2.2- Rates of Reaction Flashcards
(a)
Factors affecting rates of reaction
- concentration of a solution (pressure of a gas)
- surface area of a solid
- temperature
- catalyst
- light (in some reactions)
(a)
Collision theory
Collision theory explains reaction rates by considering how often and effectively particles collide. For a reaction to occur, molecules must collide with the correct orientation and enough energy to overcome the activation energy. The rate of reaction depends on the frequency of these effective collisions—any factor that increases them will speed up the reaction.
(b)
how to calculate rates from experimental data and how to establish the
relationship between reactant concentrations and rate
The rate of reaction is calculated by measuring the amount of reactant used up or product formed over time. A graph is plotted, and the gradient of the line gives the rate. If the graph is a curve, a tangent must be drawn to determine the initial rate.
(c)
Exothermic energy profile
In an exothermic reaction, the products have less energy than the reactants, and the excess energy is released as heat, making ΔH negative. However, energy (activation energy, Ea) is still needed to break bonds and start the reaction.
(c)
Endothermic energy profile
In an endothermic reaction, the products have more energy than the reactants, so heat is absorbed from the surroundings, making ΔH positive.
(c)
Activation energy
The minimum amount of energy required for a reaction to take place between two particles.
(d)
Rapid increase in rate with temperature in terms of changes in the Boltzmann energy distribution curve
Increasing temperature increases reaction rate as molecules gain kinetic energy, move faster, and collide more frequently. More molecules exceed the activation energy, leading to more successful collisions per second. On a Boltzmann distribution curve, a higher temperature (T₂) shifts the curve right and lowers its peak. When drawing, place the activation energy (Ea) line far to the right and ensure the curve never touches the x-axis.
(e)
Characteristics of a catalyst
A catalyst increases the reaction rate without being used up. It takes part in the reaction but is recovered unchanged at the end.
(e)
Homogeneous catalyst
A homogeneous catalyst is in the same phase as the reactants, often in liquid or solution form. An example is concentrated sulfuric acid catalysing ester formation from an alcohol and a carboxylic acid.
(e)
Heterogenous catalyst
A heterogeneous catalyst is in a different phase from the reactants, often a solid with gaseous or liquid reactants. An example is iron, which acts as a catalyst in the Haber process for ammonia production.
(f)
Catalysts increasing reaction rates
Catalysts increase the reaction rate by providing an alternative pathway with a lower activation energy, allowing more successful collisions.
(g)
how colorimetry can be used in studies of some reaction rates
Colorimetry can be used to study reaction rates by measuring the change in colour intensity of a solution over time.
(h)
measuring the rate of reaction when a gas is given off
Collect the gas produced in an upside down measuring cylinder in a trough of water or in a gas syringe to measure the volume of gas produced. Measure the amount of gas collected over regular time intervals.
(h)
measuring the rate of reaction when a precipitate is formed
Put a black cross below a beaker containing one reactant. Time how long it takes for the cross to disappear after the second reactant is added
(h)
Why is the precipitation method not a very accurate way to investigate rate of a reaction?
It’s subjective so people are likely to disagree over the exact point at which the cross is no longer visible.
(h)
How can you measure rate of reaction using a mass balance?
A mass balance can be used when a gas is produced, as this will cause the mass to decrease as the reaction proceeds. The experiment can be carried out on a mass balance and the rate of reaction can be calculated by recording the mass at regular time intervals.
(h)
A graph is plotted to show time and the amount of gas given off during a reaction. Describe the shape of the curve
At the start, the curve is steep as the reaction is fast due to many successful collisions. It gets less steep as reactants are used up. The graph flattens when the reaction stops and all reactants are used.
(h)
How do the concentrations of reactants and products change during a reaction?
Product concentration increases rapidly at first, then slows down and stays constant when the reaction is complete.
Reactant concentration decreases quickly at the start, then slows and levels off when the reactants are used up.
