AQA A Level Chemistry Year 2 - Rate Equations Questions and Answers

AQA A Level Chemistry Year 2 - Rate Equations Questions and Answers From what time is the initial rate of reaction measured? t=0 What is a rate equation? An expression showing how the rate of reactions is linked to the concentration of the reactants What is the rate equation for the reaction : A + B -- C Rate = k[A]^m[B]^n The product concentrations are included in the rate equation True or False False What is the order of reaction with respect to a particular reactant? The power to which the reactant is raised in the rate equation The number of moles of a reactant is the power to which it is raised in the rate equation True or False False otherwise how would you get reactants of zero order How do you calculate the overall order of a reaction? The sum of all the orders in the rate equation What is the rate constant? k the proporionality constant which links the rate of reaction ot the cncentrations in the rate equation If you double the concentration of a reactant of zero order, what happens to the rate? Stays the same 2^0 = 1 If you double the concentration of a reactant of first order, what happens to the rate? Doubles 2 = 2 If you double the concentration of a reactant of second order, what happens to the rate? Quadruples 2^2 = 4 How do you determine the units of the rate constant? Rearrange the rate = k[A]^m [B]^n equation with k as the subject cancel out the moldm-3 REMEMBER THE S^-1 Why must experiments involving rate be done at a constant temperature? k varies with temperature What does the graph of temperature against rate constant look like? Positive exponential What happens to the value of the rate constant, k, as temperature increases? Increases State the Arrhenius equation k = Ae^(-Ea/RT) What do each of the letters in k= Ae^(-Ea/RT) mean? k= rate constant A = Arrhenius constant e= mathematical constant Ea= activation energy R= gas constant T= tempeature What are the units of Ea? Jmol-1 What are the units of R? JK-1mol-1 What are the units of temperature? K Why does the Arrhenius constant have the same units of the rate equation? e^(-Ea/RT) has no units as J/JK-1mol-1 x K = 1 ALWAYS CONVERT ACTIVATION ENERGY INTO J ALWAYS CONVERT ACTIVATION ENERGY INTO J Why must you always convert activation energy into J? So the units cancel out and the rate constant has the same units as the Arrhenius constant What will have a greater effect on the rate of reaction, adding a catalyst, increasing the temperature or increasing the concentation of a reactant? Adding a catalyst What graphical equation can the Arrhenius equation be converted into? lnK = -Ea/RT + lnA What does each component of lnk = -Ea/RT + lnA represent in y=mx + c equation? lnk = y 1/T = x lnA= y intercept -Ea/R= gradient How can activation be calculated from the gradient of the line? Gradient = -Ea/R R is a constant What are the two types of rate monitoring? Initial rate monitoring Continuous rate monitoring What can be measured when using initial rate monitoring Volume of gas produced over time Concentration of coloured product over time Concentration of hydroxide/hydrogen ions over time What can be measured when using continuous rate monitoring? Hydrogen ion concentration over time Concentration of coloured reaction over time Titrating specific reactant How is the initial rate determined from a graph? (reaction where gas produced) Graph of gas/mass against time Gradient of tangent to line at t=0 In general, how does initial rate monitoring work? Several experiments done with different concentrations of reactants Concentration of product measured over time Graph plotted Tangent taken at t=0 to find initial rate New graph plotted with initial rates against reactant concentration Graph shape shows order of that reactant How can the rate of reaction be gotten from a graph of concentration against time? Gradient of tangents In general, how does continuous rate monitoring work? One experiment Concentration of reactant measured over time Graph plotted New graph plotted of rate against reactant concentration (rate found from tangents) Shape of graph tells order of that reactant Continuous rate monitoring uses reactant concentration True or False True C R I P Think CRIP ! How are titrations used in initial rate monitoring? Several experiments using different concentrations of the reactant Quenched sample titrated and concentration of product found at each time Graph plotted of concentration over time Gradients of tangents where t=0 What are the similarities between initial and continuous rate monitoring? Both start off with graphs of concentration against time Both use tangents from graphs to find the rate at a particular concentration What are the differences between initial and continuous rate monitoring? Initial uses concentration of product on graph and continuous uses concentration of reactant Initial uses several experiments with varying concentrations of reactant, continuous uses one experiment Initial uses tangents at t=0, continuous uses tangents at different concentrations What must be done before titrating in rate monitoring? Quench a sample What are 3 methods of quenching? 1. Rapid cooling 2. Adding chemical to remove reactant not monitored 3. Adding large known volume of water What will the shape of the rate against concentration graph look like for zero order reactant? straight horizontal line What will the shape of the rate against concentration graph look like for first order reactant? Straight positive diagonal line, directly proportional What will the shape of the rate against concentration graph look like for second order reactant? positive exponential line What will the shape of the concentration against time graph look like for a zero order reactant? Straight negative diagonal line