Unit 2: Chapter 4 - Rates and Energy Flashcards
How fast?
A rate of a chemical reaction tells us how fast reactants turn into products.
You can measure the rate of a chemical reaction by:
- Measuring the decreasing mass of a reaction mixture
- Measuring the increasing volume of gas given off
- Measuring the decreasing light passing through a solution
Rate of Reaction =
Amount of reactant used or amount of product formed/ Time
Collision Theory and Surface Area
There are 4 main factors which affect the rate of chemical reactions:
- Temperature
- Surface Area
- Concentration of solutions or pressure of gas
- Presence of a catalyst
A reaction needs enough energy to to react when they collide into each other. This is the ACTIVATION ENERGY.
SURFACE AREA
In smaller lumps of the reactant or powder, it increases the likelihood of a reaction to take place, rather than a large lump of solid.
The Effect of Temperature
When we increase the temperature of a reaction:
- PARTICLES COLLIDE MORE OFTEN. More energy is transferred to the particles and so the particles move faster and so speeds up the rate of reaction.
- PARTICLES COLLIDE WITH MORE ENERGY. The particles move quicker because they have taken in more energy. When these particles collide with each other, they are more likely to react because a higher proportion of particles have a higher energy than the activation energy.
Sodium Thiosulfate + HCl -> Sulfur (Cloudy)
This makes the cross disappear.
The Effect of Concentration or Pressure
More particles in a given space increases the rate of a reaction. This is because the particles are more likely to collide and speeds up the rate if reaction.
The Effect of Catalysts
A catalyst is never used in a reaction.
A catalyst lowers the activation energy by giving alternate routes for reaction.
Iron is used as a catalyst to make ammonia.
Platinum is used a catalyst to make nitric acid.
Catalysts in action
Catalysts can be expensive (platinum and gold) and are often toxic. These catalysts often escape, e.g. platinum and palladium through car exhausts.
Nano particles are in development for new catalysts. These catalysts produce a very large surface area.
Cheaper catalysts are now replacing transition metals in drugs which are toxic.
Biological enzymes are being connect to solids to be effective catalysts and can be used over and over.
Exothermic and Endothermic Reactions
When energy is transferred from reactions to the surrounding, this is called EXOTHERMIC REACTIONS.
When a reaction draws in energy from the surroundings, this reaction is called an ENDOTHERMIC REACTION.
EXOTHERMIC
For example, respiration is a type of oxidation which is an exothermic reaction.
Neutralization reactions between acids and alkalis are also exothermic.
ENDOTHERMIC
Endothermic reactions are generally less common than exothermic reactions.
Thermal decomposition reactions are endothermic. E.g. decomposition of Calcium Carbonate, which produces CO2 and CaO.
Energy and Reversible Reactions
Example of reversible reaction showing energy changes:
Copper sulfate is hydrated. When it is heated, it drives away water to produce ANHYDROUS copper sulfate.This is an ENDOTHERMIC REACTION.
When water is added to anhydrous copper sulfate, we form hydrated copper sulfate. The reaction is EXOTHERMIC. So much energy is produced that we may see steam rise as water boils.
THE AMOUNT OF ENERGY RELEASED IS EXACTLY EQUAL TO THE ENERGY ABSORBED WHEN THE REACTION GOES IN THE OPPOSITE DIRECTION.
Using Energy Transfers from Reactions
Chemical hand and body warmers use exothermic reactions to warm you up.
Supersaturated solution is reusable in warmers, but one-off warmers give off heat for longer.
COOLING DOWN
Endothermic processes can be used to cool things down. When ammonium nitrate dissolves, it takes in energy from its surrounding, making them colder.
The ammonium nitrate and water are kept separate in the bag.
