Rate and Extent of Chemical Change

What is the rate of a chemical reaction?

How quickly reactants are used up or products are formed.

What is the equation for mean rate using reactant?

Mean rate = quantity of reactant used / time.

What is the equation for mean rate using product?

Mean rate = quantity of product formed / time.

What units can rate have when mass is measured?

g/s.

What units can rate have when gas volume is measured?

cm³/s.

What factors affect the rate of a reaction?

Concentration, pressure of gases, surface area, temperature + catalysts.

What is collision theory?

Chemical reactions occur when reacting particles collide with sufficient energy.

What two things determine how fast a reaction occurs according to collision theory?

Frequency of collisions + proportion of collisions with enough energy.

What is activation energy?

Minimum energy particles must have for a successful collision.

How does increasing concentration increase reaction rate?

More particles in the same volume - collisions occur more frequently.

How does decreasing concentration affect reaction rate?

Fewer particles per unit volume - fewer collisions per second - slower reaction.

How does increasing gas pressure increase reaction rate?

Particles are closer together - collisions occur more frequently.

Why does pressure mainly affect reactions involving gases?

Gas particles can be compressed closer together.

How does increasing surface area increase reaction rate?

More reactant particles are exposed - collisions occur more frequently.

Why does powder react faster than large lumps of the same mass?

Powder has a larger surface area to volume ratio - more particles exposed for collisions.

How does increasing temperature increase reaction rate?

Particles move faster - collisions occur more frequently + more particles have energy greater than or equal to activation energy.

Why is "particles collide more often" not a complete explanation for temperature?

Higher temperature also increases the proportion of collisions with enough energy to react.

What is a catalyst?

Substance that increases reaction rate without being used up.

How does a catalyst increase reaction rate?

Provides an alternative reaction pathway with lower activation energy.

Does a catalyst change the overall energy change of a reaction?

No - it only lowers activation energy.

How is a catalyst shown on a reaction profile?

Catalysed pathway has a lower activation-energy peak.

How can rate be measured using gas production?

Measure volume of gas produced over time.

How can gas volume be measured during a reaction?

Using a gas syringe.

How can rate be measured using change in mass?

Measure decrease in mass over time as a gas escapes.

How can a balance be used to investigate rate?

Record mass at regular time intervals as gaseous product escapes.

What does a steeper gradient on a reaction graph mean?

Faster rate of reaction.

What happens to the gradient as a reaction slows?

It becomes less steep.

Why does a reaction graph eventually become horizontal?

Reaction has finished - no further change in reactant/product quantity.

How do you calculate mean rate from a graph?

Change in quantity / change in time.

How do you find the rate at a specific time from a curved graph?

Draw a tangent at that point - calculate its gradient. (HT)

How do you calculate the gradient of a tangent?

Change in y / change in x using a large triangle on the tangent. (HT)

Why should a large triangle be used to calculate a tangent gradient?

Reduces percentage uncertainty in reading values from the graph.

RP5 - What does the rate required practical investigate?

Effect of concentration on the rate of a chemical reaction.

RP5 - What two methods can be used?

Measure gas produced or observe disappearance of a cross.

RP5 - What reaction can be used for the disappearing-cross method?

Sodium thiosulfate + hydrochloric acid.

RP5 - Why does the cross disappear in the sodium thiosulfate experiment?

Sulfur precipitate forms - solution becomes cloudy.

RP5 - How is the disappearing-cross experiment carried out?

Place flask over cross - add sodium thiosulfate + acid - start timer - stop when cross can no longer be seen.

RP5 - What is changed in the disappearing-cross experiment?

Concentration of sodium thiosulfate.

RP5 - What should be controlled in the disappearing-cross experiment?

Volumes, acid concentration, temperature, total volume + same cross/viewing conditions.

RP5 - What is used as a measure of rate in the disappearing-cross experiment?

1 / time.

RP5 - Why can 1/time be used as a measure of rate?

The same amount of sulfur must form to hide the cross - shorter time means faster rate.

RP5 - What should a graph of rate against concentration show?

Rate increases as concentration increases.

RP5 - What reaction can be used to investigate rate by gas production?

Magnesium + hydrochloric acid.

RP5 - How can gas production be measured?

Collect hydrogen in a gas syringe + record volume at regular time intervals.

RP5 - What is changed when investigating concentration using magnesium and acid?

Concentration of hydrochloric acid.

RP5 - What should be controlled in the magnesium experiment?

Volume of acid, mass/length/surface area of magnesium + temperature.

RP5 - Why should the bung be fitted quickly in a gas experiment?

Prevents gas escaping before measurements begin.

RP5 - What is a major source of error when collecting gas?

Gas can escape before the apparatus is sealed.

RP5 - Why should experiments be repeated?

Identify anomalies + calculate a mean to improve reliability.

What is a reversible reaction?

A reaction where products can react to form the original reactants.

How is a reversible reaction represented?

Using two half arrows pointing in opposite directions.

How can the direction of some reversible reactions be changed?

By changing the conditions.

How are energy changes related in forward and reverse reactions?

One direction is exothermic and the other is endothermic.

If the forward reaction is exothermic, what is the reverse reaction?

Endothermic.

Is the energy transferred in the forward reaction different in size from the reverse reaction?

No - same amount of energy, transferred in opposite directions.

What is equilibrium?

State in a reversible reaction where concentrations of reactants and products remain constant.

When can equilibrium be reached?

In a closed system.

What is a closed system?

System where reactants/products cannot escape or enter.

What is dynamic equilibrium?

Forward + reverse reactions continue at equal rates, so concentrations remain constant.

Do reactions stop at dynamic equilibrium?

No - both reactions continue.

Are concentrations of reactants and products equal at equilibrium?

Not necessarily - they are constant, but do not have to be equal.

What is equal at dynamic equilibrium?

Rates of forward + reverse reactions.

What is Le Chatelier's principle?

If a system at equilibrium experiences a change, the equilibrium shifts to oppose the change. (HT)

What factors can change the position of equilibrium?

Concentration, pressure + temperature. (HT)

What happens if concentration of a reactant is increased?

Equilibrium shifts towards products to use up some added reactant. (HT)

What happens if concentration of a reactant is decreased?

Equilibrium shifts towards reactants to replace some removed reactant. (HT)

What happens if concentration of a product is increased?

Equilibrium shifts towards reactants to use up some added product. (HT)

What happens if a product is removed?

Equilibrium shifts towards products to replace some removed product. (HT)

When does changing pressure affect equilibrium?

When gases are involved + there are different numbers of gas molecules on each side. (HT)

What happens when pressure is increased at equilibrium?

Equilibrium shifts to the side with fewer gas molecules. (HT)

What happens when pressure is decreased at equilibrium?

Equilibrium shifts to the side with more gas molecules. (HT)

What if both sides have the same number of gas molecules?

Changing pressure does not change the equilibrium position. (HT)

How do you determine which side has fewer gas molecules?

Count the balancing numbers of gaseous substances on each side. (HT)

What happens when temperature is increased at equilibrium?

Equilibrium shifts in the endothermic direction to take in added energy. (HT)

What happens when temperature is decreased at equilibrium?

Equilibrium shifts in the exothermic direction to release energy. (HT)

If the forward reaction is exothermic, what happens when temperature increases?

Equilibrium shifts left towards reactants. (HT)

If the forward reaction is endothermic, what happens when temperature increases?

Equilibrium shifts right towards products. (HT)

Does a catalyst change the position of equilibrium?

No.

What does a catalyst do to a reversible reaction?

Increases rates of forward + reverse reactions equally - equilibrium is reached faster.

How should you answer an equilibrium-change question?

Identify the change - state which direction opposes it - state whether product/reactant yield increases.