Flashcards in Kinetics Deck (27)
What is kinetics?
The experimental study of reaction rates and factors affecting rates.
What are some of the applications of kinetics?
Cooking food and food preservation, farming and growth of crops, construction, archaeological dating, fuels, the chemical industry and reaction mechanism studies
What is the definition of rate of reaction?
The rate of a chemical reaction is the change in concentration of reactant or product per unit time (units are usually moldm^-3 s^-1)
What is the explanation of the shape of the graph of concentration of reactant against time?
Gradient greatest at the start as this is when the most reactant is available and concentration of reactants are known, then gradient decreases as the concentration of reactants decreases until the gradient becomes 0 when all the reactants have been used up
How can the instantaneous rate of reaction be found from a graph of reactant concentration against time or product concentration against time?
Draw tangents to the curve at the point where the rate is being found and calculate the gradient
What are the requirements of collision theory?
- The first requirement is for molecule A to collide with molecule B.
- The second requirement is that the molecules need to collide with sufficient energy to cause a reaction. Collisions that cause the reaction to occur have to be high energy in order to break covalent bonds and start the reaction
What is the general way to increase rate of reaction?
Increase the number of collisions per second - the collision frequency - of molecules with sufficient energy to react
What is a homogeneous reaction?
A reaction that takes place in a single state, e.g. all species present are gases, or all species are in the same solution
How does changing the pressure affect the rate of reaction and why?
An increase in pressure at constant temperature increases the reaction rate of gas reactions, as there are more gas particles per unit volume. Kinetic energy of the particles does not change, so proportion of collisions does not change, but there are more collisions per second, so rate increases.
There is no affect on rate of reaction between solids, liquids or solutions
How does changing the concentration affect the rate of reaction and why?
For reactions in solution, an increase in concentration means that the frequency of collisions between solute molecules is increased as an increased concentration means more solute particles in a given volume.
How does particle size affect rate of reaction?
In heterogeneous systems involving a solid, a larger surface area of the solid gives a faster rate of reaction.
Powder < granules < lumps/chips
How does temperature affect rate of reaction?
Increased temperature always increases rate of reaction because the molecules have higher kinetic energy so collide more frequently but it also means that a greater fraction of the molecules have the energy necessary to react on collision/energy greater than the activation energy which is the main reason that rate increases.
What does the Maxwell-Boltzmann distribution curve look like?
Asymptotic, bell shaped curve which starts at the origin but does not touch the x axis at the right hand side. The activation energy can be marked on a point on the curve with a vertical line and the area under the graph at the right of this point gives the fraction of molecules with energy greater than the activation energy.
Plotted with "fraction of molecules with a given energy" on y axis, and "energy" on x axis
How does increasing temperature change the Maxwell-Boltzmann distribution curve?
It moves the peak to the right hand side and lower. Area under the graph after the activation energy will increase
What is activation energy?
Activation energy is defined as the minimum combined energy that molecules must have on collision for a reaction to occur
What is a catalyst?
A catalyst is substance that alters the rate of a chemical reaction but remains permanently chemically unchanged at the end of the reaction
How does a catalyst work?
A catalyst speeds up the rate of reaction by providing an alternative route of lower activation energy.
How does the addition of a catalyst change the Maxwell-Boltzmann distribution curve?
The activation energy moves to the left on the x axis, meaning there is a greater area under the curve, indicating a greater fraction of molecules with energy > activation energy
How does an energy reaction profile diagram illustrate the difference between a catalysed and uncatalysed reaction?
The line on an uncatalysed reaction goes straight from reacts to products with a high activation energy, but there is a lower line just above the reactants line with a catalysed reaction which allows for a lower activation energy, and the line goes from reactants to that line to products, showing that, although the route is longer, the activation energy is lower
What are the economic advantages of catalysts?
- Increase the rate of a chemical reaction, allowing more of a desired product to be made in a given time
- Allow reactions to be performed effectively at lower temperatures, saving energy and money
What is a phase?
A phase is one of the 3 states of matter, solids, liquids and gases. Chemical systems often have more than 1 phase. Each phase is distinct but doesn't need to be pure; a solid in equilibrium with its saturated solution is a 2 phase system.
What is a heterogeneous catalyst?
One that is in a different phase from the reactants. Generally it is a solid, while reactants are gases or in solution
What is the benefit of a heterogeneous catalyst?
It can be easily removed from the product when a homogeneous catalyst cannot as easily
How can a rate of reaction be estimated experimentally?
- Measuring the time for a certain amount of the mixture to react
- Measuring the time for a certain amount of product to be formed
The assumption is then made that 1/time is a measure of the rate - although this is not strictly true as the rate will decrease from the moment reactants are mixed
How can production of a gas be used to measure rate of reaction?
The time taken to produce a certain volume of gas (collected over water or in a gas syringe) can be measured.
A reduction in mass can also be measured (using a conical flask with cotton wool to stop solution spraying out while allowing gas to be released on a mass balance)
What is the issue with measuring loss in mass to calculate rate of reaction if the gas given off is hydrogen?
Hydrogen gas has a very low Mr and so any mass decrease will be very small and difficult to measure