Chapter 10 - Reaction rates and equilibrium Flashcards
What is the rate of a chemical reaction?
Give units
Change in concentration of a reactant or product in a given time.
rate = change in concentration/ time
units - moldm^-3 s^-1
Describe the general rate throughout a reaction.
- Rate fastest at the start of the reaction - each reactant at it’s highest concentration
- Rate of reaction slows down as reaction proceeds, because the reactants are being used up as their concentrations decrease
- once one of the reactants has been completely used up, the concentrations stop changing and the rate of reaction is zero.
Draw concentration-time graph of a general reaction
concentration increases at a decelerating rate until it stops increasing (flat line)
What is the collision theory?
Two reacting particles must collide for a reaction to occur.
What conditions need to be met for a collision to be successful?
- Particles collide with correct orientation
- Particles have sufficient energy to overcome activation energy barrier of the reaction
What is the effect of increasing concentration of reactant on rate of reaction?
Increases
Why does increasing the concentration of a reactant, increase rate of reaction?
Increase in concentration increases number of particles in the same volume, particles closer together, and therefore collide more frequently - so more successful collisions.
How does increasing the pressure of a gas affect the rate of reaction?
Increases
Why does increasing the pressure of a gas increase the rate of reaction?
Increases the concentration of the gas, so collide more frequently (more successful collisions)
How can the progress of a reaction be measured in?
- Monitoring the removal of a reactant or formation of a product
- Monitoring the volume of a gas produced at a regular time intervals using gas collection, or monitoring loss of mass of reactants using a balance.
What is a catalyst?
Substance that changes the rate of a chemical reaction without undergoing any permanent change itself.
Provides alternative reaction pathway of lower activation energy.
What is a homogeneous catalyst?
Catalyst that has the same physical state as the reactants. Catalyst reacts with the reactants, to form an intermediate, which then breaks down to give product and regenerates catalyst.
What is a heterogeneous catalyst?
Catalyst has different physical state from the reactants Usually solids in contact with gaseous reactants or reactants in solution. Reactant molecules absorbed onto the surface of the catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst by desorption.
How do catalysts provide economic and sustainability benefits?
Reactions with catalysts require less energy, therefore less electricity or fossil fuels are used. Making product faster, using less energy can increase profitability. Less fossil fuels are used so CO2 emissions are cut - mitigating global warming.
(improved sustainability out-weights the toxicity of some catalysts)
What is the boltzmann distribution?
Shows spread of molecular energies.
What are the main features of a boltzmann distribution?
- No molecules have zero energy - curve starts at the origin.
- Area under the curve is equal to the total number of molecules
- No maximum energy for a molecule - the curve does not meet the x-axis at high energy.
Draw the boltzmann distribution of a typical reaction.
How does a boltzmann distribution curve change when temperature is increased?
- More molecules have energy greater than activation energy, increasing rate of reaction
- Graph is now stretched over a greater range of energy values, peak of graph is at lower y value and further along x-axis
What effect does a catalyst have on the boltzmann distribution?
Ea is at a lower energy, so more molecules are in the Ea section.
What is a dynamic equilibrium?
When rate of forward reaction is same as the reverse reaction, in a closed system
What happens to concentrations of reactants and products when in dynamic equilibrium?
Stays the same
What is Le chatelier’s principle?
States that when a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change.
What is the position of equlibrium?
Indicates the extent of the reaction
What is the effect of increasing concentration of reactants of a equilibrium system?
Shifts the equilibrium to the right creating more product
What effect does increasing pressure have on an equilibrium (with gases)?
Shifts equilibrium to side with fewest gas molecules.
What effect does increasing temperature have on an equIlibrium?
Shifts equilibrium to the endothermic direction to aborb heat energy.
What effect does a catalyst have on an equlibrium?
Increases rate of forward and reverse reactions, so equilibrium reached faster. No effect on position of equilibrium
Give general reversible reaction
aA + bB cC + dD
What is the equilibrium constant? Give the equation
Kc = [C]^c [D]^d/ [A]^a [B]^b
[products]/[reactants]
(if reaction is aA +bB cC +dD)
What do values of Kc > 1, <1 or =1 mean?
- Value of 1 = position of equilibrium is halfway between products and reactants.
- Kc value > 1 indicates position of equilibrium towards the products
- Kc value < 1 indicates position of equilibrium towards the reactants.
How is the concentration of ozone in the stratosphere maintained? Use equations
O3 O2 + O
Raye of ozone decomposition (almost) equals rate of ozone formation
How initial rate of dynamic equilibrium increased?
- Temp increase
- Pressure increase
- Catalyst added
