Rates of Reaction and Energy Changes Flashcards
Core Practical: Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions:
In these experiments, you are investigating the effect on rate of changing the size of the marble chips (smaller chips=larger surface area= faster rate) and also the effect of changing the concentration of hydrochloric acid (greater concentration= greater number of particles in a given volume= faster rate).
Practical methods for determining the rate of a given reaction
Rates of reactions can be measured using the amount of product used, or amount of product formed over time.
Quantity of reactant or product can be measured by the mass in grams or by a volume in cm3.
To measure reactant used: if the product is a gas, which will be given off, you can carry out the reaction on a set of weighing scales and measure how much mass is lost.
To measure product formed: if the product is a gas, you can measure the volume of gas produced in a gas syringe.
How reactions occur when particles collide
Chemical reactions only occur when the reacting particles collide with enough energy, the minimum amount of energy required is called the activation energy.
In order to increase the rate of a reaction, you need to increase the frequency/energy of collisions, so that more of them reach the activation energy.
This can be done by: increasing temperature, pressure, concentration, surface area or by using a catalyst.
The effects on rates of reaction of changes in temperature
Increasing the temperature increases the rate of reaction. As increasing temperature increases the kinetic energy of particles, so they collide more frequently and energetically.
The effects on rates of reaction of changes in concentration
Increasing concentration of reacting solutions increases the rate of reaction, as it increases the number of particles in a given volume and so increases the frequency of collisions.
The effects on rates of reaction of changes in pressure
Increasing pressure in reacting gases increases the rate of reaction, as it increases the number of particles in a given volume so increases the frequency of collisions.
The effects on rates of reaction of changes in surface area of solid reactants
Increasing the surface area of solid reactants increases the rate of reaction, as it increases the frequency of collisions so increases the rate of reaction.
What are catalysts?
Catalysts are substances that speed up chemical reactions without being changed or used up during the reaction. They are the same and have the same mass at the end of the reaction.
Explain how the addition of catalyst increase the rate of a reaction in terms of activation energy
Catalysts decrease the activation energy; this increases the proportion of particles with energy to react, leading to more particles with energy to react, leading to more frequent successful collisions and so an increased rate of reaction.
Catalysts lower the activation energy by providing a different pathway for a chemical reaction that has a lower activation energy
Recall that changes in heat energy accompany the following
changes: salts dissolving in water
Salts dissolving in water is either exothermic or endothermic.
Recall that changes in heat energy accompany the following
changes: neutralisation reactions
Neutralisation reaction is exothermic.
Recall that changes in heat energy accompany the following
changes: displacement reactions
Displacement is an exothermic or endothermic reaction.
Recall that changes in heat energy accompany the following
changes: precipitation reactions
Precipitation is an exothermic reaction.
What happens when these reactions take place in a solution?
When these reactions take place in solution,
temperature changes can be measured to reflect the heat changes.
Describe an exothermic change
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
Examples of exothermic reactions include; combustion, many oxidation reactions and neutralisation.
Everyday examples of exothermic reactions include; self-heating cans (e.g for coffee) and hand warmers.
Describe an endothermic change or reaction as one in which heat energy is taken in
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogen carbonate.
Describe the making and breaking of bonds
During a chemical reaction:
Energy must be SUPPLIED to BREAK bonds in the reactants.
Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in).
Energy is RELEASED when bonds in the products are FORMED.
Energy taken in to break < energy released when formed = EXOTHERMIC (because overall energy has been released).
Calculate the energy change in a reaction given the energies of bonds (in kJ mol-1)
Sum of energy taken in to break bonds – sum of energy released to form bonds = overall energy change
If the energy out > energy in, the energy change will be negative showing an exothermic reaction and if the energy out < energy in, the energy change will be positive showing an endothermic reaction.
Explain the term Activation energy
Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
Activation energy = minimum amount of energy that particles must have to react.
Draw and label reaction profiles for endothermic and exothermic reactions, identifying activation energy
For the exothermic diagram, the products have less energy than the reactants, because the energy has been released to the surroundings.
For the endothermic diagram, the reactants have less energy than the products, because the energy has been taken in from the surroundings.
