Chemical reactions: an introduction Chapter 6

Evidence for a chemical reaction Chemical reaction often gives a visual signal A color change A solid forms Bubbles are produced (gas)-DEMO A flame occurs-DEMO Change in temperature IMPORTANT! One of these signals does not ALWAYS mean that a chemical reaction has taken place. Examples?

Chemical equations In a chemical reaction, the atoms of different chemicals are rearranged. We write this as a chemical equation. CH4 + O2  CO2 + H2O Ex: Reaction of methane and oxygen Reactants-those chemicals present before the reaction; left of the arrow CH4 & O2 Products-those chemicals present after the reaction; right of the arrow CO2 & H2O

Chemical equations In a chemical reaction, atoms are neither created or destroyed Any atoms present in the reactants must be accounted for in the products. There must also be the same number of each type of atom on the product side as on the reactant side of the arrow.

Balancing chemical equations This chemical equation does not have the same number of each type of atom on each side of the arrow…therefore, it is not balanced. CH4 + O2  CO2 + H2O Reactants = 1 C; 4 H; 2 O Products = 1 C; 2 H; 3 O We can balance the equation by adding coefficients to the front of the compounds.

Balancing chemical equations How can we balance this equation? CH4 + O2  CO2 + H2O CH4 + 2O2  CO2 + 2H2O

Physical States of chemicals In a chemical equation, we often indicate the physical states of the reactants and products with the following symbols. (s) solid (l) liquid (g) gas (aq) dissolved in water-an aqueous solution

Physical states of chemicals The equation below demonstrates how we use these symbols: K(s) + H2O(l)  H2(g) + KOH(aq) -Solid potassium is dissolved into liquid water. Hydrogen gas is produced while potassium hydroxide remains dissolved in the water.

Write the unbalanced chemical equations of the following reactions Solid mercury (II) oxide decomposes to produce liquid mercury metal and gaseous oxygen. Solid carbon reacts with gaseous oxygen to form gaseous carbon dioxide. Solid zinc is added to an aqueous solution containing dissolved hydrogen chloride to produce gaseous hydrogen that bubbles out of the solution out of the solution and zinc chloride that remains dissolved in the water.

Balancing Chemical Equations When balancing a chemical equation, the identities (formulas) of the compounds must never be changed. H2(g) + O2(g)  H2O(l) Can’t make H2O2 to balance the equation. H2O2 is peroxide, while H2O is water. 2H2(g) + O2(g)  2H2O(l)

How to Write and balance equations Read the description of the chemical reaction. What are the reactants, the products, and their states? Write the appropriate formulas. Write the unbalanced equation that summarizes the information from Step 1. Balance the equation by inspection, starting with the most complicated molecule. Do not change the identities (formulas) of any of the reactants or products. Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. Note: An atom may be present in an element, a compound, or an ion. Also, use the smallest coefficient s that balance the equations.

Write the balanced chemical equation for the following reactions Nitrogen monoxide gas reacts with hydrogen gas to form nitrogen gas and water. Lithium metal reacts with chlorine gas to form solid lithium chloride. Iron metal reacts with oxygen gas in the air to form iron (III) oxide. Diboron trioxide reacts with water to form boric acid.

Homework Complete the 3 Self-Check Exercises on pg. 158 and the 3 Self- Check Exercises on pg. 165.