Chemical Reactions Chapter 7

Chap 7 Pretest Which of the following is an example of a physical change? A. Wood burns and becomes ash. B. A steel nail rusts over time. C. Ice melts and becomes water. D. Milk curdles when acid is added to it.

Chap 7 Pretest Which of the following characteristics can you determine about a substance based on its chemical formula? A. The number and types of atoms that make up a substance. B. The mass of an unknown sample of the substance. C. The melting point of the substance. D. The density and state of the substance at room temperature.

Chap 7 Pretest How do you find the atomic mass of an element? The atomic mass of each element is listed on the periodic table.

Chap 7 Pretest Which conversion factor would you multiply 0.020 m by in order to express the quantity in centimeters? A. 1000 m/1 km B. 1 km/1000 m C. 1 m/ 100 cm D. 100 cm/1 m

Equation Analogy Imagine that you work at a skateboard shop and you are in charge of assembling the skateboards. Every skateboard requires one deck (the board), two trucks (the mounted axels), and four wheels. Your boss asks you to make five skateboards. How many trucks do you need? The following diagram shows the “recipe” for one skateboard. What do you notice about the relative amounts of each part on either side of the arrow? + + 1D + 2T + 4W D1T2W4 One deck plus two trucks plus four wheels makes one skate board.

7.1 – Describing Reactions Chemical equations are used to describe reactions. Reactants Products C + O2 CO2 The coefficients show the molar ratios of the reactants and products, and they can be changed in order to balance the equation. 2Cu + O2 2CuO Chemical equations are balanced to show that mass is conserved in the chemical reaction. Only the coefficients, not the subscripts, should change when balancing chemical equations. subscript coefficient

7.1 – Balancing Chemical Equations Step 1 – count the number of atoms of each element on each side of the equation. Step 2 – change one or more coefficients until the equation is balanced. Start by balancing an element that appears in only one reactant and product. Once one element is balanced, proceed to balance another, and another, until all elements are balanced. Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas. Coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=2 oxygen atoms.

7.1 – Describing Reactions Chemists use the mole, which is an amount containing 6.02 x 1023 particles, to count large numbers of small particles. Also known as Avogadro’s number. The mass of one mole of a substance is called a molar mass. The molar mass of an element is the same as its atomic mass expressed in grams. A compound’s molar mass is the sum of the atomic masses of its component elements expressed in grams.

7.1 – Describing Reactions Molar mass is used to convert between molar and mass amounts of one substance. A mole ratio is used to convert between molar amounts of two different substances in a chemical reaction.

Models of Reactions The following drawings represent reactants and products of three different chemical reactions. Reaction A reactant products Reaction B reactants product Reaction C reactants products

Models of Reactions 1. Synthesis means “putting something together”. Which drawing represents a synthesis reaction? Reaction B 2. Decomposition means “taking something apart”. Which drawing represents a decomposition reaction? Reaction A 3. Replacement means “something taking the place of the other”. Which drawing represents a replacement reaction? Reaction C

7.2 – Types of Reactions Synthesis reaction – two or more substances react to form a single substance. “putting something together” A + B AB 2Na + Cl2 2NaCl Decomposition reaction – a single substance reacts to form two or more substances. “taking something apart” AB A + B 2H2O 2H2 + O2 reverse of synthesis reaction

7.2 – Types of Reactions Single replacement reaction – one element takes the place of another element in a compound. A + BC B + AC Cu + 2AgNO3 2Ag + Cu(NO3)2 Double replacement reaction – two different compounds exchange positive ions and form two new compounds. AB + CD AD + CB CaCO3 + 2HCl CaCl2 + H2CO3

7.2 – Types of Reactions Combustion reaction – a substance reacts rapidly with oxygen, often generating light and heat. CH4 + 2O2 CO2 + 2H2O 2H2 + O2 2H2O Oxidation-reduction (redox) reaction – any reaction that involves a transfer of electrons. 2Ca + O2 2CaO Ca Ca2+ + 2e- O + 2e- O2-

7.3 – Energy Changes in Reactions During a chemical reaction, chemical bonds in the reactants are broken and chemical bonds in the products are formed. Breaking bonds requires energy. Forming bonds releases energy. A chemical reaction can be classified as: Exothermic (releases energy to the surroundings) Endothermic (absorbs energy from the surroundings) Energy is conserved during chemical reactions. Total amount of energy before and after the reaction is the same.

7.4 – Reaction Rates Reaction rate – the time-rate at which reactants change into products. Reaction rates are generally increased by the following: Rise in temperature Increased surface area Concentration of reactants, agitation, and catalysts

7.5 – Equilibrium Equilibrium – when there is no change in the concentrations of reactants and products, and reactants and products are forming simultaneously. Can only take place when the forward and reverse reactions occur at the same rate. Products Reactants Equilibrium can also by physical in nature. Example: liquid water and water vapor inside a closed container. Factors affecting chemical equilibrium include: Shifts in temperature, pressure, and concentration.