Chemistry 18 - Kinetics 2 Questions & Answers

Chemistry 18 - Kinetics 2 Questions & Answers What are the main factors that affect reaction rate? Temperature, concentration (solutions), pressure (gases), surface area (solids), catalysts. What is the rate of reaction? The change in concentration of any of the reactants or products with unit time. Usually it is assumed that a rate is measured by following the concentration of a product, because the concentration of the product increases with time. Previous Play Next Rewind 10 seconds Move forward 10 seconds Unmute 0:00 / 0:15 Full screen Brainpower Read More How does the rate of reaction change over time? Initially the rate of reaction is at its fastest, as there is a high concentration of reactant being formed into product. As the amount of product increases and the amount of reactant decreases, the rate of reaction slows as there is less product being formed. Eventually the rate of reaction will level off once all of the product has been formed from all of the reactant. How do you measure the rate of reaction at any instant? Draw a tangent to the curve at a specific time and then find its gradient (change in y/change in x) What is the unit for rate of reaction? mol dm⁻³ s⁻¹ What are the two different ways of measuring rates of reaction? Monitoring over time and end-point measurements What methods could you use for monitoring over time? > Measuring the concentration/mass/volume of a product/reactant over time. > Using a gas syringe to measure the volume of a gas formed over time. What methods could you use for end-point measurements? > Time taken for an 'X' to disappear > Time taken for a solid to dissolve > Time taken for a colour change to occur What does the rate of a chemical reaction depend on? The concentrations of some or all of the species in the reaction vessel (reactants and catalysts). These do not necessarily all make the same contribution to how fast the reaction goes. What is the rate equation? An equation that describes how the rate of reaction at a particular temperature depends on the concentration of species involved in the reaction. It only tells us about the contribution of the species that DO affect the rate of reaction. It is possible that one or more of the species that appear in the chemical equation will not appear in the rate equation and vice versa (e.g. catalysts) How do you find rate equations? From experiments only. How do you write a rate equation? rate = k[A]ᴹ[B]ᴺ where: > k is the rate constant. This is different for every reaction and varies with temperature. > [A] is the concentration of reactant A > [B] is the concentration of reactant B > M is the order of reaction with respect to A > N is the order of reaction with respect to B What is the order of a reaction? The power to which the concentration of a reactant must be raised to give the experimentally observed relationship between concentration and rate. It tells us how the rate depends on the concentration of that species. What is the overall order of a reaction? The sum of the orders of all the species that appear in the rate equation (M+N) What is a zero order reaction (w.r.t a species)? A reaction where rate ∝ concentration⁰ This means that the rate is constant and independent of the concentration of the reactant (increasing the concentration of the reactant has no effect on the rate). A zero order reactant does not appear in the rate equation. What is a first order reaction (w.r.t a species)? A reaction where rate ∝ concentration This means that doubling the concentration of the reactant will double the rate of reaction. What is a second order reaction (w.r.t a species)? A reaction where rate ∝ concentration² This means that doubling the concentration of the reactant will increase the rate of reaction by a factor of 4. What are the units of k for a zero order reaction? rate = k What are the units of k for a first order reaction? s⁻¹ What are the units of k for a second order reaction? mol⁻¹ dm³ s⁻¹ What are the units of k for a third order reaction? mol⁻² dm⁶ s⁻¹ How do you calculate k? rate = k[A]ᴹ[B]ᴺ k = rate / [A]ᴹ[B]ᴺ > Find the order of reaction w.r.t. A whilst [B] is constant > Find the order of reaction w.r.t. B whilst [A] is constant (if [A] or [B] are never constant, use the order of the other reactant and the rate to see what the order of the reactant must be in order to get that rate) > Put values into the equation for k and calculate > Put units of k How do you determine orders of reaction from experimental data (e.g. a table in an exam)? If rate is not affected by the concentration of a species, the reaction is zero order w.r.t to that species. The species does not occur in the rate equation. If rate ∝ concentration of a species, the reaction is first order w.r.t. to that species. The species occurs in the rate equation to the power of 1. If rate ∝ concentration² of a species, the reaction is second order w.r.t. to that species. The species occurs in the rate equation to the power of 2. What is the half life of a reactant? The time taken for the concentration of the reactant to halve. How do you determine orders of reaction from a concentration-time graph? If the graph is a straight line, the reaction is zero order w.r.t. to the reactant. If the graph is a curve, the reaction is first or second order w.r.t to the reactant. > first order = 2 or more half lives drawn on the graph are constant. > second order = half lives are not constant. What is the gradient of a concentration-time graph? The rate of reaction (at a specific time). What is the shape of a rate-concentration graph for a zero order reaction? A horizontal line. What is the shape of a rate-concentration graph for a first order reaction? A straight line through the origin. What is the shape of a rate-concentration graph for a >1 order reaction? A curve (not a straight line). The order cannot be found directly - it could be 2, but it is not necessarily 2. If you plot a graph of rate-concentration² and it is a straight line, then the order is 2 What is the initial rate method? A series of experiments that are carried out at constant temperatures. Each experiment has a different combination of initial concentrations of reactants and catalysts. The experiments are planned so that between any pair of experiments, the concentration of only one of the species varies and the rest stays the same. For each experiment, the concentration of one reactant is followed and a conc-time graph is plotted. The gradient of the tangent at t=0 is the initial rate. Comparing the initial rates and concentrations for pairs of experiments allows the order w.r.t. each reactant to be found. What is the iodine-clock reaction? Used to measure the rate of reaction between hydrogen peroxide and potassium iodide in acidic conditions to form iodine. A solution of hydrogen peroxide is added to a solution containing potassium iodide, sodium thiosulfate, and starch. Hydrogen peroxide reacts with iodide ions and H+ ions (acidic conditions) to form iodine. As soon as the iodine is produced, it reacts with the thiosulfate ions and starch to give a dark blue-black colour. The time it takes for the colour change to occur is recorded for varying concentrations of one of the reactants (KI) What is the effect of temperature on the rate constant, k? As temperature increases, k increases. Small changes in temperature produce very large charges in reaction rates. Roughly every 10K rise in temperature doubles the reaction rate. The rate constant allows you to compare the speeds of reactions at a given temperature. The larger the value of k, the faster the reaction, and the higher the temperature must be. Why does the rate constant depend on temperature? Temperature is a measure of the average kinetic energy of particles, which will only react together if their collisions have the activation energy to start bond breaking. The shape of a Maxwell-Boltzmann graph changes with temperature. As temperature increases, a greater proportion of molecules have enough energy to react What is the Arrhenius equation? An equation that shows how the rate constant varies with temperature. k = A