Chemical Reactions Chapter 11

Writing chemical equations To write a word equation: write names of reactants left of the arrow separated by plus signs; write names of products right of the arrow separated by plus signs. Hydrogen + oxygen  water

Skeleton equation Chemical equation that does not indicate amounts of the reactants and products. Not balanced! H2(g) + O2(g)  H2O(l) Catalyst is a substance that speeds up the reaction but is not consumed in the reaction. Neither reactant or product and written above the arrow.

Balancing equations Coefficients: small whole numbers that are placed in front of the formulas in an equation in order to balance it. A balanced equation is where each side of the equation has equal numbers of atoms of each element and mass is conserved.

Balancing equations First write the skeleton equation Then use coefficients to balance the equation so that it obeys the law of conservation of mass. Never change subscripts. Simplify if possible

Chemical reactions The five general types of reactions are: Combination Decomposition Single-replacement Double-replacement Combustion *Chemical change occurs with all of them!

Combination Reactions Two or more substances react to form a single new substance. 2Mg(s) + O2(g)  2MgO(s)

Decomposition A single reactant forms 2 or more products 2HgO(s)  2Hg(l) + O2(g)

Single-Replacement Reactions One element replaces a second element in a compound. Both the reactants and products have a single element and a compound. Activity series: lists metals in order of decreasing reactivity. Page 333 A reactive metal will replace any metal listed below it in the activity series. A reactive halogen will replace any halogen below it.

Single-replacement reactions Br2(aq) + NaI(aq)  NaBr(aq) + I2(aq) Br2(aq) + NaCl(aq)  No reaction

Double-Replacement Reactions Exchange of positive ions between two compounds. Aqueous solutions Form a precipitate (solid), a gas, or a molecular compound such as water Na2S(aq) + Cd(NO3)2(aq)  CdS(s) + 2NaNO3(aq) 2NaCN(aq) + H2SO4(aq)  2HCN(g) + Na2SO4(aq) Ca(OH)2(aq) + 2HCl(aq)  CaCl2(aq) + 2H2O(l)

Combustion Reaction An element or a compound reacts with oxygen often producing energy in the form of heat and light. Always involves oxygen as a reactant. The other reactant is usually a hydrocarbon (compound composed of carbons and hydrogens). Products are carbon dioxide and water for complete combustion.

Combustion Reaction 2C8H18(l) + 25O2(g)  16CO2(g) + 18H20(l) Reactions between oxygen and some elements other than carbon can also be combustion reactions. 2Mg(s) + O2(g)  2MgO(s)

Predicting the products of a chemical reaction The number of elements and/or compounds reacting is a good indicator of possible reaction type and thus possible products. Figure 11.10 on page 338-339