Module 5 - Physical Chemistry and Transition Elements Flashcards
Define rate of reaction.
Rate of reaction - change in conc of a reactant or product in a given time.
Define order of reaction.
Order of reaction - power to which conc of a reactant is raised in the rate equation.
Define rate constant.
Rate constant - constant that links the rate of reaction with the concentrations of reactants raised to powers of their orders in the rate equation.
Define overall order.
Overall order - sum of orders with respect to each reactant.
Define half-life.
Half-life - time taken for concentration of a reactant to decrease by half.
Define rate-determining step.
Rate-determining step - slowest step in the reaction mechanism of a multi-step reaction.
Define initial rate.
Initial rate - instantaneous rate at the start of the reaction, when t=0.
What is the special feature of first order reactions?
Constant half-life.
What happens to half-life of zero order reaction when reactant conc is decreased?
Half-life increases.
What happens to half-life of first order reaction when reactant conc is decreased?
Half-life remains constant.
What happens to half-life of second order reaction when reactant conc is decreased?
Half-life increases.
How do you calculate the rate constant using the half life of a first order reaction?
k = 0.693 / half-life.
How do you calculate k from a zero order conc-time graph?
Zero order - k = -slope of conc-time graph.
How do you calculate k from a zero order rate-conc graph?
Zero order - k = rate at any concentration.
How do you calculate k from a first order conc-time graph?
First order - k = slope of ln(conc) vs time.
How do you calculate k from a first order rate-conc graph?
First order - k = slope of rate vs conc graph.
How do you calculate k from a second order conc-time graph?
Second order - k = slope of 1/concentration vs time.
How do you calculate k from a second order rate-conc graph?
Second order - k = slope of rate vs concentration.
Outline method for using colorimeter to monitor concentration change.
Make known conc solutions of X. Measure absorbance of each using colorimeter. Plot absorbance against conc to make calibration curve. Measure absorbance due to X during experiment. Use calibration curve to convert absorbance to concentration.
When can you use ‘loss of mass method’ for monitoring concentration?
If a relatively heavy gas is formed, eg CO2, and not hydrogen because the mass change would be too small to measure.
When shouldn’t you use a gas syringe?
If a lot of gas is evolved.
When shouldn’t you use ‘mass loss method’?
If the gas is toxic.
When shouldn’t you use the inverted cylinder method?
If gas reacts/is soluble in water.
How do you monitor the progress of a reaction when brown iodine is produced?
Use a colorimeter to measure the increase in absorbance of light.
How do you monitor the progress of a reaction when a gas is produced?
Use a gas syringe or a measuring cylinder over water to measure the volume of gas produced.
What is the continuous monitoring method?
Measuring concentration of reactants over time to plot conc-time graph from which we can calculate initial rate/any rate.
Describe the clock method.
Measure time taken for an obvious change from start of reaction. Average rate = initial rate. Initial rate is proportional to 1/t. Repeat clock reaction several times with different conc. Plot 1/t against concentration.
How do you improve accuracy of clock method?
Measure average rate during the first part of reaction; the shorter the time period, the less the rate changes.
How does the rate equation relate to the rate-determining step?
Rate equation tells us which species react in the rate-determining step and in what proportions (orders = moles).
What is the most important rule when suggesting mechanisms?
Never have 3 or more particles colliding in one step.
Define oxidising agent.
Oxidising agent - reagent which oxidises (takes electrons from) another species.
Define reducing agent.
Reducing agent - reagent which reduces (adds electrons to) another species.
When shouldn’t you use the inverted cylinder method?
If gas reacts/is soluble in water.
