Exothermic & Endothermic Reactions Flashcards

All 27 cards in this deck

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, so the temperature of the surroundings increases.

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, so the temperature of the surroundings decreases.

Name three types of reaction that are exothermic.

Combustion, many oxidation reactions and neutralisation.

Give two examples of endothermic reactions.

Thermal decomposition, and the reaction of citric acid with sodium hydrogencarbonate.

Which type of reaction is used in hand warmers and self-heating cans?

Exothermic — energy is transferred to the surroundings, warming them up.

True or false? A sports injury cold pack works using an exothermic reaction.

False. Some sports injury packs use an endothermic reaction, which takes in energy and cools the surroundings.

In an exothermic reaction, how does the energy of the product molecules compare with the energy of the reactants?

The products have less energy than the reactants, by the amount of energy transferred to the surroundings (energy is conserved).

What are the steps to measure the temperature change of a neutralisation reaction?

e.g. $25$ cm$^3$ of acid added to $25$ cm$^3$ of alkali

1. Measure $25$ cm$^3$ of alkali into a polystyrene cup with a lid and record its temperature.
2. Add $25$ cm$^3$ of acid, stir, and record the highest temperature reached.
3. Temperature change = highest temperature − starting temperature.

Why does a polystyrene cup give more accurate results than a glass beaker in a temperature-change investigation?

Polystyrene is a better thermal insulator, so there is less energy transfer to the surroundings.

True or false? Repeating a temperature-change experiment in a metal container instead of a polystyrene cup gives a smaller temperature change.

True. Metal is a better conductor, so more energy is transferred to (or absorbed from) the surroundings.

Give the independent and dependent variables in this investigation.

e.g. different masses of zinc powder added to copper sulfate solution

Independent: mass of zinc added.
Dependent: highest temperature reached (or temperature change).

On a graph of highest temperature against mass of solid added, where do you read the initial temperature of the reaction mixture?

Where the line of best fit, extrapolated back, meets the $y$-axis.

On a temperature against mass graph, what does the point where the temperature stops changing show?

The reaction is complete — one reactant has been used up and the added solid is now in excess, so no further reaction occurs.

What is meant by activation energy?

The minimum amount of energy that particles must have to react (the energy needed for a reaction to occur).

What two conditions must be met for reacting particles to react?

They must collide with each other, and collide with at least the activation energy.

How does a reaction profile show that a reaction is exothermic?

The products line is below the reactants line — the products have less energy than the reactants.

On a reaction profile, between which two points is the activation energy drawn?

From the reactants line up to the top (peak) of the curve.

What are the steps to draw a reaction profile for an exothermic reaction?

e.g. the combustion of methane

1. Draw the reactants level on the left and the products level lower down on the right.
2. Join them with a curved line that rises to a peak, then falls to the products.
3. Label the activation energy (reactants to peak) and the overall energy change (reactants level to products level).

How does a reaction profile for an endothermic reaction differ from one for an exothermic reaction?

The products level is above the reactants level, so the overall energy change arrow points upwards.

What is shown on the vertical axis of a reaction profile?

Energy (the horizontal axis shows the progress of the reaction).

Is breaking bonds in the reactants exothermic or endothermic?

Endothermic — energy must be supplied to break bonds.

What happens to energy when bonds in the products are formed?

Energy is released (bond making is exothermic).

How is the overall energy change of a reaction found from bond energies?

Overall energy change = energy needed to break bonds in the reactants − energy released forming bonds in the products, in kJ/mol.

In an exothermic reaction, how do the bond breaking and bond making energies compare?

The energy released from forming new bonds is greater than the energy needed to break existing bonds.

True or false? In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released when new bonds form.

True. That is why energy is taken in overall and the energy change is positive.

What are the steps to calculate the overall energy change from bond energies?

e.g. H–H + Cl–Cl $\rightarrow$ 2 H–Cl (H–H $436$, Cl–Cl $243$, H–Cl $432$ kJ/mol)

1. Add the bond energies of all bonds broken: $436 + 243 = 679$.
2. Add the bond energies of all bonds made: $2 \times 432 = 864$.
3. Energy change = broken − made $= 679 - 864 = -185$ kJ/mol, so exothermic.

True or false? In an exothermic reaction some energy is destroyed, so there is less energy in the universe after the reaction.

False. Energy is conserved — the total energy in the universe is the same before and after; it is only transferred to the surroundings.