Rates Of Reaction Flashcards
How is rate of reaction calculated?
Rate of reaction = amount of reactant used or product formed / time
Rate of reaction (mol/s) = Moles of reactant used or product formed / time
What are the various units for rate of reaction?
Can include g/s or cm% or mol/s
Generally, mass/time, volume/time, moles/time
Name three common ways of measuring rate of reaction
- Loss in mass of reactants
- Volume of gas produced
Time for a solution to become opaque
Describe measuring the rate by monitoring mass loss
Place the reaction flask on a balance. In these reactions (e.g. metal carbonate + acid) a gas is given off, so record the decrease in mass in time intervals (note hydrogen is too light). Plot a graph of mass vs time.
Describe measuring the rate by monitoring the volume of a gas
Connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals. Plot a graph of volume vs time.
Describe measuring the rate by monitoring the disappearance of a cross
Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
How to find a rate of reaction at some time, t, from a graph of amount of reactant vs time?
- Pick a point corresponding to the time t, and find the tangent to the curve at this point.
- The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate.
Gradient of tangent can be expressed in change in y values over change in x values.
State five factors affecting the rate of a chemical reaction
- Concentration of reactants
- Pressure of gases (volume)
- Surface area
- Temperature
- Catalysts
What is the collision theory?
Chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to activation energy).
Describe and explain the effect of increasing temperature on the rate of reaction
T increases = faster reaction
As T increases, kinetic energy of particles increases, i.e. more energetic collisions
Also, they move faster, so they collide more frequently
However, there is no straight line relationship between rate and temperature, i.e. they are not directly proportional to each other
Describe and explain the effect of increasing concentration on the rate of reaction
Conc. increases = faster reaction,
More reactants = more frequent collisions
Describe and explain the effect of increasing pressure of a gas on the rate of reaction
Increasing the pressure of reacting gases, is the same as increasing concentration. It increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.
Note that volume and pressure are inversely proportional to each other. Increasing the volume retards the reaction.
Describe and explain the effect of increasing surface area
If solid reactants are in smaller pieces, they have a greater surface area.
Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction, e.g. block of magnesium reacts slower with acid then magnesium powder.
What is a catalyst and how does it work? How does it affect the reaction profile?
A catalyst changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy).
How is rate of reaction calculated?
Rate of reaction = amount (e.g. grams, cm°) of reactant used or product formed time
Rate of reaction (mol/s) = Moles of reactant used or product formed
time
