Reactions Rates, Reaction Equilibrium, Thermodynamics PDF

Collision theory explains why reactions occur at different rates. According to collision theory in order for a chemical reaction to occur, reacting atoms must (1) collide with (2) sufficient energy, and (3) with the proper orientation.  Rates of Reaction The rate of rea...

Collision theory explains why reactions occur at different rates. According to collision theory in order for a chemical reaction to occur, reacting atoms must (1) collide with (2) sufficient energy, and (3) with the proper orientation.  Rates of Reaction The rate of reaction is the rate at which the reactants of a chemical reaction form the products. As the reaction proceeds, the concentration of reactants decrease while the concentration of products increase  Activation Energy The minimum energy requirement for colliding particles to react is called the activation energy. ○ The activation energy is the ‘hill’ that reactants must overcome before products can form. Factors That Affect Reaction Rates - Chemical Nature of Reactants Reactants must have compatible chemistry to react. ○ Example: Noble gases do not react because they are already stable.  Factors That Affect Reaction Rates - Temperature Increasing temperature increases the speed and energy of particles. ○ Faster-moving particles collide more often and with more energy, increasing the likelihood of a reaction.  Factors That Affect Reaction Rates - Concentration Concentration refers to how much solute is dissolved in a solution ○ Increasing the concentration of reactants increases the number of particles in a given space. ○ This leads to more frequent collisions.  Factors That Affect Reaction Rates - Surface Area Breaking a solid into smaller pieces increases its surface area. ○ More surface area allows more collisions with reactant particles.  Factors That Affect Reaction Rates - Catalysts Catalysts provide an alternate reaction pathway that requires less energy ○ Lowers the activation energy They speed up the reaction without being consumed in the process.  What is enthalpy? Enthalpy (H) is the chemical potential of a substance. ○ Unit: Kilojoules (kJ) Enthalpy change (ΔH) is the change in energy that occurs as a result of chemical reaction. ○ ΔH = H(products) – H(reactants) ○ Units = kilojoules per mole (kJ/mol)  Enthalpy change of a reaction ΔH is positive ΔH is negative Endothermic Exothermic The reaction The reaction absorbs energy releases energy  What is a potential energy diagram? The energy changes that occur during a chemical reaction is shown in a diagram called a potential energy (PE) diagram. ΔH is negative = Exothermic ΔH is positive = Endothermic  Enthalpy changes in chemical equations Enthalpy change (ΔH) is written to the right of a chemical equation. ○ (+) = endothermic reaction | (-) = exothermic reaction  Activity 5.4A - Defining Chemical Equilibrium Background In many chemical systems, reactions can proceed in both the forward and reverse directions. ○ These are called reversible reactions.  Activity 5.4A - Background, continued In a forward reaction, reactants combine to form products. In the reverse reaction, products break down to reform the original reactants. These reversible reactions are represented with a double arrow: ⇌ or ↔ Eventually, these reactions can reach a state called equilibrium. Activity 5.4A - Defining Chemical Equilibrium - Class Discussion What happened to the amount of reactants? ○ The amount of reactants decreased What happened to the amount of products? ○ The amount of products increased. Note: This is not the same data from the activity. Activity 5.4A - Defining Chemical Equilibrium - Class Discussion At what time do the amounts of reactants and products stop changing? Why did the concentrations stop changing even though the reaction was still occurring? ○ Reactants were combining to form products at the SAME RATE that products were breaking down to reform reactants.  What is equilibrium? Equilibrium: the rates of the forward and reverse reactions are equal ○ To reach equilibrium, the amount of reactants and products do not have to be equal. ○ After equilibrium is reached, the amounts of reactants and products will be constant. Rate (forward) = Rate (reverse)  What happens to a reaction that has reached equilibrium if the conditions change? Le Chȃtelier’s Principle is used to predict the effect of a change on the chemical equilibrium. ○ It states that a disturbance in the reaction will shift the reaction to counteract the change and restore the equilibrium.  There are three key disturbances to equilibrium Concentration Pressure Temperature What happens if the concentration of a substance is changed? When reactant or product is added Stress: the concentration of reactant or product increase Counteract: consume the added substance Shift: opposite side of the added substance. To reestablish balance Consume the added reactants At equilibrium Reactant Produce more (Balanced) concentration products increases What happens if the concentration of a substance is changed? When reactant or product is removed Stress: the concentration of reactant or product decreased Counteract: produce the removed substance Shift: same side of the removed substance. To reestablish balance Consume products Produce reactants At equilibrium Reactant (Balanced) concentration decreases  What happens if the pressure is changed? When pressure is increased Stress: pressure increased Counteract: reduce pressure Shift: To the side with fewer gas molecules How many moles of gas? Reactant: Products 1 mole of 2 moles of gas gas  What happens if the pressure is changed? When pressure is decreased Stress: pressure decreased Counteract: increase pressure Shift: To the side with more gas molecules How many moles of gas? Reactant: Products 1 mole of 2 moles of gas gas  What happens if heat is added or removed in a reaction? In an EXOTHERMIC reaction, heat is a PRODUCT A + B ⇌ C + D + HEAT Heat is added in an EXO reaction Stress: Heat is added Counteract: consume the added heat Shift: To the reactants Heat is removed in an EXO reaction Stress: Heat is removed Counteract: produce heat Shift: To the products  What happens if heat is added or removed in a reaction? In an ENDOTHERMIC reaction, heat is a REACTANT A + B + HEAT ⇌ C + D Heat is added in an ENDO reaction Stress: Heat is added Counteract: consume the added heat Shift: To the products Heat is removed in an ENDO reaction Stress: Heat is removed Counteract: produce heat Shift: To the reactants  Lab 5.5 Observing Le Chatelier's Principle Background Some equilibrium reactions are colorimetric reactions. ○ This means that the substances in the reactions are colorful. How are they useful? ○ Colorimetric reactions are useful for observing Le Chatelier’s Principle because we can analyze color changes to determine the direction of the equilibrium shift.  Lab 5.5 Observing Le Chatelier's Principle Background, continued Example: ○ In the equilibrium reaction below, Co(H 2O) 62+ is red and CoCl 42- is blue. Co(H 2O) 62+ (aq) + 4 Cl - (aq) → CoCl 42- (aq) + 6 H 2O (l) You might predict the color of the solution at equilibrium to be _____________________________. ○ Because at equilibrium, reactants and products are both present. Phase change that require energy When ice gains enough energy, the hydrogen bonds that hold it together are disrupted enough to allow the molecules to move apart and enter the liquid phase This is called melting. Phase change that require energy When an ice cube melts, the temperature of the water produced remains constant. Once all of the ice has melted, additional energy added to the system increases the kinetic energy of the liquid molecules. Phase change that require energy In liquid water, some molecules have more energy than others. Those molecules that have the energy required to overcome the forces of attraction escape from the liquid and enter the gas phase. Phase change that require energy Vaporization is the process by which a liquid changes to a gas or vapor. When vaporization occurs only at the surface of a liquid, the process is called evaporation. Phase change that require energy Evaporation takes place even in a sealed container. In a partially filled container, water vapor collects above the liquid and exerts pressure on the surface of a liquid. The pressure exerted by a vapor over a liquid is called vapor pressure. Phase change that require energy Evaporation takes place even in a sealed container. In a partially filled container, water vapor collects above the liquid and exerts pressure on the surface of a liquid. The pressure exerted by a vapor over a liquid is called vapor pressure. Phase change that require energy The temperature at which the vapor pressure of a liquid equals the external or atmospheric pressure is called the boiling point. At the boiling point, molecules throughout the liquid have enough energy to vaporize. Bubbles of vapor collect below the surface of a liquid and rise to the surface. Phase change that require energy Many substances have the ability to change directly from the solid phase to the gas phase. Sublimation is the process by which a solid changes directly to a gas without first becoming a liquid. Dry ice (COz) is an example of this process. Phase change that release energy When enough energy has been A cooling curve is a graphical removed, hydrogen bonds between plot of the changes in water molecules keep the molecules temperature with time for a fixed or frozen into set positions. material over the entire Freezing is the reverse of melting. temperature range through which The freezing point is the temperature it cools. at which a liquid is converted into a Cooling Curve for Pure Metals is crystalline solid. The freezing point of shown here. water is 0°C. Phase change that release energy When a water vapor molecule loses energy, its velocity decreases. The water vapor molecule is more likely to form a hydrogen bond with another water molecule. The process by which a gas or vapor becomes a liquid is called condensation. Condensation is the reverse of vaporization. Phase change that release energy The process by which a substance changes from a gas or vapor to a solid without first becoming a liquid is called deposition. Deposition is the reverse of sublimation. Frost is an example of deposition. The graph of temperature against time is called a heating or cooling curve. Heating and cooling curves show how the temperature changes as a substance is heated or cooled.  Heating and Cooling Curves, continued Phase changes of a substance can be determine by the horizontal lines or plateau on the curve, where the temperatures of the system are constant.  Heating and Cooling Curves, continued Each substance has a different heating and cooling curve. How to read a heating curve - SOLID TO LIQUID The first phase change is melting ○ Changing from solid to liquid The temperature stays the same while a substance melts. ○ This temperature is the melting point of the substance Both solid and liquid exist in different ratios ○ It starts with 100% solid ○ Then it ends with 100% liquid How to read a heating curve - LIQUID TO GAS The second phase change is boiling or vaporization ○ changing from liquid to gas The temperature stays the same while a substance boils. ○ This temperature is the boiling point of the substance Both liquid and gas exist in different ratios ○ It starts with 100% liquid ○ Then it ends with 100% gas How to read a cooling curve - GAS TO LIQUID The first phase change is condensation ○ changing from gas to liquid The temperature stays the same while a substance condenses. Both gas and liquid exist in different ratios ○ It starts with 100% liquid ○ Then it ends with 100% gas How to read a cooling curve - LIQUID TO SOLID The second phase change is freezing (changing from a liquid to a gas). The temperature stays the same while a substance freezes. ○ This temperature is the freezing point of the substance Both liquid and solid exist in different ratios ○ It starts with 100% liquid ○ Then it ends with 100% solid  Phase Diagrams A phase diagram combines graphs of pressure versus temperature for the phase changes of a substance. These diagrams tell us the physical states that exist under specific conditions of pressure and temperature.  Components of a Phase Diagram Regions or areas labeled solid, liquid, and gas represent single phases. Lines or curves represent two phases existing together. The triple point is the point where all three phases coexist. A critical point is where the distinction between the liquid and gas phases disappears.  Warm-Up Discussion What does the line separating solid and liquid represent? ○ Melting phase change If a substance is at a high Solid pressure Liquid pressure and low temperature, which phase is it in? ○ solid Gas  Heat Capacity and Specific Heat Capacity Heat Capacity: The amount of heat required to change the temperature of a substance by 1 °C. Specific Heat Capacity (c): The amount of heat required to raise the temperature of one gram of a substance by 1 °C. Which substances, water or metal, do you think would heat up faster? ○ Consider past experiences ○ Discuss with your team  Activity 5.8A Investigating Specific Heat Capacity Results: Which material required the most heat energy to show a small temperature change? ○ Water What does this suggest about its specific heat? ○ Consider that specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by 1 °C. ○ Suggests that water has the highest specific heat capacity of the tested materials  Calculating specific heat We can calculate heat energy of a substance with the formula: q: heat energy (Joules) m: mass (grams) c: specific heat (J/g°C) ΔT: change in temperature (°C) Then we can rearrange the formula to solve for specific heat (c)