Rates of reaction & energy changes Flashcards
Collision theory
When reactants come together the kinetic energy they possess means their particles will collide and some of these collisions will result in chemical bonds being broken and some new bonds being formed.
Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules.
We can use collision theory to explain why these factors influence the reaction rate.
Not all collisions result in a chemical reaction.
Most collisions just result in the colliding particles bouncing off each other
Collisions which do not result in a reaction are known as unsuccessful collisions.
The minimum amount of energy needed is called the activation energy.
Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds.
If they do not have sufficient energy, the collision will not result in a chemical reaction.
If they have sufficient energy, they will react, and the collision will be successful.
Concentration
Increasing the concentration of a solution will increase the rate of reaction.
This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions per second.
If you double the number of particles you will double the number of collisions per second.
The number of collisions is proportional to the number of particles present.
Temperature
Increase in the temperature, the rate of reaction will increase.
This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction.
The effect of temperature on collisions is not so straight forward as concentration or surface area; a small increase in temperature causes a large increase in rate.
For aqueous and gaseous systems, a rough rule of thumb is that for every 10 degree (Kelvin) increase in temperature the rate of reaction approximately doubles.
Surface area
With an increase in the surface area of a solid reactant, the rate of reaction will increase.
This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second.
If you double the surface area you will double the number of collisions per second.
Pressure
Increasing the pressure of a gas increases the rate of reaction.
Increasing the pressure means that there are the same number of reactant particles in a smaller volume.
This causes more collisions per second.
Leading to more frequent and successful collisions per second.
Therefore, the rate of reaction increases.
Catalysts
Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction.
The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation.
Normally only small amounts of catalysts are needed to have an effect on a reaction.
Different processes require different types of catalysts but they all work on the same principle of providing an alternate route for the reaction to occur.
They do this by lowering the activation energy required, hence providing a reaction pathway requiring less energy.
Catalysis is a very important branch of chemistry in commercial terms as catalysts increase the rate of reaction (hence the production rate) and they reduce energy costs.
Enzymes
Enzymes are biological catalysts made from protein.
Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature).
Important reactions that are biologically catalysed include respiration, photosynthesis and protein synthesis.
The production of alcohol by the fermentation of sugars occurs in the presence of a biological catalyst, yeast enzymes:
C6H12O6 + enzymes → 2CO2 + 2C2H5OH
This reaction is very important to the production of alcoholic drinks such as beer and wine.
Not only do enzymes work at low temperatures, but they are very selective and will only work on very specific molecules when presented with a mixture of reactants.
Heat energy and temperature changes
Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons.
This is done by chemical bonding where old bonds are broken, and new bonds are formed.
This process involves the transfer of energy into and out of reaction mixtures.
The terms used to describe this are the system (what happens in the chemical reaction) and the surroundings.
The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy.
In the majority of reactions, the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound.
The changes in heat can be observed and measured with a thermometer and simple calorimeter.
Examples of heat changes in reaction
Salts dissolving in water:
These can either take energy in or give it out
Neutralisation reactions:
These always give energy out
Displacement reactions:
These can either take energy in or give it out
Precipitation reactions:
These always give energy out
Exothermic reactions
In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases.
Combustion, oxidation, and neutralisation reactions are typical exothermic reactions.
Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction.
Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers.
Endothermic reactions
In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases.
These types of reactions are much less common than the exothermic reactions.
Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions.
Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling.
Breaking and forming bonds
Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process.
The opposite occurs for forming bonds as it releases energy back to the surroundings in an exothermic process.
Endothermic bonds
If more energy is absorbed than is released, this reaction is endothermic.
More energy is required to break the bonds than that gained from making the new bonds.
The change in energy is positive since the products have more energy than the reactants.
Therefore an endothermic reaction has a positive change in energy.
Exothermic bonds
If more energy is released than is absorbed, then the reaction is exothermic.
More energy is released when new bonds are formed than energy required to break the bonds in the reactants.
The change in energy is negative since the reactants have more energy than the products.
Therefore an exothermic reaction has a negative change in energy.
Bond energy calculations
Each chemical bond has a specific bond energy associated with it.
This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.
This energy can be used to calculate how much heat would be released or absorbed in a reaction.
To do this it is necessary to know the bonds present in both the reactants and products.
We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved.
Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
Energy change = Energy taken in - Energy given out
