Rates and equilibrium Flashcards
(25 cards)
Which factors can affect rate of reaction?
Temperature, concentration, pressure, surface area
Explain the effect of temperature on rate of reaction
Increasing temperature increases rate of reaction because the reacting particles have more energy, resulting in more frequent collisions with more energy, which means more successful collisions.
Explain the effect of surface area on rate of reaction
More particles are available for reactions in a given time, so collisions are more frequent and the rate of reaction increases. Surface area can be increased by using smaller pieces of reactant.
Explain the effect of concentration on rate of reaction
The higher the concentration (in a solution), the closer the particles are crowded, so they collide more frequently, resulting in more successful collisions and a higher rate of reaction.
Explain the effect of pressure on rate of reaction
The higher the pressure (as gases react), the closer the particles are crowded, so they collide more frequently, resulting in more successful collisions and a higher rate of reaction
How do you calculate rate of reaction?
Product formed OR reactant used up / time
How do you calculate rate of reaction at a specific point in time?
Find the point on the graph corresponding with the time and, if the line is curved, draw a tangent. The gradient of the tangent (or of the line itself if it’s straight) is the rate of reaction.
What is a closed system (in chemistry)?
A system in which mass can’t enter or leave but energy can
What are the requirements to achieve equilibrium?
In a closed system, the rate of the forward reaction is equivalent to the rate of the reverse reaction
In a reaction exothermic in the forward direction, what will happen to the position of equilibrium when temperature is increased, and why?
Because the reverse reaction is endothermic, it counteracts the change, so it is favoured. Due to this, more reactant will be formed and equilibrium shifts left.
How can you tell if a reaction is exothermic in the forward direction?
The change in energy is negative.
What is the effect of changing pressure in a reaction involving gases in equilibrium?
Equilibrium will shift in the direction with the least number of molecules
While a system is in equilibrium, what is the effect of increasing the concentration of a reactant?
The system is no longer at equilibrium because reactant has been added. Since there is a higher concentration of reactants, the forward reaction is favoured and more product than reactant will be formed until equilibrium is reached again.
What is a catalyst?
A substance that increases the rate of a chemical reaction without being used up in the process. A catalyst reduces activation energy, and gives a surface for molecules to attach so they collide more frequently. This results in more successful collisions and a higher rate of reaction.
While a system is in equilibrium, what is the effect of decreasing the concentration of a reactant?
The system is no longer at equilibrium because there is a lower concentration of reactant. Since there is a higher concentration of product, the reverse reaction is favoured, producing more reactant, until equilibrium is reached again.
What is Le Chatelier’s principle?
If a system is at equilibrium and a change is made to the system, the system responds to counteract the change.
If a reaction is endothermic in the forward reaction, what happens if you increase temperature, and why?
Because the forward reaction is endothermic, it counteracts the change, so it will be favoured and more product will be formed, and equilibrium will shift to the right.
If there are more molecules in the reactant side in a reversible reaction (gases in equilibrium), what is the effect of increasing pressure?
Equilibrium will shift to the right because there are less molecules on that side.
What is the point of the haber process?
It produces ammonia, which can be used to make fertilizers and explosives.
What temperature is used in the haber process and why?
The reaction is exothermic in the forward direction, so a lower temperature would favour the forward reaction because it counteracts the change, increasing yield of ammonia. Furthermore, a higher temperature requires more energy, which is expensive.
However, a lower temperature results in a far too slow rate of reaction.
So, a compromise temperature - about 450 degrees Celsius - is used.
What pressure is used in the haber process and why?
Since there are fewer molecules on the right hand side, a higher pressure would result in a higher yield of ammonia.
However, high pressures require expensive equipment to maintain and bear risk of explosion.
So, a compromise pressure - of about 200 atmospheres - is used
What catalyst is used in the harber process and what is its role? Does it affect the position of equilibrium?
An iron catalyst is used to increase the rate of reaction. Since it increases the rate of both the forward and reverse reaction equally, it has no effect on the position of equilibrium.
Detail, fully, an experiment to test the effect of varying concentrations on rate of reaction. Describe hazards and countermeasures.
Place a conical flask with 20ml (measured with a measuring cylinder) of sodium thiosulfate on top of a white tile with a cross drawn on it. Add 20ml of 0.2 mol/dm^3 hydrochloric acid to the flask and start the timer immediately, swirling the flask to ensure all particles react. Watch the cross through the top of the flask, stopping the timer when it disappears. The quicker it was, the faster the rate of reaction.
Repeat the experiment with varying concentrations of HCl, increasing in increments of 0.2 mol/dm^3 up to 1.4 mol/dm^3. Record the time it takes for the cross to disappear for every concentration of HCl.
The acid can be harmful to the eyes, so wear goggles.
Explain collision theory.
For a chemical reaction to occur, particles must collide with sufficient energy (called activation energy).
