Compounds and reactions

What Are Compounds?

• Chemical bonds distinguish compounds from mixtures.
• A compound is held together by chemical bonds.
• A chemical bond is the attractive force that holds atoms or ions together.
• A compound always has the same chemical formula.
• Some compounds are made of molecules.
• Some compounds made of molecules are solids, others are liquids, others are gases.
• The strength of attractions between molecules varies.

Attractions between water molecules are called hydrogen bonds.

What Holds Bonded Atoms Together?

• Ionic bonds are formed between oppositely charged ions.
• When melted or dissolved in water, ionic compounds conduct electricity.
• A covalent bond is a bond formed when atoms share one or more pairs of electrons.
• Covalent compounds can be solids, liquids, or gases.
• Bonds in which atoms share electrons equally are called nonpolar covalent bonds
• Atoms do not always share electrons equally.
• An unequal sharing of electrons forms a polar covalent bond.
• Atoms may share more than one pair of electrons.

Chemical Reactions Change Substances

• Chemical reactions occur when substances undergo chemical changes to form new substances.
• Production of gas and change of color are signs of chemical reactions.
• Chemical reactions rearrange atoms.
• A reactant is a substance or molecule that participates in a chemical reaction.
• A product is a substance that forms in a chemical reaction.
• Energy must be added to break bonds.
• Forming bonds releases energy.
• An exothermic reaction is a chemical reaction in which heat is released to the surroundings.
• An endothermic reaction is a chemical reaction that absorbs heat.

Classifying Reactions

• A synthesis reaction is a reaction in which two or more substances combine to form a new compound.
• Synthesis reactions have the following general form: A + B → AB
• Example: In the following synthesis reaction, the metal sodium reacts with chlorine gas to form sodium chloride, or table salt.

2Na + Cl 2 → 2NaCl

• A decomposition reaction is a reaction in a single compound breaks down to form two or more simpler substances.
• Decomposition reactions have the following general form: AB → A + B
• Example: The following shows the decomposition of water.

2H 2O → 2H 2 + O 2

• When the number of atoms of reactants matches the number of atoms of products, then the chemical equation is said to be balanced.
• Balancing equations follows the law of conservation of mass.
• You cannot balance chemical equations by changing chemical formulas themselves, because that would change the substances involved.
• To balance chemical equations, numbers called coefficients must be placed in front of the chemical formulas.