Chapter 18: Speed of Reaction Flashcards
How can we measure the speed of a reaction?
1) Measuring the volume of gas (product) formed per unit time
- Collect and measure the volume of gas produced at regular time intervals
- SOR = volume of product formed/ time taken
2) Measuring the mass of reactant(s) used up per unit time
- SOR = mass of reactant used up/ time taken
What is the criteria that needs to be met if one wants to measure the speed of a reaction using the method of measuring the volume of gas formed per unit time?
The gas evolved in the reaction must be a heavy gas so that the mass lost is significant. Hence, if H2 gas (Mr = 2) is the product, this method is not suitable.
Not recommended for: H2 gas, soluble gases, Cl2, SO2 and NH3
Recommended for: CO2 gas (heavy gas)
State the collision theory.
For a chemical reaction to occur, reacting particles must collide with each other and collide with energy that is equal to or greater than the activation energy. Collisions that result in the formation of products are known as effective collisions.
State the factors that affect the SOR.
1) Particle size of solid reactants
2) Temperature of reacting system (replace “reacting system” according to the context of the question)
3) Pressure of gases in gaseous reactions
4) Presence of a catalyst
5) Concentration of aqueous limiting reagent
Explain how the particle size of solid reactants affect SOR.
Smaller particle size → larger surface area of contact between reactant particles → increase in FREQUENCY of effective collisions → SOR increases
Explain how the temperature of reacting system affects SOR.
Increase in temperature → particles gain more KE and hence move faster → more particles possess energy that is more than or equal to the activation energy → increase in frequency of effective collisions → SOR increases
Explain how the pressure of gases in gaseous reactions affects the SOR.
Higher pressure → particles closer together → greater number of particles in contact with one another → increase in frequency of effective collisions → SOR increases
What is a catalyst?
A catalyst increases the speed of a chemical reaction, and remains chemically unchanged at the end of the reaction.
List the 7 characteristics of catalysts.
1) Lowers the activation energy of a chemical reaction
2) Selective in action → one catalyst cannot be used to speed up all reactions
3) Only a small amount is required to speed up reaction
4) Increase SOR but NOT yield of products
5) Impurities can affect catalysts’ performance in reactions
6) Remains chemically unchanged at the end of the reaction
7) Is not used up in a reaction. Same amount present in the beginning and at the end.
Explain how the presence of a catalyst affects the SOR.
Presence of catalyst → provides an alternative pathway with a lower activation energy → more particles possess energy that is more than or equal to the LOWERED activation energy → frequency of effective collisions increases → SOR increases
Explain how the concentration of an aqueous solution affects the SOR.
More concentrated solution → more reactant particles per unit volume → increase in frequency of effective collisions between particles → higher speed of reaction
What is concentration? State the units for concentration.
- Amount of matter per unit volume (dm^3)
- Units: mol/ dm^3 or g/dm^3
