Energy Changes in Reactions p

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1 Energy Changes in Reactions p Heat vs. temperature: Heat is a form of energy, it is transferred from one system to another Temperature is an indication of the intensity of heat, it measures the agitation of particles Transfer of energy the natural tendency in nature is to move toward higher levels of disorder (entropy), this is why heat ALWAYS flows from hot to cold (think of standing by a campfire) when your hands feel warm they are receiving the energy of a warmer substance when your hands feel cold, you are feeling the loss of energy to a colder substance there is no visible movement as heat transfer occurs Conservation of Energy Energy can be transferred or transformed, but never created or destroyed when studying energy changes, the type of "system" must be considered a system is a location being observed (beaker, appliance, humans, solar system, etc) systems can be in contact with and influenced by surroundings which are part of their environment Types of systems 1. Open: in contact with surroundings, exchanges matter and energy with it (beaker, can be heated or cooled or added to) 2. Closed: no exchanges of matter but allows energy to be exchanged with surroundings (a tied balloon, no matter can be added but can be heated or cooled) 3. Isolated: no exchanges of matter or energy with surroundings (a thermos is temporarily an isolated system) 1

2 Calorimetry used to determine quantities of heat involved in a reaction calorimeter is a closed system (internal chamber) that allows the transfer of energy to water surrounding the core by measuring the temperature change of the water, energy change can be calculated if the water warms up, the reaction in the chamber has released energy if the water cools down, the reaction in the chamber has taken away energy from the water Calculating heat energy Q = mcδt Q = heat energy (J) m = mass (g) c = specific heat capacity (J/g o C Δt = change in temperature (T f T i ) specific heat capacity refers to the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius Why are coastal regions typically more temperate? Why does the desert experience such drastic temperature changes? 2

3 1. Calculate the quantity of thermal energy absorbed by a 5.00 kg block of concrete (use table p.134) when its temperature increases from 17.1 o C to 35.5 o C. Q = m = c = ΔT= 2. A 1.35 g piece of aluminum is heated to 205 o C and then removed from the heat. After a few seconds, the aluminum has released 176 J of heat. What is its final temperature? Q = m = c = ΔT = Calculating energy transfer in mixtures when mixing two sustances, heat will flow between them until the final temperature of both is the same the heat energy lost by one substance is gained by the other in other words, Q = +Q mcδt = +mcδt HOT COLD the hotter substance ALWAYS loses heat the colder substance ALWAYS gains heat the final temperature of BOTH substances is ALWAYS the same 3

4 Examples 1. Calculate the mass of water at 10.0 o C needed to cool a 10.0 g piece of glass from 95.0 o C to 30.0 o C. 2. A 13.7 g piece of copper is heated in an oven to a temperature of o C. It is then removed and dropped into a beaker containing ml of water at 22.0 o C. What is the final temperature of the mixture? Energy Balance the sum of energy required to break bonds (+) and reform new ones ( ) the amount of energy depends on the types of bonds being broken (refer to your reference table) draw the Lewis diagram to figure out whether a bond is single double or triple ΔH = ΔH bonds broken + ΔH bonds formed Calculate the enthalpy change for the following reaction: H 2 (g) + Cl 2 (g) 2 HCl (g) Bonds broken: Bonds formed: ΔH = This reaction is 4

5 More complex molecules: 1. Place the least electronegative atom in the center 2. Hydrogen always goes on the outside! 3. Place two electrons between each pair to show they are bonded together 4. Fill the outer atoms (make sure they have 8 electrons around them) 5. Calculate bond energy 1. CO2 2. H2O 3. CH4 4. C2H6 Phase change (endo/exo) Endothermic reactions absorb energy while exothermic ones release energy during a change of state (fusion or solidification, vaporization or condensation) energy is absorbed or released, but there is no overall change in temperature (see p. 151) the energy required to change state depends on mass to calculate total energy involved in a reaction involving changes of state, we must calculate a value for each step energies for change of state are provided in your reference table 5

6 1. Sketch a graph for the transformation of a 15.0 g block of ice at 10.0 o C into water vapour at o C. How much energy is involved in this transformation? 2. What is the change in energy involved in cooling a 57.0 ml sample of water at 34.0 o C and then solidifying it? Sketch the graph. 6

7 Thermochemical equation in all chemical reactions there is an endothermic portion and exothermic portion in order to break bonds, energy must be absorbed (ENDO) as new bonds form, energy is released (EXO) to determine whether the reaction is endothermic or exothermic overall, we need to look at the ratio of the two thermochemical equations express the enthalpy change for a reaction they can be written in two ways 1. CaCO 3 (s) CaO (s) + CO 2 (g) ΔH = 178 kj/mol 2. CaCO 3 (s) kj CaO (s) + CO 2 (g) this reaction is endothermic, ΔH = POSITIVE, there has been an overall intake of energy energy is a REACTANT for an exothermic reaction ΔH = NEGATIVE due to an overall loss of energy energy is a PRODUCT Enthalpy enthalpy (H) is the total energy of a system (sum of potential and kinetic) enthalpy change (ΔH) is the energy exchanged between a system and its environment during a reaction, also referred to as the heat of reaction ΔH = H products H reactants (measured in kj) if there is an overall decrease in energy (products have less energy than reactant), ΔH = NEGATIVE if there is an overall increase in energy (products have more energy than reactants), ΔH = POSITIVE in the case of calorimetry, ΔH = Q (the enthalpy change of th system is the negative value of the energy gained or released by the water) 7

8 1. You carry out a reaction in a calorimeter containing ml of water at 21.6 o C. At the end of the reaction, the water reaches a temperature of 27.2 o C. What is the enthalpy change for this reaction? Is this reaction releasing energy or absorbing it? Calculating enthalpy change using stoichiometry using a thermochemical equation we can determine the enthalpy change for the reaction of a given mass of substance Consider the following equation: H 2 (g) + F 2 (g) 2 HF (g) kj for the reaction of one mole of H 2 or one mole of F 2, OR for the formation of 2 mole of HF, kj are released. What is the enthalpy change for the reaction of 15.6 g of hydrogen? What is the enthalpy change for the formation of 65.0 g of HF? Re write the thermochemical equation in ΔH format. 8

9 Enthalpy diagrams in an endothermic reaction, enthalpy of reactants less than enthalpy of products in an exothermic reaction, enthalpy of reactants greater than enthalpy of products gap between the two is the overall change (ΔH) activated complex is an intermediate cluster of colliding atoms during a reaction (all bonds broken) E p is at its highest at this point in the reaction activation energy (E A ) is the minimum energy required for a reaction to occur energy diagrams allow us to visualize the activation energy and the overall energy change for a reaction Plot the energy diagram for the combustion of methane if the activation energy is kj, the enthalpy change is 802 kj/mol, and the E p of the reactants is kj. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) 9

10 Spontaneous Reactions occurs when there is zero activation energy (theoretical as there is always an input of energy but it can be very small) the higher the activation energy, the longer the reaction will take Reversible reactions some reactions may occur in a forward and reverse direction must be in a closed system we can read the energy diagram in the forward or reverse direction Sketch the energy diagram for the following reaction given that the ΔH reverse = 63.0 kj, E A of the forward reaction is kj and the E p of the products in the forward reaction is kj. NO (g) + O 3 (g) NO 2 (g) + O 2 (g) 10

11 Molar heats of reaction amount of energy involved in the reaction of one mole of a substance subscript of ΔH designates the type of reaction ΔH d represents molar heat of dissolution (also ΔH sol'n ) ΔH n represents the molar heat of neutralization ΔH c represents the molar heat of combustion Calculating heat of dissolution the heat of reaction (ΔH) is equal to Q of the water (if the water heats up, the dissolution has given up energy and vice versa) knowing molar heat of reaction can also allow us to determine initial or final temperature of water in a calorimeter 1. You dissolve 24.6 g of BeCl 2 in 75.0 ml of water at 23.6 o C in a calorimeter. The final temperature of the water is 38.5 o C, calculate the molar heat of dissolution of beryllium chloride. 11

12 2. You dissolve 3.46 g of KNO 3 in ml of water at 22.7 o C. Given that the molar heat of dissolution of potassium nitrate is 32.0 kj/mol, what will the final temperature of the water be? Molar heat of neutralization energy absorbed or released by the neutralization of one mole of acid or base temperature change allows us to calculate molar heat knowing molar heat allows us to calculate initial or final temperature we must know number of moles being neutralized n = c x v also remember that acid + base = salt + water the total mass of water (needed to calculate Q) is the sum of the mass of the acid AND base ΔH n = Q water if the resulting water heats up, the neutralization has released energy and vice versa 12

13 1. You neutralize ml of 0.65 M NaOH with ml of 0.65 M HCl. The initial temperature of the solutions is 24.6 o C, and the final temperature of the mixture is 27.2 o C. What is the molar heat of neutralization of NaOH? 2. The molar heat of neutralization of HNO 3 is 37.0 kj/mol. Calculate the final temperature of the solution when ml of 0.25 M HNO 3 is neutralized by ml of 0.25 M LiOH both initially at 20.6 o C. 13

14 Reaction mechanism a series of steps through which a chemical reaction occurs the overall heat of reaction is the sum of all the heats of the intermediate steps the speed at which the reaction occurs depends on the slowest step we can use an energy diagram to represent multi step reactions, this allows us to see the "rate determining step" the slowest step will be the one with the highest activation energy the overall change in enthalpy is still the difference between the starting point and the end point Hess' Law allows us to calculate overall enthalpy change by finding the sum of intermediate steps we must manipulate intermediate steps to end up with the "target equation" we can cancel out parts of equations on opposite side of arrow (reactants vs. products) if same state we can flip an equation if needed (sign of ΔH changes) we can multiply or divide an equation (same to ΔH) when target equation is achieved, the sum of all ΔH values is equal to the overall enthalpy change for the reaction 14

15 1. Determine the enthalpy change for the combustion of propane (C 3 H 8 ) in the following equation using the intermediate steps. C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) ΔH=? 2. Determine the molar heat of reaction using heats of formation: Fe2O3 (s) + 2 Al (s) Al2O3 (s) + 2 Fe (s) 15