Energy and Physical Processes - Exothermic and Endothermic Reactions

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1 Chem 2A/2B Term 3 Notes A 1 Coghlan Chemistry 2A/2B Term 3 Notes A Energy and Physical Processes - Exothermic and Endothermic Reactions An in the E P of the (the particles are further ) may cause a L G change. To achieve this change, energy in the form of must be to the system. H 2 O(l) + Energy H 2 O(g) Because energy is to the system, the change is endothermic. Conversely CO 2 (g) CO 2 (s) + Energy Because energy is from the system, the change is exothermic. The energy changes involved in such processes are much than the energy changes involved in chemical because only interparticular are involved (hydrogen, dipole-dipole and dispersion). To a chemical bond (ionic, or ) requires (it is an endothemic process). To a chemical bond (, or metallic) energy (it is an process). This energy involved is in the chemical bond and is called, H.

2 Chem 2A/2B Term 3 Notes A 2 Coghlan Enthalpy and Chemical reactions For a chemical reaction to occur, bonds must be and. Breaking bonds is an process and the enthalpy of the system. Making bonds is an process and the enthalpy of the system. The difference in total enthalpy for a reaction, H, is final enthalpy (products) take initial enthalpy (reactants). H = H products - H reactants If H products > H reactants then the overall H for the reaction is and the reaction is. If H products < H reactants then the overall H for the reaction is and the reaction is. Endothermic Exothermic Endothermic reactions Exothermic reactions Breaking and Making chemical bonds Energy is required to break bonds holding molecules or compounds together breaking of chemical bonds is an endothermic process. Energy is released when bonds are made making of bonds is an exothermic process.

3 Chem 2A/2B Term 3 Notes A 3 Coghlan So for an overall endothermic reaction, the energy required to break bonds is greater than the energy released when bonds are made. So for an overall exothermic reaction, the energy required to break bonds is less than the energy released when bonds are made. Potential Energy Diagrams for reactions(energy Profile diagrams) Exothermic Using CO + NO 2 as an example Moving along the Reaction from left to right a. CO & NO 2 one another. b. CO & NO 2 are close enough to. If they have sufficient E K, the complex is formed. c. Activated complex is for med. If sufficient energy is available, will form. d. CO 2 & NO have separated and E K.

4 Chem 2A/2B Term 3 Notes A 4 Coghlan Endothermic The of the energy barrier ( energy) has no effect on the net heat released/absorbed. must be put into the system to overcome the energy barrier. This energy is in an reaction. This energy is in the in an endothermic reaction. Investigate an endothermic reaction 1. Pour the citric acid solution in a styrofoam coffee cup. Use a thermometer or other temperature probe to record the initial temperature. 2. Stir in the baking soda (sodium bicarbonate). Track the change in temperature as a function of time. 3. The reaction is: H 3C 6H 5O 7(aq) + 3 NaHCO 3(s) --> 3 CO 2(g) + 3 H 2O(l) + NaC 6H 5O 7(aq) EC Set 15 Enthalpy change examples and Energy in society Burning of fuels Heat packs Evaporative air conditioners Cold packs

5 Chem 2A/2B Term 3 Notes A 5 Coghlan Respiration Photosynthesis Food Rates of Reaction The rate of a chemical reaction is the rate at which reactants are Or The rate at which are made Or H t (the rate of energy or released). Rate of Reaction = Collision Theory This theory states that for a reaction to occur there must be a. A between reactants. b. The collision must have energy. c. The collision must have the correct. At constant T, as the concentration of the increases, the number of per unit increases, therefore the rate of reaction. Consider a reaction According to theory, there must be a collision between ions with the geometry this is very. However the reaction does.

6 Chem 2A/2B Term 3 Notes A 6 Coghlan To explain this, a reaction mechanism has been proposed for all reactions. This is a of reactions by which a process occurs. The step in a reaction mechanism is called the rate step. The intermediate reaction products are called. A acid reacts more than a dilute one. Factors affecting reaction rates 1. of reactants 2. of reactants 3. Catalyst Subdivision. Nature of reactants Bonds are broken Bonds are made Reactions which involve the and of many bonds tend to be. Conversely, reactions that involve no bond are fast.

7 Chem 2A/2B Term 3 Notes A 7 Coghlan Also, some substances are inherently more reactive than others due to their e - or. reactivity of N 2 can be explained by the triple bond. Concentration of Reactants The the rate of collisions between species the the reaction will be. blowing on a small fire to make it burn faster; supercharging a car engine State of Subdivision For reactions involving different, the state of subdivision of effects reaction rate. If a solid is more divided, there will be more area available and hence more per unit time. The spraying of fine mist into an speeds up the reaction rate. Temperature As the temperature of a reaction, individual reacting species possess more and more. This means that when reacting species, the chance of them having energy to form the complex is so the reaction rate. A effect of increasing temperature is the increase in velocity of the species and hence a more rapid rate of collision. Food will much faster at a temperature because of the reaction. Catalyst The process of increasing the rate of through the use of a catalyst is known as. When a catalyst is added, a new is provided with a activation and different activated.

8 Chem 2A/2B Term 3 Notes A 8 Coghlan Although the catalyst emerges after reaction unchanged, it was probably during of the reaction steps and then changed at another step. Catalysts in a car s converter (Pt, Rh) help remove from the fumes. Enzymes are catalysts that are active in many lifesustaining. There are many applications of catalysts Fe in the of NH 3, in the manufacture of H 2 SO 4. EC Set 16 Writing ionic equations 1. Acid + Base ( ) Acid + Base Salt + H 2 O 2. Acid + Metal Oxide (Neutralisation) Since oxides are ; Acid + Base Salt + H 2 O 3. Acid + Metal Only for the following metals; Acid + Metal Salt + H 2 sodium and HCl

9 Chem 2A/2B Term 3 Notes A 9 Coghlan 4. Acid + Carbonate Acid + Carbonate Salt + H 2 O + CO 2 5. Acid + Hydrogencarbonate ( ) Acid + Hydrogencarbonate Salt + H 2 O + CO 2 6. Acid + Sulfite For concentrated acids only. Acid + Sulfite Salt + H 2 O + SO 2 Concentrated Sulfuric acid + sodium sulfite 7. Acid + Sulfide Group I & II and Fe, Ni, Co, Zn, Mn sulfides only Acid + Sulfide Salt + H 2 S Iron(II) sulfide + Hydrochloric acid

10 Chem 2A/2B Term 3 Notes A 10 Coghlan 8. Decomposition Heating carbonates and hydrogencarbonates 9. Precipitation Two solutions produce a precipitate. sodium iodide and mercury(ii) chloride AA , Set 9 AA , Set 10, 11 Limiting Reagent Sometimes the masses of both reagents are given one of them will usually be present in excess i.e. when the reaction stops there is some of one reactant still left. The reagent in excess must be identified first. Ex/ 7.38 g of calcium carbonate (limestone) is reacted with 2.63 g of hydrochloric acid. What mass of carbon dioxide is generated?

11 Chem 2A/2B Term 3 Notes A 11 Coghlan Ex/ Now calculate the volume of the carbon dioxide produced at STP. The key to knowing you are dealing with a limiting reagent problem is if the problem gives you data concerning two different substances (usually both reactants). AA , Set EC Set 23 Percentage Composition Ex/ Find the % composition of sulfuric acid. Ex/ What is the % composition of ethanoic acid (CH 3 COOH)?

12 Chem 2A/2B Term 3 Notes A 12 Coghlan AA , Set 14 Empirical Formulae The empirical formula of a compound is the whole number ratio of the of atoms/ present. The empirical formula of a compound is the simplest whole number ratio of the number of of atoms/ions present. Empirical Formula Molecular Formula Ex/ A compound consists of 45.1% oxygen, 33.3% chlorine and the remainder sodium. If its molar mass is g mol -1, determine its empirical & molecular formula.

13 Chem 2A/2B Term 3 Notes A 13 Coghlan Ex/ A 5.00 g of a compound contains 1.86 g of C, 0.39 g of H and 2.75 g of Cl. Determine its empirical formula. Ex/ Water of Crystallisation The common form of magnesium sulfate is a hydrate known as Epsom salts a common laxative. A sample of Epsom salts is heated carefully in a clean, dry crucible and the following results were obtained; Mass of crucible + lid after heating = g Mass of crucible + lid + Epsom salts = g Mass of crucible + lid + Epsom salts after 25 minutes heating = g Mass of crucible + lid + Epsom salts after 5 minutes more heating = g a. Calculate the formula of the hydrated magnesium salts. b. The crystals of the Epsom salts appear to melt soon after heating, yet their actual MP is over 1000 o C. Explain this observation. EC Set 24, 25