Rate of Reaction Flashcards

All 44 cards in this deck

What two things can be measured over time to find the rate of a chemical reaction?

The quantity of a reactant used up, or the quantity of a product formed.

What units are used for rate of reaction when the quantity is measured as a mass or as a gas volume?

g/s (mass) or cm$^3$/s (volume).

What are the steps to calculate the mean rate of a reaction?
e.g. 48 cm$^3$ of gas collected in 60 s

1. Find the change in quantity of product: $48$ cm$^3$
2. Divide by the time taken: $48 \div 60$
3. Mean rate $= 0.8$ cm$^3$/s

Name three experimental methods used to follow the rate of a reaction.

Measuring the volume of gas produced, measuring the loss in mass, and following a colour change or turbidity (cloudiness).

In a 'loss of mass' rate experiment, why does the mass of the flask and contents decrease?

A gas is produced which escapes from the flask.

True or false? The mean rate of a reaction is the same as the rate at every moment of the reaction.

False. The mean rate is an average; the actual rate is fastest at the start and slows as reactants are used up.

What unit is used for rate of reaction when the quantity of reactant or product is measured in moles?

mol/s (moles per second).

What are the steps to calculate a mean rate of reaction in mol/s?
e.g. 0.0030 mol of gas collected in 60 s

1. Find the change in moles: $0.0030$ mol
2. Divide by the time taken: $0.0030 \div 60$
3. Mean rate $= 5 \times 10^{-5}$ mol/s

What does a rate of reaction of $2.8 \times 10^{-5}$ mol/s mean?

$2.8 \times 10^{-5}$ moles of product are formed (or reactant used up) each second.

True or false? For the same reaction, the rate in mol/s and the rate in cm$^3$/s are different numbers.

True. They measure the same reaction using different quantities, so the numerical values differ.

On a graph of quantity of product against time, when is the reaction fastest?

At the start, where the curve is steepest.

Why does the line of best fit on a product-against-time graph become horizontal?

The reaction has stopped (no more product formed) because a reactant has been used up.

Why does the curve on a product-against-time graph get less steep as the reaction goes on?

The concentration of the reactants decreases, so the collision frequency and rate decrease.

What are the steps to plot the results of a rate experiment as a graph?
e.g. volume of gas collected every 20 s

1. Put time on the x-axis and volume of gas on the y-axis.
2. Plot each point accurately (to within ½ a small square).
3. Draw a smooth curve of best fit, not dot-to-dot.

True or false? A line that levels off at a higher value always means a faster reaction.

False. The height it levels off at shows the total amount of product; the steepness of the line shows the rate.

What does the slope (gradient) of a tangent drawn to a rate curve tell you?

The rate of reaction at that specific time.

What are the steps to find the rate of reaction at a specific time from a curved graph?
e.g. the rate at 40 s on a gas volume–time graph

1. Draw a tangent that just touches the curve at 40 s.
2. Read a y-step and an x-step from the tangent, e.g. 30 cm$^3$ and 60 s.
3. Rate = y-step ÷ x-step = $0.5$ cm$^3$/s

What is the formula for the gradient of a tangent on a rate graph?

Gradient = y-step ÷ x-step (change in quantity ÷ change in time).

What unit does the gradient of a tangent give on a graph of gas volume (cm$^3$) against time (s)?

cm$^3$/s.

True or false? Joining two points on a curve with a straight line and finding its gradient gives the rate at a specific time.

False. That gives the mean rate between those times; a tangent is needed for the rate at a specific time.

List the five factors that affect the rate of a chemical reaction.

Concentration of reactants in solution, pressure of reacting gases, surface area of solid reactants, temperature, and the presence of a catalyst.

What effect does increasing the temperature have on the rate of a reaction?

It increases the rate of reaction.

What effect does breaking a solid reactant into smaller pieces have on the rate of reaction?

It increases the rate, because the surface area of the solid is increased.

What effect does increasing the pressure of reacting gases have on the rate of reaction?

It increases the rate of reaction.

True or false? Doubling the concentration of a reactant in solution roughly doubles the rate of reaction.

True. There are twice as many reactant particles in the same volume, so roughly twice the collision frequency.

True or false? Using smaller pieces of the same mass of a solid reactant increases the total mass of product formed.

False. The same amount of product is formed, just in a shorter time.

What two methods are used in the required practical investigating the effect of concentration on rate?

Measuring the volume of gas produced, and following a change in colour or turbidity (the disappearing cross).

What are the steps of the 'disappearing cross' method for investigating concentration and rate?
e.g. sodium thiosulfate + hydrochloric acid

1. Place the flask over a paper cross, add fixed volumes of thiosulfate and acid and start the timer.
2. Time how long until the cross can no longer be seen through the cloudy mixture.
3. Repeat with different concentrations of thiosulfate, keeping volumes and temperature the same.

In the sodium thiosulfate and hydrochloric acid reaction, why does the mixture become cloudy?

Sulfur is produced, which is an insoluble solid (a precipitate).

In the required practical on concentration and rate, what are the independent, dependent and one control variable?

Independent: concentration of the solution; dependent: volume of gas collected (or time taken); control: temperature (also volumes and mass of solid).

When collecting a gas from a conical flask, why must the stopper be inserted as quickly as possible?

To reduce the amount of gas that escapes before it can be collected.

When collecting gas over water from a reaction flask, what apparatus error must be avoided with the delivery tube?

The delivery tube must not be left in the reaction mixture (the acid) — it must lead into the collecting vessel.

What does collision theory state about when a chemical reaction can occur?

Reactions occur only when reacting particles collide with each other and with sufficient energy.

What is activation energy?

The minimum amount of energy that particles must have in order to react.

Using collision theory, why does increasing the concentration of a solution increase the rate of reaction?

There are more reactant particles in the same volume, so collisions are more frequent.

Using collision theory, why does increasing the temperature increase the rate of reaction?

Particles have more energy and move faster, so collisions are more frequent and a greater proportion have enough energy to react.

Using collision theory, why do smaller pieces of a solid reactant react faster than larger lumps?

They have a larger surface area to volume ratio, so more particles are exposed and collisions are more frequent.

True or false? Increasing the pressure of reacting gases speeds up the reaction because the collisions become more energetic.

False. The particles are closer together, so collisions are more frequent — not more energetic.

What is a catalyst?

A substance that changes (increases) the rate of a reaction without being used up in the reaction.

How does a catalyst increase the rate of a reaction?

It provides a different pathway for the reaction that has a lower activation energy.

How can you identify a catalyst in a reaction?

It speeds up the reaction but is not used up, and it does not appear in the chemical equation.

What name is given to the catalysts that act in biological systems?

Enzymes.

On a reaction profile, how is a catalysed reaction shown compared with the uncatalysed reaction?

A lower 'hump' (lower activation energy); the overall energy change stays the same.

True or false? Adding a catalyst increases the amount of product a reaction makes.

False. It only makes the same amount of product form faster.