Chemical Reactions BIOB111 CHEMISTRY & BIOCHEMISTRY. Session 3

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1 Chemical Reactions BIOB111 CHEMISTRY & BIOCHEMISTRY Session 3

2 Session Plan Physical & Chemical changes Chemical equations Balancing chemical equations Classification of chemical reactions Redox reactions Formula Mass The Mole Avogadro s Number Molar Mass

3 Physical vs. Chemical Change Revision Physical change Change in the state of matter solid, liquid & gas The chemical identity & composition do not change water molecules are present in ice, liquid water & water vapor Chemical change Change in the chemical composition of a substance. The molecular structure of the original substance changes, while it undergoes a chemical reaction forming one or more new substances (methane gas reacts with O 2 to produce CO 2 & H 2 O). The properties of the original & new substances will also change.

4 Chemical Reactions CHEMISTRY = the study of structure, properties & transformation of matter. In a chemical reaction 1 or more original substances = REACTANTS are converted to 1 or more new substances = PRODUCTS. The bonds between atoms in the reactants are broken >>> atoms are rearranged & new bonds are formed between atoms = products. A chemical reaction is usually written in a chemical equation.

5 Chemical Equation Chemical equation: Is a diagram of a chemical reaction. Reactants are shown on the left & product on the right. The arrow shows the progress of the chemical reaction. Burning of coal can be written as the following equation: REACTANTS PRODUCTS C(s) + O 2 (g) CO 2 (g) Timberlake 2013, p210

6 Chemical Equation H 2 (g) + O 2 (g) H 2 O(g) chemical equation is not balanced different amounts of atoms of the same element on each side of the equation. LAW OF CONSERVATION OF MATTER Atoms can neither be created nor destroyed, only rearranged. 2H 2 (g) + O 2 (g) 2H 2 O(g) chemical equation is balanced same amount of atoms of the same element on each side of the equation.

7 Chemical Equation Subscript Identifies the number of atoms of a certain element present in a specific molecule. Subscripts never change in the formula. Coefficient Identifies the number of molecules present in the specific chemical equation. Always written in front of the formula. It is always a whole number (1 is usually not written)

8 Balancing of Chemical Equations 1. Write the equation with the correct formulas. NH 3 (g) + O 2 (g) NO(g) + H 2 O(g) 2. Determine if the equation is balanced. No, not all atoms are balanced. 3. Balance with coefficients in front of the formulas. 4NH 3 + 5O 2 4NO + 6H 2 O 4. Check that atoms of each element are equal on both sides of the chemical equation. 4 N (4 x 1N) = 4 N (4 x 1N) 12 H (4 x 3H) = 12 H (6 x 2H) 10 O (5 x 2O) = 10 O (4O + 6O)

9 Balancing of Chemical Equations N 2 + H 2 NH 3 Fe + O 2 Fe 2 O 3 CH 4 + O 2 CO 2 + H 2 O

10 Balancing of Chemical Equations N 2 + 3H 2 2NH 3 4Fe + 3O 2 2Fe 2 O 3 CH 4 + 2O 2 CO 2 + 2H 2 O

11 Types of Chemical Reactions Combination Decomposition Displacement Exchange Combustion Stoker 2014, p240

12 Types of Chemical Reactions Combination Decomposition 2HgO(s) 2Hg(l) + O 2 (g) Displacement Zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) Exchange AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) Combustion CH 4 + O 2 CO 2 + H 2 O

13 Combustion Reaction The most common type of chemical reaction. A substance reacts with oxygen (usually form the air), producing energy (usually in the form of heat / light). Organic hydrocarbons produce CO 2 + H 2 O: Methane CH 4 + O 2 CO 2 + H 2 O Propane C 3 H 8 + O 2 CO 2 + H 2 O

14 Redox Reactions OIL RIG (Oxidation-Reduction Reactions, a common type of Chemical Reaction). OXIDATION Loss of 1 or more electrons REDUCTION Gain of 1 or more electrons Redox reaction: CuSO 4 (aq) + Zn(s) ZnSO 4 (aq) + Cu(s) Placing Zinc strip in copper (II) sulphate solution {Cu 2+, SO 4 2- }, A redox reaction occurs. Copper metal begins to deposit on the strip. Oxidation & reduction are complementary processes, which run simultaneously because if there is a loss of electrons (oxidation) these electrons must be accepted somewhere else (reduction).

15 REDOX reaction In an oxidation-reduction reaction (redox) electrons from one reactant transferred to another Oxidation Is Loss of Electrons (OIL) Zn(s) Zn 2+ (aq) + 2e - Reduction Is Gain of Electrons (RIG) Cu 2+ (aq) + 2e - Cu(s)

16 Redox Reactions Timberlake 2013, p Electrons H atoms O atoms OXIDATION Loss of electrons Loss of H atoms Gain of O atoms REDUCTION Gain of electrons Gain of H atoms Loss of O atoms

17 Redox in Biochemistry Biological oxidation involves the loss of (electrons) and hydrogen atoms Coenzymes transfer hydrogen atoms to another compound Coenzymes are derived from vitamins Common coenzymes of living cells that carry H atoms NAD + (nicotinamide adenine dinucleotide) NADP (nicotinamide adenine dinucleotide phosphate) FAD (flavin adenine dinucleotide) Biological reduction is the addition of electrons and hydrogen atoms Molecules that undergo reduction become more energy rich

18 Key concept: chemical reactions What happens during a chemical reaction (hint: think about the reactants and products?) How do atom rearrangements facilitate the conversion of reactants into products? Is it possible for products to be converted back into reactants? Why/why not? G

19 Concept: Chemical reactions Context: Chemical reactions change the chemical structure of the reactant compounds & in doing so convert them into new product compounds. Question: Which of the following accurately describes the chemical reaction shown below? CH 4 + 2O 2 CO 2 + 2H 2 O a) The reactants CO 2 & H 2 O undergo a chemical change to become the products CH 4 & O 2 b) The reactants O 2 and CO 2 undergo a chemical change to become the products CH 4 & H 2 O c) The reactants CH 4 & O 2 undergo a chemical change to become the products CO 2 & H 2 O d) The reactants CH 4 & H 2 O undergo a chemical change to become the products O 2 and CO 2

20 Concept: Bond breakage/formation in chemical reactions Context: Chemical reactions involve the breakage and formation of chemical bonds which rearranges the molecular structures of the compounds involved. Through undergoing a chemical reaction a reactant compound is converted into a product compound. Question: Which of the following accurately describes a chemical reaction? a) Bond breakage within the reactant compound is not needed to convert reactants to products b) The products are formed before bond breakage occurs during a chemical reaction c) Bond breakage & formation must occur in both the reactants & products during a chemical reaction d) Some of the bonds connecting the atoms within the reactant compounds must be broken before new product compounds can form via the formation of new bonds

21 G Cu + 2Ag + Cu Ag Key concept: redox reactions When the Ag + positively charged cation gains one electron, what happens to its charge? Gaining/losing electrons is called oxidation/reduction? In the above reaction, which atom/ion is oxidised/reduced? Is the reaction reversible?

22 Concept: Redox reactions Context: Many metabolic reactions such as the citric acid cycle rely on redox reactions to convert reactants into products. In a redox reaction both oxidation and reduction occur simultaneously. Question: Which of the following accurately describes how redox reactions proceed (example of a redox reaction show below)? Cu + 2Ag + Cu Ag a) Oxidation and reduction of the two reactants causes them both to lose electrons b) The oxidised reactant loses electrons, while the other reactant is reduced thereby gaining electrons c) Oxidation and reduction of the two reactants causes them both to gain electrons d) The reduced reactant loses electrons, while the other reactant is oxidised thereby gaining electrons

23 Concept: Redox reactions Context: In the chemical reaction below oxygen (O 2 ) & methane (CH 4 ) react to form carbon dioxide (CO 2 ) & water (H 2 O). The chemical reaction is classed as a redox reaction as one of the reactants is oxidised and one is reduced. Question: In the reaction below which of the reactants is oxidised/reduced & why? CH 4 + 2O 2 CO 2 + 2H 2 O a) In the redox reaction, CH 4 is oxidised as it loses hydrogen & gains oxygen, whereas O 2 is reduced as it gains hydrogen b) In the redox reaction, O 2 is oxidised as it gains hydrogen, whereas CH 4 is reduced as it loses hydrogen and gains oxygen c) In the redox reaction, CH 4 is oxidised as it gains hydrogen & loses oxygen, whereas O 2 is reduced as it loses hydrogen d) In the redox reaction, O 2 is oxidised as it loses hydrogen, whereas CH 4 is reduced as it gains hydrogen and loses oxygen

24 Formula Mass The sum of the atomic masses of all the atoms present in the chemical formula of a substance. Formula mass of calcium chloride CaCl 2 = amu Amu = atomic mass units Atomic mass of Ca = amu Atomic mass of Cl = amu Can find in the periodic table (35.45 x 2) = amu

25 Mass & Amount Units The quantity of a substance in a sample is given in mass units or amount units. Mass of a substance is measured in grams. Amount of a substance is specified by numbers or collection terms. 1 dozen eggs = 12 eggs 1 case of VB = 24 cans VB 1 ream of paper = 500 sheets

26 The Mole = Chemist s Counting Unit 1 mole = 6.02 x particles x = Avogadro s number Definition of a mole: The amount of a substance that contains as many particles (atoms, molecules or formula units) as there are atoms in exactly 12 grams of pure 12 C. 1 mole C = 6.02 x C atoms = g C 1 mole of anything = 6.02 x units of that thing.

27 The Mole A mole of atoms 1 mole of Na = 6.02 x Na atoms 1 mole of O = 6.02 x O atoms A mole of ionic compound 1 mole of NaCl = 6.02 x NaCl formula units 1 mole of K 2 SO 4 = 6.02 x K 2 SO 4 formula units A mole of covalent compound 1 mole of CO 2 = 6.02 x CO 2 molecules 1 mole of H 2 O = 6.02 x H 2 O molecules

28 Calculating with the Mole Avogadro's number is used to convert particles of a substance to moles of that substance. How many moles of CO 2 are represent 2.50 x molecules CO 2? Can use a ratio? moles of CO 2 = 2.50 x CO 2 1 mole of CO 2 = 6.02 x CO x molecules CO 2 x 1 mole CO 2 = 4.15 moles CO x molecules CO 2 There are 4.15 moles of CO 2 represented in 2.50 x molecules CO 2.

29 Molar Mass Mass of one mole of a substance in grams. The Molar Mass of an Element = the Atomic Mass of the element (see Periodic Table below elemental symbol) expressed in grams. The Molar Mass of a Compound = the sum of the Molar Masses of all the elements in the formula. Calculate the Molar Mass of H 2 O? Atomic mass of H = 1g Atomic mass of O = 16g Calculation: (1x2) + 16 = 18g/mol The Molar Mass of H 2 O is 18g/mol.

30 Exercise Mole (n)= mass (grams) Molar mass (Mr) n= m/mr Calculate the mass of a 2.5-mole sample of ethane, C 2 H 6. n = 2.5 mol m =? g Mr = g/mol (from periodic table) n=m/mr m=n Mr m=2.5 mol g/mol m=75 g Calculate the number of moles in 50.0 g of H 2 O. n =? mol m = 50 g Mr = g/mol (from periodic table) n=m/mr n=50 g/ (g/mol) m=2.78 g

31 Chemical Formulas & The Mole In a chemical formula the subscripts indicate: The number of atoms of each element present in 1 formula unit of the substance. E.g. H 2 O has two hydrogen and one oxygen atoms Chemical formula also tells you the number of moles of atoms of each element present in one mole of a substance. E.g. for one mole of H 2 O molecules, there are two moles of hydrogen and one mole of oxygen atoms In 1 molecule of glucose C 6 H 12 O 6 In 1 molecule: 6 atoms C 12 atoms H 6 atoms O In 1 mole: 6 moles C 12 moles H 6 moles O

32 Aspirin C 9 H 8 O 4 Subscripts indicate: Number of atoms Number of moles of an element, within a substance. Timberlake 2013, p228

33 Exercise How many moles of carbon atoms and hydrogen atoms are present in a 2.5-mole sample of ethane, C 2 H 6? æ 2 mol C ö 2.5 mol C2H 6 ç = 5.0 mol C atoms 1 mol C H è 2 6 ø æ 6 mol H ö 2.5 mol C2H 6 ç = 15 mol H atoms 1 mol C H è 2 6 ø There are 5 moles of O atoms & 15 moles of H atoms in a 2.5-mole sample of ethane.

34 The Mole Concept Summary Stoker 2014, p163

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36 Readings & Resources Stoker, HS 2014, General, Organic and Biological Chemistry, 7 th edn, Brooks/Cole, Cengage Learning, Belmont, CA. Stoker, HS 2004, General, Organic and Biological Chemistry, 3 rd edn, Houghton Mifflin, Boston, MA. Timberlake, KC 2013, General, organic, and biological chemistry: structures of life, 4 th edn, Pearson, Boston, MA. Alberts, B, Johnson, A, Lewis, J, Raff, M, Roberts, K & Walter P 2008, Molecular biology of the cell, 5 th edn, Garland Science, New York. Berg, JM, Tymoczko, JL & Stryer, L 2012, Biochemistry, 7 th edn, W.H. Freeman, New York. Dominiczak, MH 2007, Flesh and bones of metabolism, Elsevier Mosby, Edinburgh. Tortora, GJ & Derrickson, B 2014, Principles of Anatomy and Physiology, 14 th edn, John Wiley & Sons, Hoboken, NJ. Tortora, GJ & Grabowski, SR 2003, Principles of Anatomy and Physiology, 10 th edn, John Wiley & Sons, New York, NY.

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