- We can find out the rate of a chemical reaction by following the amount of reactants used up over time.
- We can find how quickly the reactants are used up.
- We can find out how quickly the products are made.
- The slope of the line at any given time on the graphs drawn from such experiments tells us the rate of reaction at that time. The steeper the slope, the faster the reaction.
- Measuring the amount of reactants which are used up over time or the amount of products made over time are two ways of finding out the rate of the reaction. The slope of the lines on graphs drawn from these experiments tells us about the rate at any given time.
Collision Theory and Surface Area.
There are 4 main factors which affect the rate of chemical reactions.
- Surface area
- Concentration of solutions or pressure of gases
- Prescene of a catalyst
- Reactions need enough energy to react when they collide- The collision Theory!
- The smallest amout of energy needed before a reaction can take place is activation energy.
- The particles inside a large lump of solid are not in contact with the solution so they cannot react. In smaller lumps- reactions can take place more quickly as more particles are exposed to attack.
Effect of temperature
Collision theory tells us that if you increase the temperature:
Particles collide more often:
When we heat up a substance, energy is transferred to its particles. This means the particles move around faster, meaning more sucessful collisions.
Particles collide with more energy
Particles that move around more quickly have more energy. Their collisions will be very energetic. When we increase the temperature there will be more successful collisions- this is because more particles have reached activation energy.
If we increase the temperature by 10 degrees the rate of the reaction will double.
The effect of concentration
Increasing the concentration of reactants in a solution increases the amount of reactants in a solution. More particles of reactants are moving around in the same volume of solution. The more 'crowded' the solution is the more likely it is that the particles will collide. More successful collisions.
Increasing the pressure has the same effect. It squashes the gas particles closer together. We have more particles of gas in a given space, therfore more successful collisions.
The effect of catalysts
Sometimes a reaction may only work at high temperatures- expensive. We can use a catalyst.
Increases the rate of reaction, but not chemically changed.
Not used up in the reaction.
We need different catalysts for different reactions. Many catalysts involve transition metals.
We normally use catalysts in the form of powders- speeds up reaction, biggest possible surface area.
Catalysts in action
Catalysts are often expensive metals. Cheaper to use a catalyst rather than to pay for the energy needed without one. Without a catalyst would mean higher temperatures and pressures.
Many catalysts used are transition metals and are toxic. If they escape into the environment they can harm living organisms, they can poison them. Chemists are working to create harmless catalysts.
Nanoparticles- arrange atoms-best shape-catalysing- small mass= huge surface area. Fuel cells will run cars.
Catalysts used in drugs. Metal is bonded to an organic molecule. Now catalysts can make this without the metal- needed to make stable compound. Research resulted in breakthrough-cheaper catalysts. No risk of catalysing drug with a toxic transition metal.
Enzymes are very effecient catalysts. Help clean our clothes. Biological enzymes break apart stain molecules such as proteins at lower temperatures. Low temperature saves energy. Enzymes are soluble- scientists bind to a solid- no time or money wasted- can be used again.
Exothermic and Endothermic reactions
In any chemical reaction, energy is involved. Chemical bonds broken and new ones made. Energy from reacting chemicals to surrounding area- exothermic. Energy from surrounding area to reacting chemicals=endothermic.
Exothermic examples- fuels burning, respiration, neutrilisation between acids and alkilis.
Endothermic examples (less common)- Thermal decomposition e.g, calcium carbonate.
Start with reactants end with products. Reversible reactants can go from the product to the original reactant. E.g Ammonuim chloride can break down into ammonia and hydrogen chloride. Ammnoia and hydrogen chloride can react to form ammonium chloride again.
Reversible reaction sign shows two reactions going on at the same time. One forwards, one backwards. Speed will eventually balance out and so a reversible reaction will reach a steady state where both reactions are happening. Overall amounts of each chemical are not changing- Dynamic Equilibrium.
Using energy transfers from reactions
Chemical hand warmers and body warmers can be very useful. Use exothermic reactions to warm you up. Some hand warmers can only be used once- oxidation of iron to release energy. Similar to rusting. Others are based on the formation of crystals from solutions of salt. A small metal disk used to start reaction- press this a few times small particles of metal are scraped off. They start off crystalisation. Also used in self heating cans.
Endothermic reactions can be used to cool things down. Chemical cold packs usually contain ammonium nitrate and water. When ammonium nitrate dissolves takes in energy from its surroundings. Only used once but are ideal if there is no ice avaliable- treat knock or strain.