Chapter 10 Flashcards
What is the rate of a chemical reaction?
It measures how fast a reactant is being used up or how fast a product is formed. It can be defined as the change in concentration of a reactant or product in a given time.
How do you calculate the rate of reaction?
concentration change/ time the units are moldm3s
When is the rate of reaction, fastest at?
It is fastest at the beginning of the reaction as each reactant is at its highest concentration. On a graph this is the steepest part,
When does the rate of reaction slowdown?
It slows down as the reaction continues because the reactants are being used and their concentration is decreasing. On the graph this is getting less steep as the reaction progresses.
Where does the reaction plateau?
It plateaus once one of the reactants is completely used up as the concentration stop changing and the rate is zero. On a graph the curve eventually becomes a straight parallel line to the X axis as the reaction is complete.
What are the four ways you can alter a chemical reaction rate ?
Concentration (or pressure when reactants are gases), temperature, use of a catalyst, surface area of solid.
What is the collision theory?
It states that reacting particles must collide for a reaction to occur. Usually only a small proportion of collisions result in a chemical reaction, most result in molecules being chemically unchanged as they bounce off one another.
What two conditions need to be met in order for a collisions to be effective?
That the particles collide in the correct orientation. The particles containing a greater amount of energy than the activation energy so can overcome the energy barrier of the reaction.
How does changing the concentration effect the rate?
When the reactant concentration increases the rate of reaction increases. An increase in concentration increases the number of particles in the same volume so the particles get closer and collide more frequently in a given time. So there will be more effective collisions (correct orientation and sufficient energy). Compare to a lower concentration having a slower rate of reaction
How does changing the pressure affect the rate?
When the gas is compressed into a smaller volume the pressure increases and so does the rate. The concentration of molecules increases as the same number of molecules occupy the same volume. Gas molecules are closer together and collide more frequently leading to more effective collisions.
How is the progress of a chemical reaction followed?
By monitoring the mood removal of a reactant (decrease in concentration) and following the formation of a product (increase in concentration)
Two methods can be used when determining the rate of reaction with a reaction that produces a gas?
Monitoring the volume of gas produced at regular time intervals using gas collection. Monitoring the loss of mass of reactants using a balance. These are boys proportional to the change in concentration of a reactant or product so will give a measure of the rate of reaction.
Describe and experiment monitoring production of gas using gas collection?
- The reactant e.g hydrogen peroxide is added to the conical flask and a bung is replaced.
- The initial gas volume is measured and recorded from the measuring cylinder.
- A catalyst e.g Manganese dioxide is quickly added to the flask and the bung is replaced. A stopclock is started.
- The volume of gas produced is recorded at regular intervals until the reaction is complete
- When it is complete no more gas is produced
- A graph is plotted with total volume of gas produced against time. You can find the initial rate by drawing a tangent at t=0.
Describe the experiment and monitoring the loss of mass of reactants using a balance?
The mass of the flask and contents is initially recorded, as well as at regular intervals, once no more gas is produced and the loss in mass stops, the reaction is complete. A graph can be plotted.
E.g calcium carbonate and HCl
What is a catalyst?
It is a substance that changes the rate of a chemical reaction without undergoing any permanent change itself. The catalyst is not used up in the reaction. The catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place. At the end of the reaction the catalyst is regenerated.
What do catalysts do?
They increase the rate of reaction by providing an alternative pathway of lower activation energy.
What are the two types of catalyst?
Homogeneous catalyst and heterogeneous catalyst
What is a homogeneous catalyst?
The catalyst has the same physical state as the reactants. The catalyst reacts with the reactants to form an intermediate. The intermediate then breaks down to give the products and regenerate the catalyst.
What are the two examples of homogeneous catalysts?
Making esters with sulphuric acid as a catalyst- the reactants and the catalyst are all liquids (ethanol and ethanoic acid are the reactants and the sulphuric acid is the catalyst) Ozone depletion (Cl radicals as a catalyst) the reactants ozone and the catalyst Cl radicals are both gases
What are heterogeneous catalysts?
A heterogeneous catalyst has a different physical state from the reactants. These are usually solids in contact with gaseous reactants or reactants in a solution. Reactant molecules are adsorbed (weakly bonded) onto the surface of the catalyst where the reaction takes place. After the reaction, the molecules leave the surface of the catalyst by desorption.
What is the percentage of all chemical material is produced by catalyst?
90%
Why a catalyst used?
They are used in many industrial chemical reactions as they increase the rate by providing an alternative reaction pathway with a lower activation energy which reduces the temperature required for the process and any energy requirements. If a chemical process requires less energy then less electricity or fossil fuels are used. Which makes the production faster with using less energy cutting down costs and increasing profitability.
What are the advantages of catalysts?
It has economic advantages as they increase profitability which outweigh any costs associated with developing a catalytic process. The modern focus on sustainability requires industry to operate processes at lower energy so if you are pollutants and a higher atom economy is. As the combustion of fossil feels would be decreased that results in a reduction of CO2 omissions and other atmospheric pollutants which are linked to global warming.
What are disadvantages of catalysts?
Some catalyst which are used in industry are toxic.
How have heterogeneous catalysts been used to reduce atmospheric pollution?
A catalytic converter contains a catalyst within a honeycomb mesh which has a large surface area in which reactions occurring. The exhaust gases are passed over the catalyst and any harmful gases are convert into less harmful ones. The combustion in a petrol engine forms, monoxide and nitrogen monoxide where the converter converts carbon monoxide into carbon dioxide (oxidised) and nitrogen monoxide into nitrogen gas (reduced) both products are non-toxic and can be released into the atmosphere . Hydrocarbons are oxidised into water and CO2
What is the rate of molecular energies in gases known as?
Boltzmann distribution.
