Balancing Equations

Chemical Changes A change in which new substance is formed Every chemical change involves a chemical reaction One or more substances combine or break down to form new substances Existing atoms are rearranged to form new compounds Substance is changed forever – look different at the end The product also has different properties Energy is absorbed or given off (released) Breaking of bonds in substances and they look different in the end

Describing Chemical Reactions A chemical reaction is a change in which one or more substances are converted into new substances. A chemical reaction occurs when you bake a cake.

Mechanics of chemical reactions Two molecules meet with enough energy, bonds break Simultaneously, new bonds form Time for breaking/forming bonds is almost immeasurable

Chemical reaction Evidences of chemical reaction taking place: Production of heat or light Appearance of gas bubbles Formation of a new solid (precipitate) Color change

Describing Chemical Reactions The substances that react are called reactants. The new substances produced are called products. This relationship can be written as follows:

Conservation of Mass The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants. For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products.

The Law of Conservation of Mass (Matter): mass of the substances is conserved No mass is lost or gained in a chemical rxn. Matter can not be created or destroyed in a chemical rxn. Lavoisier – 1770’s – total mass of the products equal the total mass of the reactants A closed system must be used to capture the products

Writing Equations Scientists have developed a shorthand method to describe chemical reactions. A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols. Rearrangement of one or more reactants to form one or more products Bonds are breaking and reforming Chemical equation is like a recipe – tells ingredients involved and amount needed

EXAMPLE The physical state of each chemical can be indicated by using the symbols (g) gas (l) liquid (s) solid (cr) crystalline (aq) aqueous = dissolved in water

Balancing Equations Conservation of Atoms – the number of each type of atom on the reactant side must equal the number of each type of atom on the product side IS THIS EQUATION BALANCED?

Coefficients = # of molecules present Finding out which coefficients to use to balance an equation is often a trial-and-error process.

Try Your Balancing Act Step 1 Write a chemical equation for the reaction using formulas and symbols. Step 2 Count the atoms in reactants and products.

Try Your Balancing Act Step 3 Choose coefficients that balance the equation. Remember, never change subscripts of a correct formula to balance an equation.

Try Your Balancing Act Step 4 Recheck the numbers of each atom on each side of the equation and adjust coefficients again if necessary. Key to chemical equations: New atoms are not created in a rxn; atoms are rearranged in a rxn

Diatomic Molecules Certain elements exist as diatomic molecules, they are H2, F2, N2, Cl2, O2, Br2 At2 I2 Remember most atoms do not like to be alone

Classification of Reactions Combustion Synthesis Reactions (combination or addition) Decomposition Reactions Single Displacement or Replacement Reactions Double Displacement or Replacement Reactions

Combustion Reactions – an element or compound reacts with oxygen, often producing heat or light may include hydrocarbons (carbon and hydrogen) – methane (CH4), propane (C3H8), butane (C4H10), octane (C8H18): when this occurs the most probable products are carbon dioxide and water

Examples: Fuel (Carbon Compound) + O2  CO2 + H2O Ex: C6H12O6 + O2  CO2 + H2O Ex: CH4 + 2O2  CO2 + 2H2O + heat + light Combustion reaction between an element and oxygen is also an example of a combustion reaction

Synthesis Reactions – Combination or Addition In a synthesis reaction, two or more substances combine to forms one single product. The product must be a compound. The generalized formula for this reaction type is as follows: A + B  AB. ex: 2H2 + O2  2H2O ex: Ca + S  CaS

Decomposition Reactions – a single compound is broken down into two or more products Most require heat, light or electricity Hard to predict products unless they are binary AB  A + B (KEY = one reactant) ex: 2KClO3  2KCl + 3O2 ex: CaCO3  CaO + CO2 ex: 2H2O  2H2 + O2

Single Displacement (or Replacement) When one element replaces another element in a compound The metals can replace each other A + BC  AC + B ex: Mg + CaCl2  MgCl2 + Ca ex: Zn + H2SO4  ZnSO4 + H2 The non-metals can replace each other AC + B  BC + A Ex: 2KBr + Cl2  2KCl + Br2

Double Displacement (or Replacement) – involve an exchange of positive ions between two compounds Usually take place between two ionic compounds AB + CD  AD + CB ex: NaNO3 + AgCl  NaCl + AgNO3 ex: AgNO3 + KCl  AgCl + KNO3

Double Displacement One of the following is always true: 1. one product is slightly soluble and precipitates from the solution 2. one product is a gas that bubbles out of the solution 3. one product is a molecular compound, such as water A precipitate forms and is an insoluble compound that comes out of solution during this type of reaction

Oxidation-Reduction Reactions One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons. Chemists use the term oxidation to describe the loss of electrons and the term reduction to describe the gain of electrons.

Oxidation-Reduction Reactions The substance that gains an electron or electrons obviously becomes more negative, so we say it is reduced. On the other hand, the substance that loses an electron or electrons then becomes more positive, and we say it is oxidized.

How to predict products Write the chemical formulas for the reactants Identify the type of reaction Predict the products and write their chemical formulas Write and balance the complete equation

Rate of Reaction Collision Theory = substances must come in contact with each other in order to react with each other. Factors that speed up or slow down a rxns: Surface Area Temperature Concentration Catalysts – speed up Inhibitors – slow down

What limits a reaction? Limiting reagent The reagent that is used up first limits the amount of product to be formed What if you need two eggs to make cupcakes and you only have one? You can only make half a batch of cupcakes Excess Reagent The reactant that is not completely used up because the other reactant is used up first

Energy in Reactions Exothermic = energy is released by the rxn. Takes less energy to break the bonds in the reactants than are released by the forming of bonds in the products Energy is released (gets hotter) Sometimes hard to detect the temperature change MRE’s – food wrapped in special sleeve with magnesium when water is added to the sleeve the reaction produces heat which warms the food exo = out or outer original substances→ new substances + energy

Activation Energy Activation Energy = energy that is put into an exothermic reaction to start the rxn.

Energy of a Rxn (heat change) Endothermic = energy is put into the rxn, rxn absorbs energy Takes more energy to break the bonds in the reactants then is released by the forming of bonds in the products Energy must be put in for the reaction to take place (gets colder) Instant ice pack – water and ammonium nitrate endo = inside or into thermic = heat or energy original substances + energy → new substances The energy absorbed can be in the form of light, heat or electricity