the rate and extent of chemical change Flashcards
how do you calculate rate of reaction
amount of reactant used or product formed / time
what are the various units for rate of reaction?
g/s or cm^3/s or mol/s
name 3 common ways of measuring rate of reaction
- loss in mass of reactants
- volume of gas produced
- time for a solution to become opaque
describe measuring the rate by monitoring mass loss
place the reaction flask on balance. In these reactions a gas is given off, so record the decrease in mass in time intervals. Plot a graph of mass v time
describe measuring the rate by monitoring the volume of a gas
connect a gas syringe to a reaction flask and measure the volume of a gas formed in time intervals. Plot a graph of volume v time
describe measuring the rate by monitoring the disappearance of a cross
take a piece of paper and mark a cross (X) on it. Put the reaction flask on this cross. Mix the reagents, and measure how long it takes for a cloudy mixture to conceal a cross.
how to find a rate of reaction at some time, t, from a graph of amount of reactant v time?
- pick a point corresponding to the time t, and find the tangent to the curve at this point
- the tangent is the gradient of the graph - it tells you how fast the reaction proceeds at this point. The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values over change in x values
state 5 factors affecting the rate of a chemical reaction
- concentration of reactants
- pressure of gasses (volume)
- surface area
- temperature
- catalysts
what is the collision theory?
chemical reactions can occur only when reacting particles collide with each other with sufficient energy
describe and explain the effect of increasing temperature of the rate of reaction
T increases = faster reaction
As T increases, kinetic energy of particles increases, ie, more energetic collisions they also move faster so they collide more frequently. However, there is no straight line relationship between the rate and temperature, ie. they are not directly proportional to each other.
describe and explain the effect of increasing concentration on the rate of reaction
concentration increases = faster reaction
more reactants = more frequent collisions
describe and explain the effect of increasing pressure of a gas on the rate of reaction.
increasing the pressure of reacting gasses, is the same as increasing concentration. It increases the number of gas molecules in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.
describe and explain the effect of increasing surface area
if solid reactants are in smaller pieces, they have a greater surface area. Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction eg. block of magnesium reacts slower with acid then magnesium powder.
what is a catalyst and how does it work? How does it affect the reaction profile
A catalyst changes the rate of reaction but is not used up. It increases rate of reaction of providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower maximum of the curve.
what is an enzyme?
an enzyme is a molecule that acts as a catalyst in a biological system
what is a reversible reaction?
a reversible reaction occurs when the products of a reaction can react backwards to produce the original reactants
when is dynamic equilibrium reached?
in a closed system, when the forward and reverse reactions occur at the same rate and the concentrations of reactants and products remain constant.
describe Le Chatelier’s principle
if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract change and restore the equilibrium.
describe the effect of changing the concentration of reactant and product on the position of the equilibrium.
if the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and the concentration of all the substances will change until equilibrium is reached again. If the concentration of a reactant is increased, more products will be formed until equilibrium is reached again. If the concentration of a product is decreased, more reactants will react until equilibrium is reached again.
describe the effect of changing temperature on the position of the equilibrium
if the temperature of a system at equilibrium is increased:
- the relative amount of products at equilibrium increases for an endothermic reaction
- the relative amount of products at equilibrium decreases for an exothermic reaction
describe the effect of a catalyst in the position of the equilibrium
no effect
it just speeds up both forward and backwards reactions equally so equilibrium in achieved faster.
