Rates of reaction Flashcards
Definition of rate of reaction?
The change in concentration of a reactant or product in unit time
What units is rate of reaction measured in?
mol dm ^-3.s^-1
What does increasing pressure and increasing concentration do to the rate of reaction?
Increases it as more collisions
Examples of how the progress of a chemical reaction might be monitored?
Colour change, mass change, volume of gas produced, pH, conductivity
What does rate of reaction =?
Change in concentration/change in time
How to calculate rate of reaction from a concentration time graph?
At the given time draw a tangent at that point and calculate it’s gradient (change in y / change in x)
Catalyst definition?
Substance that increases the rate of a chemical reaction without being used up in the process, providing an alternative route for the reaction with lower activation energy
What’s a heterogeneous catalyst?
It’s in a different physical state from the reactants, which is most frequently gaseous reactants and a solid catalyst
What’s a homogenous catalyst?
Catalyst in the same physical state as reactants, most frequently aqueous or gaseous state
Ways catalysts affect chemical industry?
Reduce energy consumption, positive effect on environment
Increase atom economy, reduce waste
Enzyme use in industry
Economic benefits
Describe a boltzman distribution graph?
x axis = energy
y axis = number of molecules with a given energy
Shape- straight diagonal line which then slopes down
Activation energy line- near tail
What does the area underneath a boltzman distribution represent?
Total number of particles in the system
What happens to the boltzman distribution when temperature is increased?
The peak gets lower, and moves to a higher energy. Meaning a higher proportion of particles have more energy than the activation energy
Reasons for why an increase in temperature increases rate of reaction
More particle collisions per unit time, and more particles collide have the activation energy or more
Effect of a catalyst on a boltzman distribution?
Activation energy lower, so proportion of particles with more energy than activation energy increases
Definition of dynamic equilibrium?
The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and concentration do not change
What is meant when a system is closed?
Isolated from surroundings
No material added or removed
Conditions (temp/pressure) stay constant
Le Chatelier’s principle?
When a system in dynamic equilibrium is subjected to an external change, the system readjusts it self to minimise the effect of change and restore equillibrium
Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Adding H2 change?
More ammonia produced
Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Change if take away ammonia?
More ammonia produced
Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Change if temperature increased?
More Nitrogen and Hydrogen produced as reaction is exothermic
Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Change if pressure increased?
More ammonia produced, as their are more moles of gas of N2 and H2
Starting equation
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
: DH = –92 kJ mol-1
Adding a catalyst?
No effect on equilibrium position
Formula for equilibrium constant, Kc
aA + bB ⇌ cC + dD
Kc= ([C]^c [D]^d)/([A]^a [B]^b)
How to know the units for Kc
Cancel out the amount of moles on top and bottom, then do Mol^-1.dm^3 to that power. Eg. if it was 3 would be Mol^-3.dm^9
What does increasing the pressure do to the rate?
Increases rate
particles closer together so more frequent collisions per unit time
What does increasing concentration of reactant do to the rate of reaction?
Increases rate
More particles in a given volume, so more collisions per unit time
What does adding a catalyst do to the rate of reaction?
Increases rate
Catalyst provides an alternative route with a lower activation energy, therefore a greater proportion of particles have energy equal to or greater than the activation energy so a greater proportion of collisions are succesful
