Chemistry Exam Notes PDF

‘ Unit 1: Heat, Energy and Rates  Energy and heat Temperature vs Heat: ​ Temperature: the average kinetic energy of the particles ​ Heat: the overall thermal energy in a sample Dependent on: temperature number of particles (mass) type of particles A larger obje...

‘ Unit 1: Heat, Energy and Rates  Energy and heat Temperature vs Heat: ​ Temperature: the average kinetic energy of the particles ​ Heat: the overall thermal energy in a sample Dependent on: temperature number of particles (mass) type of particles A larger object that has a lower temperature may have more heat than a smaller object with a higher temperature Systems: ​ Open system: both energy and matter can be exchanged with surroundings ​ Closed system:only energy can be exchanged with surroundings ​ Isolated system: neither energy nor matter can be exchanged with surroundings Energy Changes in Systems: ​ Thermochemistry: the study of energy changes that accompany physical and chemical changes. ​ When a chemical system undergoes a change in energy, heat (q) is transferred between substances. ​ A system that gives off heat to its surroundings is called EXOTHERMIC. ​ A system that absorbs heat from its surroundings is called ENDOTHERMIC. Measuring Energy Changes: ​ The amount of energy gained or released by a chemical/physical system can be calculated using; q =mcΔT ​ Where: q is the energy in J or kJ ​ m is mass in g or kg ​ c is the specific heat capacity in J/g°C or kJ/kg°C  ​ ΔT is the change in temperature (T2-T1) in degrees celsius First Law of Thermodynamics: ​ The total energy in the universe is constant ​ Therefore the change in the energy of the universe is zero ​ Euniverse = Esystem + Esurroundings ​ ΔEuniverse = ΔEsystem + ΔEsurroundings = 0 ​ Therefore, we can say Δ Esystem = - Δ Esurroundings Energy Transfer: ​ Remember that energy can not be created or destroyed, but it can be changed from one form to another or transferred between objects. ​ Energy is always transferred from an object of higher temperature to an object of lower temperature and will stop when the objects are at the same temperature Heat Transfer Methods: ​ Conduction:transfer through direct touch mainly in solids ​ Convection:transfer through fluids ​ Radiation:transfer through space (no direct contact) Conductors vs Insulators: ​ Conductor: a material that heat passes through easily, example: metals ​ Insulator: a material that heat does not pass through easily, example: wood Calculations Involving Energy Transfer: ​ When energy is transferred between two objects, the law of Conservation of Energy can be expressed as: q gained + q lost = 0 Or q gained = -q lost Measuring Energy Changes: ​ Most energy changes are measured using a device called a calorimeter where the reaction takes place in an isolated compartment and the energy change can then be measured. Phase changes: ​ Phase change: term is used to refer to changes among the basic states of matter: solid, liquid, and gas ​ During a phase change, temperature does not change, instead the energy absorbed during melting for example goes to weakening the attractive forces between the molecules. ​ When calculating the heat absorbed or released during a phase change you use the latent heat value (l) for the process and the mass of the substance. ​ q phase change= m x l ​ Latent Heat Value unit is J/g or kJ/kg (be careful – sometimes it is given in J/mol or kJ/mol!) Have to calculate each step of the heating of the ice. 1. -11 degrees to 0 degrees 2. Solid state to liquid state 3. Heating past 0 degrees Enthalpy: ​ Enthalpy, H, is the heat content of the system ​ Heat = Energy + (Pressure x Volume) ​ H = E + PV ​ We can’t measure the total enthalpy of a system, but we can measure the change, ΔH ​ ΔH = ΔE + ΔPV ​ If ΔPV is zero, then ΔH = ΔE = q Enthalpy: ​ The heat of a physical or chemical change in ENTHALPY is the amount of heat produced or used during a change in state or a chemical reaction ​ It is represented using the symbol ΔH and generally has the unit of kJ/mol. ​ ΔH is also equal to the difference in enthalpies of products and reactants. Types of enthalpies: ​ Enthalpy of fusion (ΔHfus) – melting/freezing phase change ​ Enthalpy of vaporization (ΔHvap) – evaporation/condensation phase change ​ Enthalpy of reaction (ΔHr) – energy change involved in a reaction ​ Enthalpy of combustion (ΔHc) – energy released when 1 mol of fuel undergoes combustion ​ Enthalpy of formation (ΔHf) – energy required to produce 1 mol of a chemical from elements Indicating the ΔH for a Reaction: ​ The ΔH for a reaction can be indicated by writing the ΔH value after the reaction.  ​ A thermochemical equation can also be written, where the amount of energy used or produced is included as a term in the chemical equation Standard Molar Enthalpy, ΔHo: ​ The same reaction carried out in different physical states can have different ΔH ​ Even different temperatures/pressures can change the enthalpy of a reaction ​ We usually use SATP ​ When SATP conditions are in use, ΔH is written as ΔHo Calculating ΔH: ​ There are multiple ways to calculate ΔH: 1.​ Using the value of q and the number of moles. 2.​ Using Hess’ Law 3.​ Using Bond Energies 4.​ Using Standard Heats of Formation 5.​ Potential Energy Diagram ​ Note ΔH for an exothermic reaction will be negative and ΔH for an endothermic reaction will be positive. Be careful!: ​ Sometimes the calculation will be -q/n and sometimes it will be q/n ​ q can be calculated for the system OR the surroundings; ​ ΔH is ONLY for the SYSTEM ​ We include the negative sign in the calculation only if q is for the surroundings! Hess’ Law: ​ ΔH for a reaction can be calculated algebraically using various related reactions with known enthalpies ​ Equations are manipulated such that the sum of the equations result in the original equation. ​ Remember: -​ Equations can be reversed; the ΔH absolute value stays the same, but the sign changes -​ You can double, triple, etc any amount, as long as you do the same thing to other parts of equation Using Standard Heats of Formation: ​ The value of ΔH can be calculated using the standard enthalpies of formation for the products and reactants ​ The standard enthalpy of formation (H°f) is the energy associated with making a substance from its elements. ​ Elements in their standard state have a H°f of zero and the values for compounds can be found in a table ​.When calculating ΔH you must first start with a balanced chemical equation WITH STATES! ​ Then ΔH is calculated by ΔH = ΣnH°f products - ΣnH°f reactants ​ The H°f for each substance must be multiplied by its molar coefficient. Calculating ΔH Using Bond Energies: ​ Since bond breaking is an endothermic process and bond forming is an exothermic process, the enthalpy of a reaction can be calculated using bond energies ​ ΔH= Σbonds broken - Σbonds formed ​ Before making the sum, the bond energies must be multiplied by the number of that bond present AND the coefficient from the balanced chemical equation ​ When writing the balanced equation, the structures of each species must be drawn to determine which bonds present. ​ The energy to break a double bond will not necessarily be double the energy to break a single bond, so make sure you use the actual energy value given! Representing Energy Changes Graphically: ​ Changes in energy for a chemical system are represented graphically using a Potential Energy Diagram ​ These graphs plot Potential Energy vs Reaction Progress (essentially, time) ​ ∆H is calculated by determining the difference in potential energy between the reactants and products.  Rates of Reaction ​ The rate of a chemical reaction is the speed at which the reaction occurs (i.e. speed at which the reactants are used or products are produced). Rate can be measured by the: 1.​ change in mass of reactants or products 2.​ concentration (mol/L) 3.​ change in pH (acids/bases) 4.​ change in conductivity (ion production) 5.​ change in colour (intensity of colour/wavelength) 6.​ change in temperature 7.​ change in volume or pressure (gases) CALCULATING AVERAGE RATE OF REACTION: ​ Average rate of reaction is the change in concentration in a given time period ​ Mathematically,[C] is concentration in mol/L and t is time (usually seconds) ​ The unit for rate is mol/L s ​ Rate is expressed in terms of the amount of a product produced or the amount a reactant is consumed ​ Reactants are used and products produced based on their molar ratios. COLLISION THEORY: ​ The KINETIC MOLECULAR THEORY may be used to explain how the various factors will affect the rate of a chemical reaction ​ As the particles in the reactants move around, they collide with each other. ​ Most collisions do not result in anything but a few will cause the bonds in the existing molecules to break apart and new bonds will form to make new molecules.  ​ Collisions that result in the formation of products are called EFFECTIVE COLLISIONS. EFFECTIVE COLLISIONS: ​ A collision must occur: -​ at the correct orientation -​ with a minimum amount of energy, ie. at least a certain amount of energy – it can have more, but it can’t have less ​ The idea of effective collisions is called the COLLISION MODEL and states that the rate of a reaction is affected by the number of effective collisions between reactant molecules. ​ According to the Collision Model, the rate of a reaction may be increased by increasing the number of total collisions or by increasing the number of effective collisions by decreasing the activation energy. THE FACTORS AFFECTING RATE OF REACTION AND THE COLLISION THEORY: 1.​ Concentration (Pressure of Gases): The greater the number of particles the greater the number of total collisions. Since the total number of collisions has increased the number of effective will also increase therefore the rate of the reaction will increase. NOTE: As a reaction proceeds, it tends to slow down. 2.​ Surface Area (Size of Solid Particles): If the size of a solid particle is decreased, there will be more surface area available for collisions to occur. 3.​ Temperature of Reactants: The temperature of a substance is a measure of the average kinetic energy of the particles. The higher the temperature, the faster the particles are moving which will increase the chance for collisions 4.​ State or Phase of the Reactants: Reactions where all reactants are in the same state (homogeneous reactions) will occur at a faster rate than reactions where reactants are in different states (heterogeneous). This is because the reactants will have a greater opportunity of colliding. The state of the reactants will also affect the rate. The relative rates are: Gases: fastest Liquids/ Solutions:fast Solids: slow Note: Stirring increases the rate of reaction 5.​ Nature of Reactants: The type of reactants determine the activation energy needed. The higher the activation energy, the slower the rate of reaction. Endothermic reactions are much slower than exothermic reactions because they tend to have higher activation energies. 6.​ Presence of a Catalyst: A catalyst works by providing an alternative pathway for the reaction which has a lower activation energy. Lowering the activation energy will only increase the number of effective collisions. The number of total collisions will not be affected Potential Energy Diagrams and Reaction Rates: ​ The lower the activation energy, the greater the rate of reaction ​ In general, exothermic reactions tend to have greater rates than endothermic reactions ​ Many exothermic reactions are self-sustaining. ​ Catalysts increase rate by providing an alternative pathway with a lower activation energy ​ There is a mathematical relationship between the rate of reaction and the factors affecting the reaction ​ This relationship should be determined empirically (must analyze experimental data).  ​ The RATE LAW for a reaction describes the relationship between rate (r) and the product of the initial concentrations of the reactants raised to some exponential values ​ To determine the rate of a reaction mathematically, a constant k, called the rate constant, must be introduced into the relationship. ​ A RATE LAW EQUATION for the reaction is then written as -​ r = k [X]m [Y]n... ​ The value for the rate constant must also be determined experimentally. ​ The exponents in the rate law equation are called the orders of reaction. ​ The sum of all the individual orders (exponents) is referred to as the overall order of reaction. What does the order of reaction mean?: ​ For 1st order reactant, if concentration is doubled, rate is doubled ​ For 2nd order, if concentration is doubled, rate is quadrupled ​ For 3rd order, if concentration is doubled, rate is increased by a factor of 8  ​ For zero order, if concentration is doubled, there is no change in the rate ​ If a reaction is zero order for one of the reactants, that reactant is left out of the rate equation. DETERMINING A RATE LAW EQUATION USING RATE DATA: ​ Rate law equations are determined by analyzing data on changes in concentration and its effect on rate. ​ The first step is to determine the orders of each reactant and then substitute the data for one trial into the equation to find the value for k. Reaction mechanism: ​ A reaction mechanism is the step or series of steps that make up a reaction ​ The steps in a reaction mechanism are referred to as elementary steps because they are elementary reactions ​ Molecularity refers to the number of reactant molecules (ions, atoms or molecules) involved in an elementary step or making up an activated complex. -​ Unimolecular: steps involving 1 molecule -​ Bimolecular: steps involving 2 molecules -​ Termolecular: steps involving 3+ molecules ​ Simple reactions will have one step where complex reactions will consist of a number of steps. ​ Lower molecular steps tend to be faster than ones with more molecules ​ Often it is difficult to determine the actual mechanism of a reaction. In this case it must be determined through experimentally determined relationships.  ​ The rate determining step is the slowest step in the reaction mechanism and will have the largest activation energy. USING REACTION MECHANISMS TO DETERMINE THE RATE LAW EQUATION: ​ The rate law expression can be determined from the reaction mechanism ​ The rate determining step is the elementary step that determines the rate law expression ​ The rate of a reaction is proportional to the concentrations of the reactants in the rate determining step raised to their molar coefficients ​ If any reaction intermediates appear in this expression, you must use the other steps to eliminate these species ​ Only reversible equations, denoted by can be used to ⇌replace reaction intermediates.