Chapter 9 An overview of chemical reaction Chemical reactions happen in nature to produce natural substance, and even living organism. Chemical reactions have been used by us to produce millions of substance that do not exist in nature. Chemical reactions are the central part of the chemical sciences.

9.1 Chemical reactions are represented by chemical equations During a chemical reaction, one or more new compounds are formed as a result of the rearrangement of atoms. The chemical reaction can be represented by a chemical equation ( 化学反应式 ), which shows the substances about to react, called reactants ( 反应物 ), to the left of an arrow that points to the newly formed substances, called products ( 产物 ).

Law of mass conservation ( 质量守恒定律 ): matter is neither created nor destroyed during a chemical reaction. The finding of oxygen resulted from this law. The chemical equation must be balanced ( 平衡 ).

9.2 Chemists use relative masses to count atoms and molecules How to measure a specific number of atoms and molecules? By relative masses or molar mass ( 摩尔质量 ). 2H 2 + O 2 = H 2 O 2 moles 1 mole 2 moles Which is Which is which is 4 grams 32 grams 36 grams Which is Which is which is 12.04×10 23 molecules 6.02×10 23 molecules 12.04×10 23 molecules

9.3 Reaction rate ( 反应速率 ) is influenced by concentration and temperature How fast is a given reaction? Kinetics ( 动力学 ) Reaction rate can be determined by how fast the product generates or the reactant disappears. time Low concentration of products, high concentration of reactants high concentration of products, low concentration of reactants Reactants Products

The factors influencing the reaction rate: (1). How often do the reactant molecules meet? Concentration! (2). How often do the encountered reactant molecules lead to a reaction? High enough kinetic energy to break the chemical bonds in the reactants, temperature! (3). …. Fig 9.7 during a reaction, reactant molecules collide with each other

Reaction rate mA + nB lC Rate = k[A] m [B] n Arrhenius Equation: k = Ae -Ea/RT –Where Ea is the activation energy, A is frequency factor, which is related to the frequency of collisions and the probability that the collisions are favorably oriented for reaction.

Broken bonds are a necessary first step in most of chemical reactions. The energy required for this in initial breaking of bonds can be viewed as an energy barrier. The minimum energy required to overcome this energy barrier is known as the activation energy ( 活化能 ).

At any given temperature, there is a wide distribution of kinetic energies in reactant molecules. When the temperature of reactions is increased, the number of reactant molecules having sufficient energy to pass over the barrier also increases, leading to increased reaction speed.

Discussion topics: reaction selectivity Sometimes, electromagnetic fields, such as sunlight, may also lead to reactions. Question: What is the advantage and disadvantage of application of electromagnetic fields in chemical reactions? Direction of collision on reaction selectivity? Synthesis of protein: group protection

Amino acids are linked through peptide bonds ( 肽键 ). A group of amino acids linked together through peptide bonds is called peptide. Peptides containing more than ten amino acids are generally called polypeptides.

9.4 Catalysts ( 催化剂 ) increase the rate of chemical reactions A third way to increase the rate of a reaction is to add a catalyst, which is any substance that increases the rate of a chemical reaction by lowing its activation energy. The catalyst may participate as a reactant, but it is then regenerated as a product.

Cl + O 3 ClO + O 2 ClO + O 3 Cl + O 2 One chlorine atom in the ozone layer can catalyze the transformation of 100,000 ozone molecules to oxygen molecules in one or two years.

Catalytic converter To convert nitrogen monoxide (NO) to nitrogen and oxygen, carbon monoxide to carbon dioxide. In 1960, a typical car emitted about 11 grams of uncombusted fuel, 4 grams of nitrogen oxide (NOx), and 84 grams of carbon monoxide per mile traveled. In 2000, emitted less than 0.5 gram of uncombusted fuel, 0.5 gram of nitrogen oxide (NOx), and 3 grams of carbon monoxide.

Discussion: Homogeneous reaction (catalysis) and heterogeneous reaction (catalysis) Advantage and disadvantage? Example: protein synthesis

9.5 Chemical reaction can be either exothermic ( 放热的 ) or endothermic ( 吸热 的 ) For the chemical reactions taking place in burning wood, there is a net release of energy. For those taking place in a photosynthetic plant, there is a net absorption of energy.

The energy absorbed as a bond breaks or released as one forms is called bond energy ( 键能 ). Each chemical bond has its own characteristic bond energy. 2H 2 + O 2 2H 2 O Type of bondNumber of molesBond energyTotal energy H-H kJ/mole+872 kJ.mole O-O1+498 kJ/mole498 kJ/mole Total energy absorbed+1370 kJ Type of bondNumber of molesBond energyTotal energy H-O kJ/mole kJ/mole Total energy released kJ Net energy of reaction = energy absorbed + energy released = kJ BondBond energy(KJ/mol)BondBond energy(KJ/mol) H-H436O-O138 H-C414Cl-Cl243 H-N389N-N159 H-O464N=O631 H-F569O=O498 H-Cl431O=C803 H-S339N=N946 C-C347C=C837 Table 9.1 selected bond energies

In an exothermic reaction, the product molecules are at a lower potential energy than the reactant molecules. The net amount of energy released by the reaction is equal to the difference in potential energies of the reactants and products