1. Chapters 10 and 17 Chemical Reactions and Reaction Rates

2. Evidence of Chemical Reactions Chemical reaction – process by which atoms of one or more substances are rearranged to form different substances Most chemical reactions have some type of evidence – Temperature change – Bubbles – Odor – Color change – Appearance of a solid

3. Representing Reactions We use equations to represent reactions on paper – Reactants – the starting substances – Products – substances formed during the reaction

4. Representing Reactions Reactions are usually written using chemical symbols – (+) separates two or more reactants or products – (  ) separates the reactants from the products – (s) identifies a solid – (l) identifies a liquid – (g) signifies a gas – (aq) identifies a water solution In a stable molecule each atom has eight electrons

5. Balancing Chemical Equations Chemical equation – statement that uses chemical formulas to show the identities and amounts of substances involved in the reaction. There must be the same number of atoms on the reactants’ side as on the products’ side

6. Balancing Chemical Equations If the numbers of atoms on each side are not the same, we need to add coefficients in order to balance the equation – Coefficient – the number written in front of the reactant or product

7. Balancing Chemical Equations Steps to balancing equations 1.Write the basic equation H 2 (g) + Cl 2 (g)  HCl(g) 2.Count the number of atoms of each element on the reactant side and product side 2 H + 2 Cl  1 H & 1Cl 3.Add coefficients to make the number of atoms the same on both sides of the equation H 2 (g) + Cl 2 (g)  2HCl(g)

8. Balancing Chemical Equations Cont. 4.Make sure all the coefficients are in the lowest possible ratio Correct: H 2 (g) + Cl 2 (g)  2HCl(g) Incorrect: 3H 2 (g) + 3Cl 2 (g)  6HCl(g) 5. Check your work

9. Classifying Chemical Reactions Exothermic – a reaction that gives off heat (Chp 9) Endothermic – a reaction that absorbs heat, gets colder (Chp 9) Synthesis – a reaction in which two or more elements combine to make a single product H 2 (g) + Cl 2 (g)  2HCl(g)

10. Classifying Chemical Reactions Combustion – when Oxygen reacts with something to give off heat and light CH 4 + 2O 2  CO 2 + 2H 2 O Decomposition – one substance is broken down into multiple simpler substances 2H 2 O  2H 2 + O 2

11. Classifying Chemical Reactions Replacement reactions – Single replacement – One element in a substance is replaced by another element 3NaCl + Fe  FeCl 3 + 3Na – Double replacement – where two substances exchange an element Pb(NO 3 ) 2 + 2NaI  PBI 2 + 2NaNO 3 Also called and exchange reaction

12. Reactions in Solutions Most reactions are done with the reactants dissolved in water – Solute – the substance that gets dissolved – Solvent – what does the dissolving Ex. Salt is a solute and water is a solvent for salt water – Most of the solutions we use will be aqueous solutions, which means that water is the solvent

13. Types of Aqueous Reactions Some reactions form precipitates – Precipitate – solid particles that will not dissolve in water Pb(NO 3 ) 2 (aq) + NaI(aq)  PbI 2 (s) + NaNO 3 (aq) Water forming reactions – An acid will react with a base to form water HCl(aq) + NaOH(aq)  H 2 O(l) + NaCl(aq) Gas forming reactions HCl(aq) + NaHCO 3 (aq)  H 2 0(l) + CO 2 (g) + NaCl(aq)

14. Affecting Reaction Rates The nature of reactants affects how fast they will react. – The more reactive element will react faster Concentration affects reaction rate – The higher the concentration of reactants the faster the reaction goes – Reactions occur because particles collide – A higher concentration means more particles in a smaller volume

15. Affecting Reaction Rates Surface area – The more surface area of the reactants the faster the reaction – More surface area means more atoms can collide Temperature – In most cases the higher the temperature the faster reaction – Molecules move faster when they are hot, so they collide more often

16. Affecting Reaction Rates Catalyst – Catalyst – a substance that increases the rate of reaction without being changed itself – It doesn’t change how much get made, just how fast – The opposite is an inhibitor – substance that slows down a reaction