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Chemical Reactions

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.

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Chemical Reactions

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  1. Chemical Reactions Chapter 7

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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. 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

  8. 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.

  9. 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.

  10. 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.

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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-

  16. 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.

  17. 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

  18. 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.

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