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Chemical Reactions: Bringing Molecules to Life 1

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Chemical Reactions: Bringing Molecules to Life 1 11/6/2019 A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the breaking and forming of chemical bonds between atoms. CHEMICAL REACTIONS: BRINGING MOLECULES TO LIFE 1 2 A reaction could involve: The substances (or substance) initially involved in a chemical ✓ ions, reaction are called reactants or reagents. ✓ compounds, Substances which are formed after chemical reaction are called ✓ molecules of a single element. products. Chemical reactions: Products usually have properties different from the reactants. ➢ breaking of bonds between atoms in molecules of The chemical reaction accompanies by chemical change. reactants ➢ pure hydrogen gas (H2) and pure oxygen gas (O2) form water ➢ formation of new bonds between different groups of liquid (H O) atoms in products. 2 ➢ an iron (Fe) from steel garbage reacts with gaseous oxygen (O2) from air and forms iron trioxide (Fe2O3) (rust). These two processes require the movement of valence electrons to maintain full shells 3 4 1 11/6/2019 CHEMICAL VS. PHYSICAL CHANGE PHYSICAL CHANGE What is the chemical change? In the physical change the appearance of a substance is In the chemical change the reacting substances form new altered but not its composition. When water becomes a entities that have different formulas and different properties. gas, or freezes to a solid it is still a water. New properties may involve a change in colour or formation If we smash a rock or tear a piece of paper, only size of of gas (bubbles), or a solid product. the material changes. When a silver tarnishes, the bright silver metal (Ag) reacts with sulphur (S) to become the blackish substance we call tarnish (AgS). 5 6 QUIZ. CHEMICAL VS. PHYSICAL CHANGES When a refrigerator or air conditioner cools the air, Among the presented below changes there is no reaction between the air molecules. When choose chemical: rusting a nail, pouring a you melt an ice cube, it is a physical change. The change milk, souring a milk, boiling a water, in temperature is a physical change. fermenting grapes, dissolving Alka-Seltzer When you put bleach in the washing machine to clean your clothes, a chemical change breaks up your stains. tablet in a glass of water. 7 8 2 11/6/2019 Chemical reaction vs. chemical equation CHEMICAL EQUATION A chemical reaction is the process that occurs when one or more A chemical equation is a symbolic representation of a chemical substances are changed into one or more new substances. reaction. Atomic symbols are used to represent the elements that take part in a reaction. Numbers are used to denote the ratios of reactants and For example: products applied in the reaction. ➢Sodium chloride (table salt) dissociates in water to form sodium and chlorine ions. ➢Methane combusts in oxygen to form carbon dioxide, heat and 9 water. 10 The tip of the arrow points in the direction in which Whereas a chemical equation is the symbolic the reaction proceeds. representation of a chemical reaction where the Usually the arrow is from reactants to products. reactant entities are given on the left hand side and A double arrow (↔) pointing in opposite directions is the product entities on the right hand side. used for equilibrium reactions. A + B →C + D Products Reactants 11 12 3 11/6/2019 Find the answer containing the proper direction of tips in chemical reaction: BALANCING CHEMICAL REACTIONS 1. - + A) CH3COOH → CH3COO + H Equations should be balanced according to the - + B) CH3COOH ↔ CH3COO + H stoichiometry, the number of atoms of each species - + C) CH3COOH ← CH3COO + H should be the same on both sides of the equation. 2. It is according to the mass action law (Guldberg- Waage, law). A) NaOH + HCl ← NaCl + H2O B) NaOH + HCl ↔NaCl + H2O C) NaOH + HCl → NaCl + H2O 13 14 Two important principles apply when we write balanced equations for reactions between species in a solution: For example, using for the above chemical reactions: the balanced equation only includes the species that 2 H2(g) + O2(g) → 2 H2O(ℓ) + - participate in forming of products NaCl(s) + H2O → Na (aq) + Cl (aq) the total charge must be the same on both sides of a K2SO4+ BaCl2 → 2KCl + BaSO4 + -2 2+ - + - balanced equation. 2K + SO4 +Ba + 2Cl → 2K +BaSO4+ 2Cl -2 2+ The total charge can be zero or non-zero, the charge of SO4 +Ba → BaSO4 both the reactant and product sides of the equation is the same. 15 16 4 11/6/2019 WRITE THE CHEMICAL REACTION IN THE IONIC FORM AND CANCEL SOLUTES WHICH DO NOT ADDITIONAL WORDS CONNECTING WITH REACT CHEMICAL REACTION AgNO + NaCl → AgCl + NaNO 3(aq) (aq) 3(aq) chemical reaction rate - the speed at which a Ag++ Cl-→AgCl↓ chemical reaction occurs (see lecture on kinetics) activation energy - the external energy that has to be Pb(NO3)2(aq) + K2SO4(aq) → PbSO4 + 2 KNO3(aq) added in order for a chemical reaction to occur (see 2+ 2- Pb + SO4 → PbSO4 ↓ lecture on kinetics) catalyst - a substance that helps a chemical reaction to occur (lowers the activation energy), but does not 17 participate in the reaction itself 18 Chemical reactions happen at a characteristic reaction Law of Conservation of Mass - states that matter rate at a given temperature and chemical concentration (mass) is neither created nor destroyed in a chemical (kinetics). Rapid reactions are often described as reaction. (thermodynamics) spontaneous, requiring no input of extra energy other Thus number of reactant atoms = number of product than thermal energy. atoms in a chemical reaction Non-spontaneous reactions run so slowly that they are considered to require the input of some type of additional energy (extra heat, light or electricity) in order to proceed to completion (chemical equilibrium) 19 20 at human time scales. 5 11/6/2019 More elaborate reactions are represented by Some relatively minor additions to the reaction can be reaction schemes, which in addition to starting indicated above the reaction arrow. Examples of such materials and products show important intermediates additions are: water, heat, illumination, a catalyst, etc. Heat, or transition states. 2CO + 2NO N2 + 2CO2 catalyst Similarly, some minor products can be placed below 2FeCl3 + 6 NH4OH →2Fe(OH)3 + 6NH4Cl ↓ temp the arrow, often with a minus sign. C2H5OH + HCOOH - H O HCOOC2H5 Fe2O3 2 21 22 An elementary reaction is the smallest division into which a chemical reaction can be decomposed to. It has no intermediate products. Elementary reaction Reactant →product TYPES OF THE REACTION 23 24 This is a sample reaction coordinate of an elementary reaction. Note that there is one transition state and no intermediates. 6 11/6/2019 The actual sequence of the individual elementary ELEMENTARY REACTION VS SERIES OF reaction is known as reaction mechanism. REACTIONS Example Single reactions often happen as part of a overall reaction is: larger series of reactions. HCOOH → CO + H2O An elementary reaction is assumed to Mechanism + occur in a single step and to pass through Step 1: HCOOH + H+ → HCOOH2 (fast) + + a single transition state. Step 2: HCOOH2 → H2O + HCO (slow) Step 3: HCO+ → CO + H+ (fast) 25 26 Complex reaction Reactant → intermediates → products ELEMENTARY REACTIONS An unimolecular elementary reaction is the type of reaction in which a molecule A dissociates or isomerises to form the product(s) ➢ N2O4(aq)→ 2 NO2(aq) Whereas in a bimolecular elementary reaction, two This is a sample reaction coordinate of a complex reaction. Note that it involves an intermediate and multiple transition atoms, molecules, ions A and B, react together to states. Again, a complex reaction can be explained in terms of elementary reactions. form the product(s) 27 28 http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Rate_Laws/Reactions/Elementary_Re actions - + ➢ HF (aq) + NH3(aq)→ F (aq) + NH4 (aq) 7 11/6/2019 QUIZ QUIZ Choose the correct statements. Which of the presented above reaction is/are a. An elementary step has 0 intermediates. unimolecular and which is/are bimolecular? A) O O + O b. An elementary step has 1 intermediate. 3 → 2 B) O + NO→ NO + O c. An elementary step has 2 intermediates. 3 2 2 H H CH C) 2 C C d. An elementary step has 0 transition states. CH CH3 H2C 2 H e. An elementary step has 1 transition state. D) f. An elementary step has 2 transition states. 29 30 CHAIN REACTION A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. Chain reactions are one way in which systems which are in thermodynamic non-equilibrium can release energy or increase entropy in order to reach a state of higher entropy. 31 32 8 11/6/2019 http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction 33 34 True or false QUIZZES QUESTION 3: QUESTION 1: All elements can bond with all other elements. Chemical reactions create and break down... QUESTION 4: A) proton orbitals; B) chemical bonds; C) atomic During reactions, both the molecules and state of matter nuclei; D) none of the above can change. QUESTION 5: o QUESTION 2: Equilibrium values are only for molecules in the same Reactions start with products and create reactants. state of matter. Examples: Liquids reacting with liquids or solids A) True; b) False reacting with solids. 35 36 9 11/6/2019 QUESTION 6: Stoichiometry is a long word that describes... A) The numer of atoms in the reactant equalling the numer of atoms in the products. B) The proces of breaking down waxes into fatty acids C) The proces of electrons jumping from one orbital to another VARIOUS TYPES OF CHEMICAL REACTIONS D) None of the above QUESTION 7: Catalysts ___________ reactions.
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