Revision notes · The rate and extent of chemical change
Rate of reaction
Calculating rates of reactions4.6.1.1
Definition: The rate of reaction is a measure of how quickly reactants are used up or products are formed over time.
You can calculate how fast a reaction occurs by measuring changes in reactant or product quantities over time.
- •Mean rate of reaction is calculated using rate = quantity of reactant used ÷ time or rate = quantity of product formed ÷ time.
- •Units of rate are commonly g/s, cm³/s, or mol/s depending on how reactants or products are measured.
- •The rate of reaction at a specific time is found by drawing a tangent to the curve on a graph and calculating its gradient.
- •Common methods to measure rate include monitoring mass loss, volume of gas produced, or time for a solution to change turbidity.
⚠️ Common mistake: Thinking a steeper line on a graph means a slower reaction, when a steeper gradient actually indicates a faster rate of reaction.
🧠 Remember: Rate = Change in quantity ÷ Time (gradient equals rate).
Factors which affect the rates of chemical reactions4.6.1.2
Definition: The factors affecting reaction rate are physical conditions that alter the speed at which reactants turn into products.
Changing environmental or chemical conditions directly changes how fast a reaction happens.
- •Increasing the concentration of reactants in solution increases the rate of reaction.
- •Increasing the pressure of reacting gases increases the rate of reaction.
- •Increasing the surface area to volume ratio of solid reactants increases the rate of reaction.
- •Increasing the temperature of the reaction mixture increases the rate of reaction.
⚠️ Common mistake: Confusing concentration (number of particles per unit volume) with the total volume of liquid.
🧠 Remember: Remember C-P-S-T: Concentration, Pressure, Surface area, Temperature.
Collision theory and activation energy4.6.1.3
Definition: Collision theory states that chemical reactions only occur when reacting particles collide with each other with sufficient energy.
For particles to react, they must collide with energy equal to or greater than the activation energy.
- •The activation energy is the minimum amount of energy that particles must have to react.
- •Increasing concentration, pressure, or surface area increases the frequency of collisions.
- •Increasing temperature increases collision frequency and makes collisions more energetic, increasing the proportion of successful collisions.
- •A collision is only a successful collision if particles collide with energy greater than or equal to the activation energy.
⚠️ Common mistake: Writing 'more collisions' instead of 'more frequent collisions' or 'more successful collisions per second'.
🧠 Remember: No energy above activation energy = no successful collision.
Catalysts4.6.1.4
Definition: A catalyst is a substance that increases the rate of a chemical reaction without being used up.
Catalysts speed up reactions without undergoing any permanent chemical change themselves.
- •Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy.
- •Catalysts are not used up during the reaction, so they can be reused and do not appear in the chemical equation.
- •Different reactions require different catalysts, and enzymes act as biological catalysts in living systems.
- •Catalysts do not increase collision frequency, but increase the proportion of collisions that are successful.
⚠️ Common mistake: Thinking catalysts get used up in the reaction, or assuming they increase the frequency of particle collisions.
🧠 Remember: Catalysts lower activation energy to provide a faster pathway.
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