rate of reaction Flashcards
what does the phrase ‘rate of reaction’ mean?
it means the speed of reaction
what can the rate of chemical reaction be defined as?
it can either be defined as the increase in the concentration of one of the products per unit time or as the decrease in the concentration of reactants per unit time (The rate of a
reaction therefore can be measured as the ‘rate of formation of a product’ or the ‘rate of consumption of a reactant’)
how can we determine the rate of a chemical reaction?
The rate of a chemical reaction can be determined by measuring any quantity that changes over the course of the reaction.
how do we find the gradient of the graph?
The instantaneous rate at any stated point in time is then
the gradient of the graph at that time.
how can we determine the rate of reaction if a gas is produced
We can collect the gas using a graduated gas syringe and measure the volume of gas produced over time or
we could allow the gas to escape and monitor the loss in mass (of the reaction flask & its contents) over time.
are the two methods (Measuring Volume of Gas produced/Measuring Change in Mass suitable in determining the reaction rate for all chemical reactions involving
production of gas?
Only measuring of volume of gas method is suitable for most reactions with a gas as product.
Measuring change in mass is more suitable when a relatively heavy gas such as carbon dioxide is evolved. It is not suitable if the gas is very light, for example hydrogen gas, as the mass loss would be too small to be
detected by the electronic balance.
how can we measure the rate of reaction if a precipitate is produced? (list the 2 methods)
- Since the concentration of mobile ions in the solution is decreasing during the reaction (due to the formation of insoluble substance – precipitate), the concentration of mobile ions in the solution over time can be measured using a conductivity meter. A graph of conductivity over time can be plotted and the instantaneous
rate of reaction can be determined by drawing a tangent to the curve and calculating the gradient. - Alternatively, the reaction can be carried out in a conical flask with a white tile/paper with a cross placed below
the flask. At the beginning of the reaction, the cross will be clearly visible when you look into the flask. As the reaction progresses and more precipitate is formed, the cross will gradually be obscured. Measuring the time
taken for the cross to disappear gives an estimate of the average rate of this reaction. The average rate can be determined by taking the reciprocal of the time taken for the cross to be obscured. The longer the time, the
slower the reaction. However, the instantaneous rate of reaction cannot be determined by this method.
how can we measure the rate of reaction if a colour change is observed?
A colorimeter can be used to measure the intensity of a colour over time.
(For example, in the reaction where zinc displaces copper from a solution of copper(II) sulfate, the intensity of
the blue colour of copper(II) sulfate can be measured over time.)
what must the particles involved in a reaction have?
(i) collide with each other;
(ii) possess sufficient kinetic energy in order to overcome the activation energy barrier of the reaction;
(iii) collide in the correct orientation, i.e. the reactive parts of the molecule must come into contact in the correct way.
what happens in ineffective collisions?
particles do not have the correct orientation and no reaction occurs
1. reactant molecules approach
each other
2. ineffective collision – no reaction occurs
3. reactant molecules separate – no product formed
what happens in effective collisions?
particles collide with the correct orientation, enabling products to form
1. reactant molecules approach
each other
2. effective collision – particles have correct collision orientation so reaction occurs
3. product molecules formed
what are the factors affecting the Rate of Reaction
(i) increasing the collision frequency
(ii) increasing the fraction of effective collisions
how does concentration affect the rate of reaction?
- An increase in concentration causes an increase in reaction rate.
- There is an increase in the number of reacting particles per unit volume of solution.
- Hence, there is an increase in the frequency (or probability) of effective collisions between reacting particles.
how does pressure affect the rate of reaction?
When the pressure of gaseous reactants in a system increases, the number of reacting particles per unit volume increases. There is an increase in the collision frequency and therefore an increase in the frequency
of effective collisions between reacting particles.
how does particle size affect the rate of reaction?
A decrease in particle size (e.g. lumps to powder) causes an increase in reaction rate.
There is an increase in the total exposed surface area of the reactant. Hence, there is an increase in the frequency (or probability) of effective collisions between reacting particles. (leads to a faster reaction, many instantaneous reactions or even explosions can occur with finely divided or powdered substances.)
