C6: The rate and extent of chemical change Flashcards
How is rate of reaction calculated?
rate of reaction = amount ( eg grams ) of reactant used or product formed / time
rate of reaction ( mol/s) = moles of reactant used or product formed / time
give the three various units for rate of reaction?
can include:
- g/s
- cm³/s or moles/time
Name three common ways of measuring ate 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 a gas is given off
- so record the decrease in mass in time intervals
- plot a graph of mass vs time
Describe measuring the rate of reaction by monitoring the volume of a gas
- connect a gas syringe to a reaction
- 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 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 reactions
- concentration of reactants
- pressure of gases ( volumes)
- 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
- As temp increases, kinetic energy of particles increases i.e. more energetic collisions.
- also , they move faster, so they collide more frequently
Describe and explain the effect of increasing concentration on the rate of reaction
- More concentrated means more particles in solution,
- therefore more frequent collisions between reactants
Describe and explain the effect of increasing pressure of a gas on the rate of reaction
- it increases the number of gas molecules in the same volume
- so increases the frequency of collisions
- therefore increases the rate of 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
what is a catalyst and how does it work?
- A catalyst changes the rate of reaction but it is not used up
- They provide another route for the reaction to take place which 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 ?
A molecule that acts as a catalyst in a biological system
What is 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
- 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
describe the effect of changing the concentration of reactant and product on the position of the equilibrium (3)
- The system is no longer at equilibrium
- if the concentration of the reactant is increased , more products will be formed until equilibrium is reached again
- if product is decreased , more reactants will react until equilibrium is reached again
Describe the effect of changing temperature on the position of the equilibrium
increasing temp:
- equilibrium moves to the direction of the endothermic reaction
decreasing temp:
- equilibrium moves to the direction of the exothermic reaction
products inc
products dec
Describe the effect of an increase in pressure on the position of the equilibrium
this applies to equilibria that involve gases
- an increase in pressure causes the equilibrium position to shift towards the side with the smaller number molecules as shown by the symbol equation for that reaction
Describe the effect of a decrease in pressure on the position of the equilibrium
- a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for the reaction
Describe the effect of a catalyst on the position of the equilibrium
- no effect
- It just speeds up both forward and backward reactions equally
- i.e. equilibrium is achieved faster
A reaction is exothermic in the
forward direction. What will occur if
the temperature is increased?
The backward reaction will increase as it is
endothermic and will reduce the temperature
