Chemistry - Rates of reaction and energy changes

7.1 Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by:
a measuring the production of a gas (in the reaction between hydrochloric acid and marble chips)

7.1 Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by:
a measuring the production of a gas (in the reaction between hydrochloric acid and marble chips)

The finer the particles of solid, the larger the surface area of marble, so the faster the reaction.
- powdered chalk
- small chips
- large chips

The greater the mass of the marble chips also allows for a greater surface area. The extra surface area gives a faster reaction and there is more gas produced..

7.1 Core Practical: Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by:
b observing a colour change (in the reaction between sodium thiosulfate and hydrochloric acid)

Place a flask over a black mark on a piece of paper which can be seen through the solution. Watch the black mark disappear through the cloudy, yellow sulphur and time how long it takes to go.

The hotter the temperature, the faster the colour change from cloudy to yellow, increasing the rate of the reaction.

7.3

Reactions occur when particles collide.

The rate of reaction is increased when the frequency and/or energy of collisions is increased.

7.4 Explain the effects on rates of reaction of changes in temperature in terms of frequency and/or energy of collisions between particles

The higher the temperature, the faster the particles move, resulting in more frequent collisions.

There is also an increase in the energy of the collisions, which means more successful collisions, increasing the rate of reaction.

7.4 Explain the effects on rates of reaction of changes in concentration in terms of frequency and/or energy of collisions between particles

The higher the concentration, the more particles of reactant in a volume.

Collisions are more likely, so the reaction rate increases.

7.4 Explain the effects on rates of reaction of changes in surface area to volume ratio of a solid in terms of frequency and/or energy of collisions between particles

With a larger surface area:volume ratio, the particles around the solid have more area to work on, increasing the frequency of collisions will increase. So the reaction rate increases.

7.4 Explain the effects on rates of reaction of changes in pressure (on reactions involving gases) in terms of frequency and/or energy of collisions between particles

Increasing the pressure makes the particles more crowded.

The frequency of collisions will increase, so the rate of reaction will increase.

7.6

A catalyst is a substance that speeds up the rate of a reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction.

ACTIVATION ENERGY

Minimum energy needed for particles to react when they collide.

7.7 Explain how the addition of a catalyst increases the rate of a reaction in terms of activation energy

A catalyst decreases the activation energy needed for a reaction to occur, by providing an alternative activation pathway.

So, more particles have the minimum amount of energy needed for a reaction when they collide, increasing the rate of the reaction.

7.8

Enzymes are biological catalysts. Enzymes are used in the production of alcoholic drinks.

7.9

Changes in heat energy accompany these changes:
- salts dissolving in water
- neutralisation reactions
- displacement reactions
- precipitation reactions

When these reactions take place in solution, temperature changes can be measured to reflect the heat changes.

7.10

An exothermic change or reaction is one in which heat
energy is given out.

7.11

An endothermic change or reaction is one in which
heat energy is taken in.

7.12

The breaking of bonds is endothermic.

The making of bonds is exothermic.

7.13

The overall heat energy change for a reaction is exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants.

7.13

The overall heat energy change for a reaction is endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants.

7.14 Calculate the energy change in a reaction given the energies of bonds (in kJ mol-1)

energy change = energy required to break bonds - energy required to form bonds

7.15 Explain the term activation energy

Minimum energy needed for particles to react when they collide.