Reactions of Matter

Chemical Reactions Reactants  Products A Chemical Reaction is a change in which one or more substances are converted into new substances. Reactants  Products Law of Conservation of Mass – Matter cannot be created or destroyed, therefore the mass of the reactants must equal to the mass of the products. https://www.youtube.com/watch?v=IW7SUBw_aAA

Writing Equations A chemical equation is a way to describe a chemical reaction using chemical formulas other symbols. Example : Aqeuos copper sulfide reacts with iron fillings releasing heat and forming solid copper and iron sulfide solution. Cu2S (aq) + Fe (s)  FeS (aq) + 2Cu (s) + ∆H

Equation Symbols

Co-efficient The numbers in front of the chemical formulas are called co-efficient and symbolize the number of units of each substance taking part in the reaction. Example : 2H2 (g)+ O2 (g)  2H2O(l) In the above reaction 2 units(moles) of hydrogen gas and 1 unit of Oxygen react to form 2 units of water. Now since each unit is made up of 2 atoms of H2 and we have 2 units of Hydrogen gas (H2), we actually have 4 hydrogen atoms How many Hydrogen and Oxygen atoms are formed in the products? 4H 2O

Rules for Counting Atoms 1. Coefficients multiply all the interior atoms. Ex.. 2N2O6 4 Nitrogen 12 Oxygen 2. When there are brackets around atoms you have to multiply out the components. Ex… (NO2)3 Nitrogen = 3 Oxygen =6 3. Brackets and Coefficients, multiply out the brackets and then multiple the results by the coefficient. Ex… 3Na(NO2)3 Sodium =3 Nitrogen = 9 Oxygen =18

What is wrong with this equation? Based on the Law of conservation of mass what do you think should happen to the number of atoms on both sides of an equation? Look at the equation below: Mg(OH)2 + HCL  H2O + MgCl2 Have all the atoms totally been conserved? –No 1 Mg 1 O 3 H 1 CL  1 Mg 1 O 2H 2CL So as you see, we have lost a hydrogen atom and gained a chlorine atom in the reaction. This cannot occur in a chemical equation due to the law of conservation of mass.

Balancing Equations A balanced equation must have equal numbers of each atom on both sides. We use coefficients to balance out equations.

Steps for Balancing Equations Steps to Balancing out an equation : 1. Under each side of the equation write down how many of each atom you have. Use a new row for each atom. Write the number of the atom under the compound it appears in. (USE PENCIL )Example Mg(OH)2 + HCL  H2O + MgCl2 Mg 1 =1 Mg 1 = 1 O 2 =2 O 1 = 1 H 2 + 1 = 3 H 2 = 2 Cl 1 =1 Cl 2 = 2

Steps for Balancing Equations Cont. … Step 2: Look at which atoms are not balanced and then decide what number they need to be multiplied by to balance the atoms. Step 3: The number that you choose has to multiply all the other atoms in that column. Do this till all the atoms are balanced. Example ___Mg(OH)2 + _2__HCL  ___H2O + __ MgCl2 Mg 1 =1 Mg 1 = 1 O 2 =2 O (2)1 = 1 H 2 + 1 (2) = 3 H (2)2 = 2 Cl 1 (2) =1 Cl 2 = 2

Writing Word Equations Steps Cont- Step 1- Identify Reactants and Products. Step 2- Draw an Arrow (Left of the arrow is reactants, Right is products) Step 3 – Identify if each chemical is ionic or covalent and then go through the steps to get the correct chemical formula. If ionic you must cross the numbers, if covalent use prefixes. Use two line method, meaning on one line write the charges or prefixes and then below it write the actual chemical formula. Step 4 – Put reactants on the reactant side and products on the products side. Put a + sign between each reactant and each product made. Step 5 – Add symbols like gas(g), aqueous (aq) etc. Step 6- Balance

Example Solid Sodium Phosphide Reacts with aqeuous Potassium Chloride to form solid Potassium Phosphide and dissolved in water Sodium Chloride .

Energy In Reactions When bonds are broken and made energy is either released or absorbed by the compound that is break or making the bonds. Molecules must collide into each other in order to react. A reaction profile shows the changes in energy of a reaction from reactants to products. Activation Energy is the energy required for a reaction to take place. There are two main types of chemical reactions based on energy : Endothermic Exothermic

Exothermic Reactions In an exothermic reaction energy is lost to the surroundings. Therefore the products are more stable (have less energy) than the reactants because they have lost energy. The difference in energy between the reactants and products is the amount of energy the reaction gives off

Endothermic Reactions In an Endothermic Reaction, energy is taken in from the surroundings. Therefore the energy of the products are less stable (more energy) than the reactants because they have gained energy. The difference in energy between the products and reactants is the amount of energy that is absorbed.

Nuclear Reactions While chemical changes involve changes in the electrons, nuclear reactions involve changes to the nucleus and involve much larger energies than chemical reactions The strong nuclear force is the attractive force that binds protons and neutrons together in the nucleus. While the nuclear force is extremely weak at most distances, over the very short distances present in the nucleus, the force is greater than the repulsive electrical forces among protons. Therefore the nucleus stays together.

Radioactivity When the attractive nuclear forces and repulsive electrical forces in the nucleus are not balanced, the nucleus is unstable. Through radioactive decay, the unstable nucleus emits radiation in the form of very fast-moving particles and energy to produce a new nucleus, thus changing the identity of the element. Nuclei that undergo this process are said to be radioactive.

Uses of Radioactive Isotopes Radioactive isotopes have several medical applications. The radiation they release can be used to kill undesired cells (e.g., cancer cells). Radioisotopes can be introduced into the body to show the flow of materials in biological processes.

Half-Life For any radioactive isotope, the half-life is unique and constant. We can construct graphs of an elements radioactivity with respect to time to tell how long the element has been around for. This is useful in radiometric dating.

Nuclear Fusion This is the most powerful process known to man. It is responsible for the making of all the element in the universe. It is the fusing together of two nuclei to release a large amount of energy. Forms a larger nuclei. Takes place in stars.

Nuclear Fission This is when the nucleus of an atom is split apart into smaller nuclei, releasing large amounts of energy. Used in nuclear reactors to create energy for everyday life. Also used in atomic bombs