Chemical Reactions

Goals Write chemical equations to describe chemical reactions. Classify and identify chemical reactions. Write ionic equations for reactions that occur is aqueous solutions.

Introduction to Reactions Writing and Balancing

Evidence of Chemical Reactions The process by which the atoms of one or more substances are rearranged to form different substances is called a CHEMICAL REACTION. temperature change exothermic = heated released endothermic = heat absorbed color change odor evolution of gas bubbles appearance of a solid (precipitate)

Representing Chemical Reactions Equations are used to represent chemical reactions. Fe(s) + Cl2(g) → FeCl3(s) Reactants are the starting substances. If there is more than one, a “+” separates the two or more reactants. Products are the substances formed. If there is more than one, a “+” separates the two or more products. The arrow indicates the direction of the reaction. It is important to know the state of the reactants and products. (s) = solid, (l) = liquid, (g) = gas, and (aq) = aqueous when dissolved in water.

Conservation of Mass Chemical Equations simply and eloquently represent what Lavoisier professes over 250 years ago: matter is neither created nor destroyed; the number of atoms for each reactant and each product is equal on both sides of the arrow = balanced chemical equation.

iron(s) + chlorine(g) → iron(III)chloride(s) Word Equation Word equation = write out the names of the reactants and products and use symbols to describe their physical states. Reactants  Products iron(s) + chlorine(g) → iron(III)chloride(s)

Practice Problems Write word equations from the following narratives: Hydrogen gas and bromine gas combine in the air to produce hydrogen monobromide gas. Gaseous carbon monoxide couples with gaseous oxygen to form gaseous carbon dioxide. Solid potassium chlorate decomposes to solid potassium chloride and gaseous oxygen.

Skeleton Equation Skeleton equation = use chemical formulas to identify the reactants and products and symbols to describe their physical states. Fe(s) + Cl2(g) → FeCl3(s) C(s) + S(s) → CS2(l)

Catalysts Catalyst is a substance that speeds up the reaction but is not used up in the reaction. A catalyst is neither a reactant or a product, so its formula is written above the arrow in a chemical equation. http://www.youtube.com/watch?v=Xkv1v7pee7E H2O2(aq)  H2O(l) + O2(g) MnO2

Practice Problems Write the skeleton equations for the following word equations: hydrogen(g) + bromine(g) → hydrogen monobromide(g) carbon monoxide(g) + oxygen(g) → carbon dioxide(g) potassium chlorate(s) → potassium chloride(s) + oxygen(g)

Chemical Equations Chemical equation = uses numbers (coefficients) and chemical formulas to identify the relative amounts of substances in a chemical reaction. 2Fe(s) + 3Cl2(g) → 2FeCl3(s)

Balancing Chemical Equations Write the skeleton equation for the reaction. H2(g) + Cl2(g) → HCl(g) Count the atoms of the elements in the reactants and products. 2 atoms of H + 2 atoms of Cl → 1 atom of H and 1 atom of Cl

Balancing Chemical Equations Caveat If a reaction involves identical polyatomic ions in the reactants and products, count the ions as if they are elements. HCl(aq) + NH4OH(aq) → H2O(l) + NH4Cl(aq) 1 atom of H and 1 atom of Cl + 1 ion of NH4+ and 1 ion of OH- → 2 atoms of H and 1 atom of O + 1 ion of NH4+ and 1 Cl- ion

Balancing Chemical Equations Change the coefficients to make the number of atoms of each element equal on both sides of the equation. Never change a subscript. H2(g) + Cl2(g) → 2HCl(g) 2 atoms of H + 2 atoms of Cl → 2 atom of H and 2 atom of Cl Write the coefficients in their lowest possible ratio. (1:1:2) is the lowest possible ratio, since the coefficients cannot be reduced and still remain whole numbers. Check your work!!! Make sure the chemical formulas are written correctly. Make sure the number of atoms for the reactants equals the number of atoms for the products.

Practice Problems Write a chemical equations for each narrative: In water, iron(III) chloride reacts with sodium hydroxide, producing solid iron(III) hydroxide and sodium chloride. Liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas. Solid zinc and aqueous hydrogen sulfate react to produce hydrogen gas and aqueous zinc sulfate.