Chemical Reactions Chapter 17

Chemical Reactions Chapter 17

I. Physical Change Changes which affect the size or shape of the substance BUT NOT its chemical properties or formulas. e.g.; ice melting. Water is still water II. Chemical Change Affects the chemical properties of one or more substances (can also affect the size or shape).

A physical change is like printing the same word in a different style of type. stampedes stampedes A chemical change, or chemical reaction, is like scrambling the letters of a word to make new words. stampedes made + steps

Evidence for Chemical Reactions Color change Gas production Precipitation Changes in temperature Endothermic reactions- absorbs energy (drop in temperature) Exothermic reactions- release energy (raise in temperature) Changes in properties

The key characteristic of a chemical reaction is the production of new materials that are chemically different from the starting materials. Chemical reactions occur when chemical bonds are either formed or broken. All chemical bonds involve valence electrons

Conservation of Mass In a chemical reaction, no atoms just disappear and no new atoms just appear. The Law of conservation of mass and energy tells us this can not happen. Therefore the number of atoms on each side of the arrow in a chemical reaction equation MUST be equal. Remember even a gas has mass.

III. Chemical Equations Use formulas (equations) and symbols (elements) to show what happens during a chemical reaction. Molecules are separated by either a + sign or the = shows the direction a reaction moves in; yields. = shows that the substance is a solid or a precipitate. All other molecules are assumed to be in liquid form (aqueous form) NaI + Pb(NO 3 ) 2 NaNO 3 + PbI 2

Chemical Symbols Water H 2 O The little 2 is a subscript This means there are 2 atoms of H and 1 atom of O in this molecule. If there is NO subscript it is 1 Hydrogen peroxide H 2 O 2 This means there are 2 atoms of H and 2 atoms of O in this molecule.

IV. Balancing Chemical Reactions REMEMBER- In a chemical reaction, no atoms just disappear and no new atoms are just formed. To balance a chemical reaction equation you can only change the number of molecules in the reaction by changing the COEFFICIENT in front of the molecule. 2Mg + O 2 2MgO The big 2 is the COEFFICIENT on both sides sides of the arrow. This means you have 2 Mg and 2 MgO REMEMBER- The little 2 is a subscript

Structure of a Chemical Equation A chemical equation uses symbols to show the reactants and products of a chemical reaction. Reactant + Reactant Product + Product The means yields H 2 + O 2. H 2 O Hydrogen + Oxygen yields Water Reactants yields Products

REMEMBER you can not change, in any way: the subscripts formulas symbols in the equation http://www.wfu.edu/~ylwong/balanceeq/balanceq.html

2 H 2 O 2 2 H 2 O + O 2 Zn + 2 HCl I 2 + 2 KCl 2 Ba + 2 H 2 O 2 H 2 + O 2 2 ZnCl 2 + H 2 KI + Cl 2 Ba(OH) 2 + H 2 H 2 O 2 NaI + Pb(NO 3 ) 2 2 NaNO 3 + PbI 2 Cu + 2H 2 SO 4 CuSO 4 + 2 H 2 O+ SO 2 http://proton.csudh.edu/lecture_help/startbalancerxns.html

AgNO 3 + NaCl AgCl + NaNO 3 AgNO 3 + NaOH Ag 2 O + NaNO 3 + H 2 O AgNO 3 + Na 2 CO 3 Ag 2 CO 3 + NaNO 3 AgNO 3 + KI AgI + KNO 3

Pb(NO 3 ) 2 + KI PbI 2 + KNO 3 Pb(NO 3 ) 2 + NaOH Pb(OH) 2 + NaNO 3 Pb(NO 3 ) 2 + H 2 SO 4 PbSO 4 + HNO 3 Pb(NO 3 ) 2 + Na 2 CO 3 PbCO 3 + NaNO 3 Pb(NO 3 ) 2 + CuSO 4 PbSO 4 + Cu(NO 3 ) 2 Pb(NO 3 ) 2 + NaCl PbCl 2 + NaNO 3

CaCl 2 + NaOH Ca(OH) 2 + NaCl CaCl 2 + Na 2 CO 3 CaCO 3 + NaCl CaCl 2 + H 2 SO 4 CaSO 4 + HCl

V. Types of Reactions 1. Synthesis- two or more elements unite to form one compound. 2 Na + Cl 2 2 NaCl 2. Decomposition- break down of one compound into two or more elements. ZnCl 2 Zn + Cl 2

3. Single displacement- one element changes places with one cation in a molecule. BeF 2 + Mg MgF 2 +Be 4. Double displacement- the cation in each of two molecules change places. AgNO 3 + NaCl AgCl + NaNO 3

VI. Controlling Chemical Reactions Every chemical reaction involves a change in energy. Some reactions release energy: Exothermic Gives off heat

Some reactions absorb energy: Endothermic Take in heat

VII. Rates of Chemical Reactions Four ways to affect the rate of a chemical reaction (SPEED). A. Number of particles (reactant side of the equation) the concentration of molecules; This will give you more collisions between the molecules. B. Temperature the temperature and then the molecules move faster; therefore you increase the number of collisions.

C. Surface size of reactant molecules Smaller size will reaction speed D. Catalyst A substance that effects the rate of a chemical reaction without itself being changed or used up in the reaction. ENZYMES- catalyst in living things (CELLS). Inhibitor- a material used to slow down the rate of a reaction.