The rate and extent of chemical change

c6 paper 2 chemistry

(5.6.1)how is rate of reaction calculated?

1. mean rate of reaction g/s or cm3/s= quantity of reactants used cm3 / time taken (s)

2. mean rate of reaction g/s or cm3/s= quantity of product formed cm3/ time taken (s)

(5.6.1)what are the various units for rate of reaction?

g/s or cm3/s or mol/s

(5.6.1)Name 3 common ways of measuring rate of reaction?

loss in mass or reactants

volume of gas produced

time for a solution to become opaque

(5.6.1) describe measuring the rate by monitoring mass loss

place the reaction flask on a balance

in the reactions a gas is given off so record the decrease in mass in time intervals

plot a graph of mass vs time

(5.6.1)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 vs time

(5.6.1) describe measuring the rate by monitoring the disappearance of a cross

take a piece of paper and mark a cross on it

put the reaction flask on this cross

mix the reagants

measure how long it takes for a cloudy mixture to conceal a cross

(5.6.1) how to find a rate of reaction at some time from a graph of amount of reactant vs time?

pick a point corresponding to the time

find the tangent to the curve at this point

the tangent is the gradient of this graph which 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

(5.6.1)state 5 factors affecting the rate of a chemical reaction

1. concentration of reactants

2. pressure of gases (volume)

3. surface area

4. temperature

5. catalysts

(5.6.1)what is the collision theory?

chemical reactions can occur only when reacting particles collide with each other with sufficient energy (more than or equal to the activation energy)

(5.6.1) describe and explain the effect of increasing temperature on the rate of reaction?

if the temperature increases, there is a faster reaction

As temperature increases, the kinetic energy of particles increases. Therefore more energetic collisions occur; particles move faster so collide more frequently

However there is no straight line relationship between rate and temperature. This means they are not directly proportional to each other.

(5.6.1) describe and explain the effect of increasing concentration on the rate of reaction?

if the concentration increases, there is a faster reaction

The more reactants, the more frequent collisions

(5.6.1) describe and explain the effect of increasing pressure of a gas on the rate of reaction?

increasing the pressure of reacting gases 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

(5.6.1) 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 which increases the rate of reaction. eg: a block of magnesium reacts slower with acid than magnesium powder

(5.6.1)What is a catalyst? how does it work and affect the reaction profile?

A catalyst changes the rate of reaction but is not used up

it increases rate of reaction by giving a different pathway for the reaction with a lower activation energy

the reaction profile for a catalysed reaction will have a lower maximum of the curve so a lower activation energy

(5.6.1) what is an enzyme?

a molecule that acts as a catalyst in a biological system

(5.6.2)What is a reversible reaction?

occurs when the products of a reaction can react backwards to make the original reactants

(5.6.2)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

(5.6.2) 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

(5.6.2) describe the effect of changing the concentration of reactant and product on the position of the equilibrium

if the concentration of 1 of the reactants or products is changed, the system is no longer at equilibrium and the concentrations 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.

(5.6.2)Describe the effect of changing temperature on the position of the equilibrium?

the relative amount of products at equilibrium increases for an endothermic reaction

the relative amount of products at equilibrium decreases for an exothermic reaction

(5.6.2)Describe the effect of changing pressure on the position of the equilibrium

IN GASES

an increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules as shown by the symbol equation for that reaction.

a decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.

pressure has no effect on the reactions where the numbers of gas molecules are equal on both sides of the equation.

(5.6.2) describe the effect of a catalyst on the position of the equilibrium

it has no effect

it just speeds up both forward and backward reactions equally. This means equilibrium is achieved faster.