Rates of Reaction and Energy Changes

What is the core practical for investigating rates of reaction?

Investigate the effects of changing conditions on reaction rates by measuring gas production (hydrochloric acid + marble chips) and observing a colour change (sodium thiosulfate + hydrochloric acid).

How can the rate of a reaction be measured using gas production?

Measure the volume of gas produced at regular time intervals and use the results to calculate or compare the rate of reaction.

How can the rate of a reaction be measured using a colour change?

Measure the time taken for a colour change to occur; a shorter time means a faster reaction.

How can the rate of a reaction be calculated?

Rate = change in quantity ÷ time taken.

How can practical methods be used to determine the rate of a reaction?

Measure a change in mass, volume, concentration or colour over a known period of time.

What happens when particles collide in a chemical reaction?

Particles must collide with sufficient energy and the correct orientation for a reaction to occur.

What two factors determine whether collisions result in a reaction?

The frequency of collisions and the energy of the collisions.

How does increasing the frequency of collisions affect reaction rate?

It increases the rate because more successful collisions occur per second.

How does increasing the energy of collisions affect reaction rate?

It increases the rate because a greater proportion of collisions have enough energy to overcome the activation energy.

How does increasing temperature affect the rate of reaction?

Particles gain kinetic energy, move faster and collide more frequently and with greater energy, so there are more successful collisions per second.

How does increasing concentration affect the rate of reaction?

There are more particles per unit volume, increasing collision frequency and therefore the number of successful collisions per second.

How does increasing the surface area to volume ratio of a solid affect reaction rate?

More particles are exposed, increasing the frequency of collisions and therefore increasing the reaction rate.

How does increasing pressure affect the rate of a reaction involving gases?

Gas particles are closer together, increasing collision frequency and therefore increasing the reaction rate.

How does decreasing temperature affect reaction rate?

Particles have less kinetic energy, so collisions are less frequent and fewer have enough energy to overcome activation energy, decreasing the rate.

How does decreasing concentration affect reaction rate?

There are fewer particles per unit volume, so collisions occur less frequently and the reaction is slower.

How does decreasing the surface area to volume ratio of a solid affect reaction rate?

Fewer particles are exposed, reducing collision frequency and slowing the reaction.

How does decreasing pressure affect the rate of a reaction involving gases?

Gas particles are further apart, reducing collision frequency and slowing the reaction.

What does a graph of mass of reactant against time show?

The mass decreases over time; a steeper gradient means a faster reaction.

What does a graph of mass of product against time show?

The mass increases over time; a steeper gradient means a faster reaction.

What does a graph of volume of gas against time show?

The volume of gas increases over time; a steeper gradient means a faster reaction.

What does a graph of concentration of reactant against time show?

The concentration decreases over time; a steeper negative gradient represents a faster reaction.

What does a graph of concentration of product against time show?

The concentration increases over time; a steeper gradient represents a faster reaction.

How can the rate of reaction be found from a graph?

Calculate the gradient: rate = change in quantity ÷ change in time.

What is a catalyst?

A substance that increases the rate of a reaction without changing the products and is chemically unchanged and unchanged in mass at the end.

How does a catalyst increase the rate of reaction?

It provides an alternative reaction pathway with a lower activation energy.

Does a catalyst change the products of a reaction?

No. It only increases the rate of reaction.

What are enzymes?

Enzymes are biological catalysts.

How are enzymes used in the production of alcoholic drinks?

Enzymes catalyse reactions involved in fermentation, converting sugars into ethanol and carbon dioxide.

What types of reactions can involve measurable heat energy changes?

Dissolving salts in water, neutralisation, displacement and precipitation reactions.

How can heat energy changes be measured in reactions taking place in solution?

Measure the temperature change of the solution.

What is an exothermic reaction?

A reaction or change that gives out heat energy to the surroundings.

What is an endothermic reaction?

A reaction or change that takes in heat energy from the surroundings.

What happens to the temperature of the surroundings during an exothermic reaction?

The temperature increases.

What happens to the temperature of the surroundings during an endothermic reaction?

The temperature decreases.

Is breaking chemical bonds endothermic or exothermic?

Endothermic because energy is required to break bonds.

Is making chemical bonds endothermic or exothermic?

Exothermic because energy is released when bonds form.

When is an overall reaction exothermic in terms of bond energies?

When more energy is released forming bonds in the products than is required to break bonds in the reactants.

When is an overall reaction endothermic in terms of bond energies?

When less energy is released forming bonds in the products than is required to break bonds in the reactants.

What equation is used to calculate the overall energy change of a reaction from bond energies?

Energy change = energy required to break bonds − energy released when bonds form.

What does a positive energy change indicate?

An endothermic reaction.

What does a negative energy change indicate?

An exothermic reaction.

What is activation energy?

The minimum energy that particles must have for a successful collision and reaction to occur.

What is shown on the y-axis of a reaction profile?

Energy.

What is shown on the x-axis of a reaction profile?

Progress of reaction.

How is activation energy shown on a reaction profile?

As the energy difference between the reactants and the highest point of the curve.

How does an exothermic reaction profile differ from an endothermic profile?

The products have less energy than the reactants in an exothermic reaction.

How does an endothermic reaction profile differ from an exothermic profile?

The products have more energy than the reactants in an endothermic reaction.

How does a catalyst affect a reaction profile?

It lowers the activation energy but does not change the energy of the reactants or products.