Module 5: rates Flashcards
How to calculate rate:
rate= change in concentration/change in time OR rate=quantity reacted or produced/time
What is rate of reaction proportional to?
Rate is proportional to the concentration of a particular reactant raised to a power e.g [A]^2
What does the power represent in [A]^n
Represents the order of reaction
What is a zero order reaction?
Concentration of a reactant has no effect on the rate
What is a first order reaction?
If concentration is doubled, rate of reaction also doubles
What is a second order reaction?
Rate increases by the square of the concentration increase; when concentration is doubled, rate of reaction quadruples
What is the rate equation?
Rate=k[A]^n[B]^n
What is k in the rate equation?
Rate constant
How to calculate the overall order of a reaction?
overall order= sum of orders of each reactant
Rules when writing rate equation:
Zero order reactants are not represented
Don’t need to write power of 1 when representing first order reactants
How to calculate units for rate constant?
- Rearrange equation to make k the subject
- Substitute units into expression for k
- Cancel common units and show final units on a single line
Remember to swap powers if units are on the bottom of the equation!
How can orders of reaction be found from experimental results?
- By monitoring physical quantity changes over time
- This can be done either through a continuous monitoring method or initial rate method
How can continuous monitoring be carried out?
- Colorimeter
- Measure volume of gas produced over time
- Change in pH
Rate of reaction definition:
Change in the amount of reactants or products per unit time
How can initial rate be measured?
- Iodine clock
- Track formation of a precipitate
What shape is a zero order time-concentration graph?
Straight line with negative gradient
What shape is a first order time-concentration graph?
Downward curve with a decreasing gradient over time
What shape is a second order time-concentration graph?
Also a downward curve, steeper at the start, but tailing off more slowly
What is half-life?
Time taken for the concentration of a reactant to half
How to calculate the rate constant from the rate?
- Draw tangent to the curve on the concentration-time graph is drawn at a particular concentration
- Gradient of tangent calculated, giving the rate of reaction
- Rearrange equation to find k, and input rate and concentration values
What shape is a zero order rate-concentration graph?
Horizontal straight-line with zero gradient
What shape is a first order rate-concentration graph?
Linear line through the origin
What shape is a second order rate-concentration graph?
Upward curve with increasing gradient
How can rate constant be calculate from iodine clock practical?
- Solution is colourless at the start and the time is measured for the black blue colour of the iodine to appear
- Experiment repeated at varying concentrations
- Initial rate is proportional to 1/t
- A graph of rate against concentration plotted
- Shape of graph identified to represent order in respect to one reactant
- Rate equation can be written and rate constant calculated
