Chemical Reactions

Forming New Substances Why do leaves change colors in the fall? Leaves contain large amounts of chlorophyll, a green pigment, in spring and summer During the Fall, chlorophyll breaks down into a colorless substance the orange and yellow colors that were always present can be seen now

Chemical Reactions A process in which one or more substances change to make one or more new substances The chemical and physical properties of the new substances are different form the original substances. Ex: leaves changing colors mixing baking power and water makes carbon dioxide bubbles that make muffins spongy

Signs of Chemical Reactions Gas formation Solid formation- precipitate or solid is formed Energy Change- energy is released a fire gives off light and thermal heat Color change

Chemical –vs-physical change Physical change liquid boiling gives off a gas Chemical change liquid boiling gives off a gas and a new substance is formed with different properties from the original substance

Chemical Bond A force that holds two atoms together Chemical reaction must take place to break the bonds New bonds form Ex: hydrogen + chlorine hydrogen chloride +

Writing Formulas Formulas for Covalent Bonds- Compound composed of two non-metals Carbon Dioxide CO2 Dinitrogen monoxide N2O Formulas for Ionic Bonds- Compound composed of a metal and non-metal Sodium Chloride NaCl Magnesium chloride MgCl2

Prefixes used in chemical Names mono- 1 hexa- 6 di- 2 Hepta- 7 tri- 3 Octa- 8 tetra- 4 nona- 9 penta- 5 deca- 10

Chemical Equations Uses chemical symbols and formulas as a shortcut to describe a chemical reaction Reactants- the starting materials in a chemical reaction Products- the substance that forms in a chemical reaction Ex: C + 02 CO2

Be accurate CO2 Carbon dioxide CO Carbon monoxide Co Cobalt

Balancing Equations Atoms are never lost or gained in a chemical reaction They are just rearranged Every atom in the reactants becomes part of the products The number of atoms of each element in the reactant equals the number of atoms of each element in the products

Ionic Compounds NaCl sodium chloride MgCl2 Magnesium Chloride A sodium ion has a 1+ charge A chloride ion has a 1- charge One sodium and one chloride ion has an overall charge of 0 (1+) + (1-) = 0 A magnesium ion has a 2+ charge. A chloride ion has a 1- charge. One magnesium ion and two chloride ions have an overall charge of (2+) + 2(-1)= 0

Formula of an ionic compound The formula of an ionic compound is written by using enough of each ion so that the overall charge is 0.

Law of Conservation of Mass Mass is neither created or destroyed in normal chemical and physical changes The total mass of the reactants is always the same as the total mass of the products Chemical equations must show the same number and kinds of atoms on both sides of the arrow

How to balance an Equation To balance an equation you must use coefficients Coefficient- is a number that is placed in front of a chemical symbol or formula Ex: 2CO is 2 carbon monoxide molecules

Balancing an Equation All atoms must be accounted for 1. Count all atoms 2. Multiply the subscript of each element in a formula by the formulas coefficient. Ex: 2H20 contains 4 Hydrogen atoms and 2 oxygen atoms 3. Only coefficients - not subscripts – are changed when balancing equations.

Balancing equations In general, to balance an equation, here are the things we need to do: Count the atoms of each element in the reactants and the products. Use coefficients; place them in front of the compounds as needed.

Practice Na + Cl2 NaCl Mg + N2 Mg3 + N2 Na = 1 Mg= 1 Cl = 2 N= 2