Chemical Reactions & Balancing Equations
Objectives Observe and classify several chemical and physical changes. Give evidence for the occurrence of a chemical reaction. Write a balanced equation for a chemical reaction. Identify a reaction as a combination, decomposition, replacement, or combustion reaction. Background Physical properties can be observed without changing the chemical composition of the substance. Common physical properties that can be measured without changing the chemical composition are melting point, boiling point, solubility and density. Taste and odor are usually considered to be physical properties although they depend on the physiology of the person making the observations. Physical changes do not cause a change in chemical composition, but only a change in appearance. When a substance simply changes from a solid to a liquid, liquid to a gas or viceversa (liquid water to solid water, for example) without a change in composition it is a change in state. This is a physical change. (The three physical states are: gas, liquid, and solid). Chemical properties describe the ability of a substance to react and change into another substance with a different chemical composition. The new substance would have new properties. Evidence that a chemical change is taking place includes: a) A color change b) A solid product, a precipitate is formed c) A gas is formed d) Energy such as heat, light or electricity is produced. Some Physical Changes Some Chemical Changes Change in state Formation of gas (bubbles) Change in size Formation of a solid (precipitates) Tearing Disappearance of a solid (dissolves) Breaking Change in color Grinding Heat is given off or absorbed Table 5.1. Comparison of Physical and Chemical Changes
Balancing a Chemical Equation In a chemical reaction, atoms in the reactants are arranged to produce new combinations of atoms in the products. However, the total number of atoms of each element in the reactants is equal to the total number of atoms in the products. In an equation, the reactants are shown on the left and the products are on the right. An arrow between them indicates that a chemical reaction takes place. Reactants → Products To balance the number of atoms of each element on the left and right sides of the arrow, we write a number called a coefficient in front of the formula containing that particular element. Consider the balancing of the following unbalanced equation. The state of the substances as gas is shown as (g). N2(g) + H2(g) → NH3(g) N2(g) + H2(g) → 2NH3(g) N2(g) + 3H2(g) → 2NH3(g) Unbalanced equation A coefficient of 2 balances the N atoms A coefficient of 3 balances the H atoms. The equation is now balanced.
Types of Reactions There are many different chemical reactions, but most can be classified into the types of reactions shown below in Table 5.2 Type of Reaction Combination Description Elements or simple compounds form a more complex product Decomposition A reacting substance is split into simpler products Single Replacement One element takes the place of another element in a compound Double Replacement Elements in two compounds switch places Combustion Reactant and oxygen form an oxide product Table 5.2. Types of chemical reactions Example Equation Cu + S → CuS CaCO3 → CaO + CO2 Mg + 2HCl → MgCl2 + H2 AgNO3 + NaCl → AgCl + NaNO3 S + O2 → SO2
A. Observing and classifying types of changes of matter Perform the following tests at your lab bench. On your report sheet, record your observations and classify each experiment as a chemical or physical change. A.1 Ice Materials: Ice, beaker i. Obtain some ice from the reagent bench. Place it in a beaker. ii. Record your observations of what happens inside and outside the beaker. Indicate the type of change that occurred. A.2 Milk and Vinegar Materials: Milk, Vinegar i. Obtain 10 mL of milk in a beaker and mix with about 3 mL of vinegar. Stir with your glass...
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