Rates ✅ Flashcards
What is the equation for rate
Rate=change in concentration/time
Rate (moldm-3s-1)
What factors affect the rate of a reaction
Concentration (or pressure when reactants are gases)
Temp
Use of a Catalyst
SA of solid reactants
Under what circumstances will an effective collision occur
The particles collide with correct orientation
Particles have sufficient energy to overcome activation barrier of the reaction
How does increasing the concentration affect the rate of reaction
When conc increases ROR is increased, as increase in conc increases the number of particles in the same volume
. The particles are closer together and collide more frequently, in given period of time there will therefore be more effective collisions
How does increasing pressure of a gas affect ROR
ROR increases as conc of gas molecules increases as same number of gas molecules occupy a smaller volume, molecules are closer together and collide more frequently, leading to more effective collisions in same time
How can the progress of a chemical reaction be followed by
Monitoring the removal of a reactant
Following formation of a product
With gases monitor volume of gas producing at regular time intervals or loss of mass of reactants using a balance
Explain monitoring production of gas using gas collection practical
- Hydrogen peroxide added to conical flask and bung placed
- Initial volume of gas in measuring cylinder recorded
- Manganese dioxide catalyst quickly added to conical flask and bung replaced stopwatch started
- Volume of gas produced in measuring cylinder recorded at regular intervals until reaction complete
- Plot graph etc to find results
Changing concentration of reactant alters ROR, state 3 other factors that affect rate. Explain how an increase in conc increases the rate (5 marks)
Temp, SA, catalyst
When conc of a reactant increases ROR increases, increase in conc means more reactant molecules in same volume, reactant molecules are closer together and collide more frequently leading to more effective collisions
What is a catalyst
A substance that changes the rate of a chemical reaction without undergoing any permanent change
How does a catalyst increase the rate of reaction
By providing an alternative reaction pathway of lower activation energy
How/what happens in a reaction with a catalyst
Catalyst not used up in the chemical reaction
Catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
At end of reaction the catalyst is regenerated
Know how to draw an enthalpy profile diagram with a catalyst
Activation energy is lower
What is a homogeneous catalyst
Has the same physical state as the reactants, catalyst reacts with the reactants to form an intermediate, the intermediate then breaks down to give the product and regenerates the catalyst
What is a heterogeneous catalyst
Has a different physical state from reactants, usually solids in contact with gaseous reactants or in solution. Reactant molecules are absorbed into surface of catalyst, where reaction takes place, after reaction product molecules leave surface of the catalyst by desorption
State the difference between a homogeneous and heterogeneous catalyst (1 mark)
Homogenous catalyst has same physical state as reactants, heterogeneous catalyst has different physical state from reactants
Describe the effect of a catalyst on activation energy of a chemical reaction and on enthalpy change of reaction (2 marks)
Lowers activation energy by providing an alternative route for the reaction, no effect on 🔺H
