Chapter 8“Chemical Reactions” Chemistry Judson High School Mr. Trotts
Describing Chemical Reactions • OBJECTIVES: • Describe how to write a word equation.
Describing Chemical Reactions • OBJECTIVES: • Describe how to write a skeleton equation.
Describing Chemical Reactions • OBJECTIVES: • Describe the steps for writing a balanced chemical equation.
All chemical reactions… • have two parts: • Reactants - the substances you start with • Products- the substances you end up with • The reactants turn into the products. • Reactants ® Products
CHEMICAL REACTION:a reaction in which one or more substances are changed to new substances.
The relationship between reactants and products can be written as:REACTANTS PRODUCTS
In a chemical reaction • Atoms aren’t created or destroyed. • A reaction can be described several ways: 1. In a sentence (every item is a word) Copper reacts with chlorine to form copper (II) chloride. 2. In a word equation (some symbols used) Copper + chlorine ® copper (II) chloride
Symbols in equations • the arrow separates the reactants from the products • Read as “reacts to form” or yields • The plus sign = “and” • (s) after the formula = solid: AgCl(s) • (g) after the formula = gas: CO2(g) • (l) after the formula = liquid: H2O(l)
Symbols used in equations • (aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution • used after a product indicates a gas has been produced: H2↑ • ¯ used after a product indicates a solid has been produced: PbI2↓
Symbols used in equations • indicates a reversible reaction (more later) • shows that heat is supplied to the reaction • is used to indicate a catalyst is supplied, in this case, platinum.
What is a catalyst? • A substance that speeds up a reaction, without being changed or used up by the reaction. • Enzymes are biological or protein catalysts.
3. The Skeleton Equation • Uses formulas and symbols to describe a reaction • but doesn’t indicate how many; this means they are NOT balanced • All chemical equations are a description that describe reactions.
Write a skeleton equation for: • Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. • Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.
Now, read these: Fe(s) + O2(g) ® Fe2O3(s) Cu(s) + AgNO3(aq) ® Ag(s) + Cu(NO3)2(aq) NO2 (g) N2(g) + O2(g)
4. Balanced Chemical Equations • Atoms can’t be created or destroyed in an ordinary reaction: • All the atoms we start with we must end up with • A balanced equation has the same number of each element on both sides of the equation.
I. CONSERVATION OF MASS:In a chemical reaction, matter is not created or destroyed but is conserved.
II. IN OTHER WORDS:The starting mass of the reactantsequals the final mass of the products.
Rules for balancing: • Assemble the correct formulas for all the reactants and products, use + and → • Count the number of atoms of each type appearing on both sides • Balance the elements one at a time by adding coefficients where needed (the numbers in front) - save balancing the H and O until LAST! (I prefer to save O until the very last) • Check to make sure it is balanced.
Never • Never change a subscript to balance an equation. • If you change the formula you are describing a different reaction. • H2O is a different compound than H2O2 • Never put a coefficient in the middle of a formula 2NaCl is okay, but Na2Cl is not.
solid lead (II) nitrate, dissolved in water, plus solid potassium iodide, dissolved in water, produces solid lead (II) iodide plus potassium nitrate, dissolved in water
A. All of this information is importantto letting a scientist know what the reactants and products are as well as their physical states.
B. A shorthand method has been developed to describe chemical reactions.
2. Coefficients(numbers to indicate how many molecules)3. Symbols(for physical state, catalysts, direction of reaction, etc.)
The reaction described above would look like this:Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)
IV. COEFFICIENTS: are the numbers placed to theleft of the formulas for the reactants and products.
A. The coefficients represent the number of units of each substancetaking part in a reaction.
Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq) B. In the reaction above, there are:1. 2 units each of KNO3(aq) andKI(aq)2. 1 unit each of Pb(NO3)2(aq) and PbI2(s)
A. The physical state of the reactantsB. Things added to help the reaction take place
C. An indicator to show which chemicals are the reactants and which are the products
Reactants ProductsC3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)1 unit propane gas;5 units oxygen gas3 units carbon dioxide gas; 4 units water vapor
VII. REACTION ENERGY -- Chemical reactions either absorb heat (endothermic) or release heat (exothermic).
A. Endothermic reactions must have energyadded(usually thermal, sometimes electrical, or light)for the reaction to take place.
1. This is due to more energy being required to break bonds than to make new bonds.
2. Some endothermic reactions cause the reaction container to feel coolto the touch.
b. Exothermic reactions have some form of energy released by the reaction (usually it is thermal or light).
1. This is due to less energy being required to break bonds than to form new bonds.
2. Exothermic reactions cause the reaction container to feel warm or even hot to the touch.