Date: SCH 4U Name: ENTHALPY CHANGES

Date: SCH 4U Name: ENTHALPY CHANGES Enthalpy (H) = heat content of system (heat, latent heat) Enthalpy = total energy of system + pressure volume H = E + PV H = E + (PV) = final conditions initial conditions In open system, P & V don t change so H = E = Q If heat gained by system +Q + H endothermic If heat lost by system -Q - H exothermic Enthalpy Changes Physical Changes = substance is merely changed from one form to another, chemical composition has not changed (ie/ cutting, pressing, dissolving in solution, evaporating, melting etc.) o Enthalpy of Solution (C) To make a solution: Step 1: solute-solute and solvent-solvent intermolecular forces are broken ( H solute & H solvent, both are +ve) Step 2: solute-solvent intermolecular forces are formed ( H mix is ve) H solution = H solute + H solvent + H mix H solution = H final H initial + H solution = endothermic - H solution = exothermic o Enthalpy of Phase Changes (D) H o melt = enthalpy of melting, solid liquid H o vap = enthalpy of vaporization, liquid gas H o cond = enthalpy of condensation, gas liquid H o fre = enthalpy of freezing, liquid solid H o melt = - H o fre H o vap = - H o cond Heating Curve of Water (E) 1) A to B solid water (ice) is absorbing heat to heat from -25 o C to 0 o C 2) B to C temperature stays at 0 o C, absorbing heat to break intermolecular forces, solid (ice) liquid (water) 3) C to D liquid water is absorbing heat to heat from 0 o C to 100 o C 4) D to E temperature stays at 100 o C, absorbing heat to break intermolecular forces, liquid (water) gas (water vapour) 5) C to D gas (water vapour) continues to heat up, absorbs heat, 100 o C to 125 o C Chemical Changes = chemical reaction occurs between atoms or between compounds when chemical bonds are broken and re-formed (see thermochemical equations & calorimetry) Nuclear Changes = nuclear reaction occurs when the nucleus of an atom changes its composition o Nuclear Fusion = nuclei join together, releases a lot of energy in the process (F) Sun s core constantly fuses hydrogen atoms together to make helium atoms Hydrogen bombs fuse lithium atoms and neutrons High temperatures are needed to overcome the repulsion between atoms and bring them together Nuclear fusion power plants would solve our energy problems: More energy per unit mass is released in fusion than fission

2 Does not produce radioactive products But need a lot of energy to fuse nuclei together, IRL scientists use more energy to make fusion happen than what is produced Scientists also can t make fusion safe on a large scale like a power plant o Nuclear Fission = unstable nuclei splits spontaneously or when hit by neutrons (G1) There are 3 types of fission or decay: o Alpha α Decay Alpha particle is released from atom 2 protons, 2 neutrons = He nucleus Low penetrating power, a few centimetres in air Can be stopped by a sheet of paper o Beta β Decay Beta particle is released from atom No mass, negative charge = electron Moderate penetrating power, can go through skin, a few metres in air Stopped by 1mm thick Al o Gamma γ Radiation Gamma rays are released No mass, no charge = energy High penetrating power, moves at the speed of light Stopped by 1m thick Pb or concrete In general, (G2) H solution < H phase change < H chemical reaction < H nuclear reaction 100s kj 10 8 kj

3 THERMOCHEMICAL EQUATIONS H r = enthalpy change of reaction H comb = enthalpy change of combustion reaction H o = enthalpy change at SATP ( o = nought) In a chemical reaction: (H) Energy is needed to break bonds (endothermic) Energy is released when new bonds form (exothermic) Overall reaction can be endothermic: o Products have more enthalpy than reactants o N 2(g) + 2 O 2(g) + 66.4 kj 2 NO 2(g) o H r = +66.4 kj Or overall reaction can be exothermic: o Reactants have more enthalpy than products o CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) +890.8 kj o H r = - 890.8 kj Standard Molar Enthalpy of Combustion Scientists have done experiments and calculated the molar enthalpy (kj/mol) for many standard reactions C 5 H 12(l) + 8 O 2(g) 5 CO 2(g) + 6 H 2 O (l) o Look up C 5 H 12(l) H comb = -3509.0 kj/mol, so: o C 5 H 12(l) + 8 O 2(g) 5 CO 2(g) + 6 H 2 O (l) + 3509.0 kj/mol 2 CH 3 OH (l) + 5 O 2(g) 2 CO 2(g) + 8 H 2 O (l) o Look up CH 3 OH (l), H comb = -726.1 kj/mol, so: o 2 CH 3 OH (l) + 5 O 2(g) 2 CO 2(g) + 8 H 2 O (l) + (2)(726.1 kj/mol) or o CH 3 OH (l) + 2.5 O 2(g) CO 2(g) + 4 H 2 O (l) + 726.1 kj/mol

4 CALORIMETRY Calorie = amount of heat needed to increase temperature of 1g of water by 1 o C, not an SI unit (1 Calorie = 1000 calorie, 1 calorie = 4.184 J, 1 Calorie = 4.184 kj) Calorimetry = study of energy changes in physical and chemical changes with the use of a calorimeter Calorimeter = an isolated system used to measure how much heat is absorbed or released during a physical or chemical change Simple Calorimeter (I) Used for reactions occurring in solution 2 polystyrene cups stacked in each other o Polystyrene is good insulator o Air trapped between cups also insulates Inner cup = where reaction occurs, usually with a known mass of water o Make sure water is at room temp before reaction 2 holes: one for stirrer, one for thermometer o Air can flow freely, system is at constant pressure o Stir reactants frequently to ensure even temp. o Record T with thermometer Assumptions: o System is isolated o Thermal energy exchanged between all components is negligible o If part of the water turns into a solution, the density and specific heat capacity remains that of water o System is at constant pressure Heat lost by system = heat gained by surroundings Heat gained by system = heat lost by surroundings Flame Calorimetry (J) Use metal cans for the inner and outer vessel Directly heated by fire, with reactants inside inner can Bomb Calorimeter = measure enthalpy change in combustion reactions, at constant volume Inner chamber contains pure oxygen for combustion o Electric coil ignites reactants Outer chamber contains known amount of water to absorb heat released from combustion reaction Calorimeter made of too much material to assume components won t absorb any heat o Must find heat capacity (C) of entire bomb calorimeter o C bomb calorimeter = C water + C thermometer + C stirrer + C container o C = mc Q =C T for bomb calorimeter

5 HESS LAW Hess Law = enthalpy change of a physical or chemical process depends only on the initial and final conditions If the process involves multiple steps, enthalpy change of overall process is sum of enthalpy changes of all the individual steps Germain Henri Hess (1802-1850) How to Combine Multiple Reaction Steps Into an Overall Equation 1. Reverse an equation if necessary. Be sure to reverse the sign of H o. 2. To compare the reactions, make sure each equation has the same number of moles of the product by multiplying by the same integer or fraction. Be sure to multiply H o by same number. 3. Cancel any compounds that appear on both sides 4. Add up any compounds that appear multiple times on same side 5. Add up H o at the end.

6 Standard Molar Enthalpy of Formation ( H o f) = enthalpy change whilst producing 1 mol compound (1.0 mol/l solution) at SATP H o f of an element in its most stable state is arbitrarily set at zero o Graphite is standard state of carbon H o f of graphite is 0 kj/mol H o f of diamond is +1.9 kj/mol o Oxygen, H o f of O 2 is zero o F 2, Cl 2, Br 2, I 2 etc. Fe, Al, K etc. Thermal Stability = how stable a substance is, ability to resist decomposition when heated H o r = enthalpy of decomposition at SATP o If value is high, takes a lot of energy to decompose it, likely won t decompose thermally stable Which has more thermal stability: methane or calcium carbonate? ENTHALPY OF FORMATION & HESS LAW standard enthalpy change of reaction = sum of product enthalpies of formation sum of reactant enthalpies of formation H o r = Σ(n H o f products) Σ(n H o f reactants) o Calculated value is the same as combining multiple reaction steps into an overall equation and their respective H o f, therefore, this equation is consistent with Hess Law

7 RATES OF REACTION Rate = change in quantity over a period of time (K) rate = quantity time reaction rate = [X] final [X] initial t final - t initial If reaction rate is +ve (slope of graph is +ve) o reaction rate is increasing over time If reaction rate is ve (slope of graph is -ve) o reaction rate is decreasing over time Average Rate of Reaction = the average rate of a reaction across many data points during a period of time (slope between any 2 points in time) Instantaneous Rate of Reaction = the specific rate of reaction at one single point in time (tangent to the slope)

8 COLLISION THEORY Collision Theory = for a reaction to occur, reactant particles must collide with each other Only a small fraction of collisions result in a reaction Effective Collisions = reactant particle collisions that result in the formation of products 1. Reactants must collide in the correct orientation o Reactants that collide in the wrong orientation will just bounce off of each other (M) 2. Collision must occur with sufficient energy o Activation Energy (E a ) = minimum energy needed for a reaction to occur (N) E a must be overcome for reactants to proceed forward from transition state and form products Activated complex = reactants in temporary transition state, highly unstable, can proceed to form products or reverse and re-form reactants Reactions requiring low E a may even proceed at room temperature Gasoline needs high E a, which is why it does not burst into flames without ignition Reversible Reactions Exothermic reactions, H r, E a(forward) < E a(reverse), so forward reaction proceeds more easily Endothermic reactions, + H r, E a(reverse) < E a(forward), so reverse reaction proceeds more easily H r = E a(forward) E a(reverse) Consider the example: Forward CO (g) + NO 2(g) CO 2(g) + NO (g) H r = -226.1kJ Reverse CO 2(g) + NO (g) CO (g) + NO 2(g) H r = +226.1kJ

9 Potential Energy Diagram = shows energy transfer and how reaction occurs Reactants with sufficient kinetic energy collide in correct orientation Kinetic energy of collision converts to potential energy Potential energy stored in partial bonds of activated complex (in transition state) Partial bonds of activated complex form chemical bonds Potential energy stored in partial bonds converts to kinetic energy and particles of product separate

10 FACTORS AFFECTING REACTION RATE In general, any condition that increases the chance of effective collisions will increase reaction rate, vice versa. Nature of Reactants Compounds in solution react faster because: o Ions are already in solution, don t need to break bonds before forming new bonds o Opposite charges attract each other Acids and bases react faster because they are oppositely charged and attract each other Covalent compounds react slower because bonds need to be broken before new ones can be formed Exothermic reactions occur faster because less E a is needed, energy released also give reactants more kinetic energy to increase effective collisions Concentration Increase concentration, increase amount of reactants in a given volume, increase number of effective collisions o As product concentration increases, rate of reaction slows down because reactant concentration decreased Temperature Increase temperature, increase number of effective collisions Pressure Increasing pressure in gases by adding more gas particles or decrease the volume they occupy, gas particles are closer together, increase number of effective collisions Surface Area Smaller pieces of reactants have more surface area, more surfaces for other reactants to collide (ie/ powdered materials have highest surface area, compared to grains or larger pieces) Catalyst = chemical compound that increases reaction rate but is not consumed in the process, is neither reactant nor product Can lower E a, more reactants have kinetic energy equal to or greater than E a, increase number of effective collisions

11 ORDER OF REACTION & RATE LAW Quantifying Reaction Rate in terms of Concentration Consider an experiment (O) where you test how different concentrations of Reactant A affects the reaction rate o No matter what the reactant concentration is, reaction rate decreases as time passes by, because as products are formed: There are fewer reactants to continually create products Reverse reaction can occur, some products may react to re-form reactants o The higher the reactant concentration, the faster the rate is If you graph all the different reaction rates at different reactant concentrations: o It may form a straight line, therefore reaction rate vs. reactant concentration is a linear relationship, y = mx+b (P) o It may form a curve, therefore reaction rate vs. reactant concentration is more like a quadratic relationship (half a parabola) rather than linear, y = mx 2 (Q) First-Order Reactions Reaction rate and reactant concentration are directly proportional to each other: o A linear relationship o If double concentration, reaction rate doubles o If triple concentration, reaction rate triples etc. rate = k [A] How do you know if a reaction is a first-order reaction? o You need to do an experiment and test how the concentration of EACH reactant affects the rate o If for each reactant, the reaction rate and concentration are directly proportional to each other, and give a linear relationship the entire reaction is deemed first-order o ex/ A (aq) + 2B (aq) 3C (aq) + 4D (aq) To test effect of different [A], make sure B is in excess, vice versa Second-Order Reactions Reaction rate and reactant concentration are not directly proportional to each other, reaction rate is proportional to the square of reactant concentration o If double reactant concentration, reaction rate increases by 2 2, 4 times as fast o If triple reactant concentration, reaction rate increases by 3 2, 9 times as fast rate = k[a] 2

12 Rate Law k = rate constant (proportionality constant) [A] = concentration of reactant A [B] = concentration of reactant B m = the order of the reaction with respect to reactant A n = the order of the reaction with respect to reactant B reaction rate = k [A] m [B] n order of overall reaction = m + n if more than one reactant, rate = k [A] m [B] n [C] p o order of overall reaction = m + n + p all constants, k, m, n are calculated from experimental data and then graphed order of overall reaction can also be: o a fraction o zero if there is a catalyst and it is saturated reaction rate wouldn t change if you increase [reactant] because catalyst only has so many sites to facilitate reaction, and if all sites are saturated, reaction is occurring at it s fastest REACTION MECHANISMS Reaction Mechanisms = series of individual (elementary) steps that occur a multi-step reaction More than one activated complex can be formed during a multi-step reaction Intermediate = a compound formed (product) and then used up (reactant) in elementary steps but does not occur in the overall final balanced chemical reaction

13 Rate-Determining Step = aka rate-limiting step, overall reaction rate can only be as fast or as slow as the slowest step