Chemical Change & Rate of Reaction (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards

A catalyst is a substance that increases the rate of a reaction without itself being altered or consumed in the reaction.

The mass of catalyst is the same at the start and end of the reaction.

True.

A higher concentration means more particles in a given volume, leading to more frequent collisions and a faster rate of reaction.

Increasing the surface area of a solid reactant increases the rate of reaction, because more surface is exposed to the other reactant.

The graph at higher concentration has a steeper gradient at the start and becomes horizontal sooner. Both experiments produce the same total amount of product.

False.

A catalyst speeds up a reaction but is not consumed. Its mass is the same before and after the reaction and it does not appear in the equation.

A higher rate of reaction means a higher rate of production, which makes the process more efficient and more economically sustainable.

Increasing the temperature of a reaction increases the kinetic energy of the particles, leading to a higher rate of reaction.

Decreasing particle size increases the surface area exposed to the other reactant. This means more frequent and successful collisions occur, increasing the rate of reaction.

The activation energy (E_a) is the minimum energy that colliding particles must have in order to react.

Collisions with energy below the activation energy are unsuccessful.

According to collision theory, two conditions must be met:

1. The particles must collide with each other.

2. The collision must have sufficient energy (at least equal to the activation energy) to break bonds.

A successful collision is one in which reactant particles collide with sufficient energy (greater than the activation energy) and rearrange to form products.

True.

In an unsuccessful collision the particles do not have sufficient energy to break the necessary bonds, so no rearrangement of atoms occurs and the particles remain unchanged.

A greater number of particles per unit volume causes more frequent collisions per second, increasing the rate of reaction.

Increasing kinetic energy means a greater proportion of collisions exceed the activation energy, so more collisions are successful. Particles also move faster, increasing the frequency of collisions.

A higher activation energy means fewer collisions have enough energy to exceed it, so fewer collisions are successful and the rate of reaction decreases.

True.

The rate of reaction is determined by how many successful collisions occur each second. Anything that increases the frequency or proportion of successful collisions increases the rate.

Collision theory states that for a reaction to occur, particles must collide and the collision must have sufficient energy to break bonds.

A higher concentration means more reactant particles per unit volume. This causes more collisions per second, leading to more frequent and successful collisions and an increased rate of reaction.

Increasing the surface area of a solid reactant exposes more surface area of particles to the other reactant, increasing the number of collisions per second.

A catalyst provides an alternative reaction pathway with a lower activation energy. A higher proportion of reactant particles then have sufficient energy to react, so more successful collisions occur per second.

An enzyme is a biological catalyst that speeds up a reaction without being consumed. Enzymes work best within specific temperature and pH ranges.

False.

Increasing temperature increases both the frequency of collisions and the kinetic energy of particles. The increase in energy is the more important effect, meaning a greater proportion of collisions exceed the activation energy.

A catalyst lowers the activation energy of a reaction, meaning more particles have sufficient energy to react.

A small temperature increase raises the kinetic energy of particles significantly, so a much larger proportion of collisions exceed the activation energy. As a rough rule, a 10 °C rise roughly doubles the rate of reaction.

False.

Concentration and surface area only affect the number (frequency) of collisions, not their energy. Temperature is the only factor that directly increases the kinetic energy of particles.

Iron beads are used to maximise the surface area available for catalysis. A greater surface area means more catalyst surface is exposed to the reactants, increasing the rate of reaction.

Three properties that can be used to monitor the rate of a reaction are:

1. Colour change

2. Loss of mass

3. Volume of gas produced

The rate of a reaction is a measure of how quickly reactants are used up or how quickly products are formed over time.

In the sodium thiosulfate and hydrochloric acid experiment, you stop the stopwatch when the cross drawn on paper can no longer be seen from above.

False.

Hydrogen has a very small relative formula mass, M_r, so the loss in mass during the reaction would be too small to measure accurately on a balance. In practice, the mass change is too small to detect on a balance, though the method may be accepted as a theoretical answer in Core-level questions.

Using the same total mass of magnesium ensures only surface area is the variable being changed. If the mass differed, results would reflect both the change in surface area and the change in amount of reactant.

In a mass-loss experiment, cotton wool is placed in the mouth of the flask to allow gas to escape while preventing materials from being ejected.

A measuring cylinder placed upside down in a trough of water is used. Gas produced travels through the delivery tube and displaces water in the cylinder, allowing the volume of gas to be measured.

True.

Diluting the sodium thiosulfate solution with water reduces its concentration. The cross-disappearance method then compares how quickly each concentration causes the solution to turn cloudy.

Cotton wool allows gas to escape so the balance reading changes, while preventing any liquid droplets or solid particles from being ejected from the flask during a vigorous reaction.

The rate of reaction is fastest at the start of the reaction. This is when the concentration of reactants is highest, so there are more particles available to collide and form products.

True.

A horizontal line indicates the rate is zero. At this point, at least one reactant has been completely used up so no more product is being formed.

A tangent is a straight line drawn to touch a curve at one point only. Drawing a tangent allows you to calculate the gradient of the curve at that point, which gives the rate of reaction at that moment.

The rate of reaction at a particular time is calculated using: rate = change in y divided by change in x (the gradient of the tangent).

The gradient decreases because the concentration of reactants falls as they are used up. Fewer particles are available to collide, so the frequency of successful collisions decreases and the rate slows.

False.

Only the limiting reactant determines the amount of product formed. Increasing the reactant in excess does not change the total amount of product.

At double the concentration, the graph has a steeper initial gradient (faster rate) and the reaction finishes sooner. If the doubled reactant is the limiting reactant, double the amount of product is formed.

On a graph monitoring the amount of product over time, the steepest gradient is at the start of the reaction and the line becomes horizontal when the reaction is complete.

A steeper gradient means a greater change in product formed (or reactant used) per unit time. This directly corresponds to a higher rate of reaction.