6.1 Rate of reaction
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How is rate of reaction calculated? (Higher)
Rate of reaction = amount (g, cm3) 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 cm3/s 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 an experiment 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 an experiment 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 an experiment 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 do you find the rate of reaction at some time, t, from a graph of amount of reactant vs time? (Higher)
● 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.
Temperature increase = faster reaction As temperature 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.
Concentration increase = 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 slows down 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 than magnesium powder.
What is a catalyst and how does it work?
A catalyst changes the rate of reaction but is not used up. It increases the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
How does a catalyst affect the reaction profile?
The reaction profile for a catalysed reaction will have a lower maximum of the curve (lower activation energy).
What is an enzyme?
An enzyme is a molecule that acts as a catalyst in a biological system.
What is a reversible reaction?
A reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants
When is dynamic equilibrium reached?
In a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
Describe Le Chatelier’s Principle
If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.