchange of ions between two compounds. Combustion reactions occur when a substance rapidly combines with oxygen, releasing energy in the form of heat and light.
Chemical reactions are represented by chemical equations, which show reactants on the left side of an arrow and products on the right. The arrow indicates the direction of the reaction. When writing chemical equations, we must ensure they are balanced, meaning the number of atoms of each element is the same on both sides. Let's look at the reaction between hydrogen and oxygen to form water. Initially, we have H₂ plus O₂ yields H₂O. Counting atoms, we have 2 hydrogen atoms on both sides, but 2 oxygen atoms on the left and only 1 on the right. To balance this, we add a coefficient of 2 in front of water, making it 2 H₂O. Now we have 4 hydrogen atoms and 2 oxygen atoms on both sides, and the equation is balanced.
Chemical reactions involve energy changes as chemical bonds are broken and formed. There are two main types of reactions based on energy changes. Exothermic reactions release energy to the surroundings, resulting in a negative change in enthalpy, represented as negative delta H. Combustion and many oxidation reactions are exothermic. Endothermic reactions, on the other hand, absorb energy from the surroundings, resulting in a positive delta H. Photosynthesis is an example of an endothermic reaction. Energy is stored in chemical bonds. Breaking bonds requires energy input, while forming new bonds releases energy. The overall energy change depends on whether more energy is released in bond formation than is required for bond breaking. In this energy diagram for an exothermic reaction, we can see that the products have lower energy than the reactants, with the difference representing the energy released. The activation energy is the initial energy barrier that must be overcome for the reaction to proceed.
To summarize what we've learned about chemical reactions: Chemical reactions involve the rearrangement of atoms to form new substances with different properties. These reactions are represented by balanced chemical equations, where the number of atoms of each element must be the same on both sides. Chemical reactions can be classified into several types, including synthesis, decomposition, single and double replacement, and combustion. All reactions involve energy changes, either releasing energy in exothermic reactions or absorbing energy in endothermic reactions. Chemical reactions are fundamental processes that occur all around us, from the metabolism in our bodies to industrial processes and even in everyday activities like cooking. Understanding chemical reactions helps us make sense of the world and develop new materials, medicines, and technologies.