# Quick question for q = mcΔt

Watch
Announcements
Thread starter 4 years ago
#1
My teacher wrote on my test that Q is always positive but in some questions, the mark scheme says the answer is negative

I'm wondering If heat is given out (exothermic reaction) or there's a rise in temperature, does that mean Q will be negative?
0
4 years ago
#2
You can get negative Q values as the Q denotes the energy output (enthalpy change) of a particular substance. If you imagine the tables we're provided for Hess' cycles, some of the values are negative for the same reason (representing a negative enthalpy change).
Or at least as far as I'm aware (I've been awarded full marks for negative values and having gone over mark schemes it states that a mark should be deducted for positive values)...

Hope this helps,
0
4 years ago
#3
If reaction is exothermic, the enthalpy change will be negative. Likewise if reaction is endothermic, enthalpy change is positive.
1
4 years ago
#4
Surely Q would be negative for exothermic reactions and positive for endothermic?
1
Thread starter 4 years ago
#5
(Original post by Cressiswell)
You can get negative Q values as the Q denotes the energy output (enthalpy change) of a particular substance. If you imagine the tables we're provided for Hess' cycles, some of the values are negative for the same reason (representing a negative enthalpy change).
Or at least as far as I'm aware (I've been awarded full marks for negative values and having gone over mark schemes it states that a mark should be deducted for positive values)...

Hope this helps,
I've seen mark schemes with both negative and positive values, but I'm unsure which to use!

Edit: I looked it up and it said 'if temp increasesthe reaction is exothermic and is given a minus sign e.g. –146 kJ mol-1'
0
4 years ago
#6
(Original post by Mina_)
I've seen mark schemes with both negative and positive values, but I'm unsure which to use!
If it's exothermic it'll be negative; if it's endothermic it'll be positive. I've found in the past that the vast majority of 'q' values do tend to be exothermic; though this is obviously not always the case. Basically, if you know for a fact it's given out heat i.e., it's being combusted - make it negative!

Again, hope this helps,
1
Thread starter 4 years ago
#7
Thanks everyone
0
4 years ago
#8
(Original post by Mina_)
I've seen mark schemes with both negative and positive values, but I'm unsure which to use!

Edit: I looked it up and it said 'if temp increasesthe reaction is exothermic and is given a minus sign e.g. –146 kJ mol-1'
Think about it exothermic gives out heat so it loses stuff so you have a minus

Obviously endothermic you have a positive value because you take heat in
0
Thread starter 4 years ago
#9
Is my teacher wrong then? There was a temperature rise
0
4 years ago
#10
short form=
q is the energy change (enthalpy change) in the products
q is positive when energy is absorbed from surroundings and the reaction is endothermic (the products' bond energies are lower than the reactants)
q is negative when energy is released from the surroundings.

also:
Breaking the original bonds requires energy (absorbed from surroundings)
Making new bonds releases energy (to the surroundings)

When the total bond energies of reactants is higher than the products, the reaction is endothermic (more energy absorbed than released) and vice versa.
0
4 years ago
#11
(Original post by Mina_)
Is my teacher wrong then? There was a temperature rise
In fact the teacher is right, because

Energy lost by the reactants= q
therefore energy transferred to the water= -q

as a result, the answer is positive because it is talking about the water not the products
0
Thread starter 4 years ago
#12
(Original post by longshot100)
In fact the teacher is right, because

Energy lost by the reactants= q
therefore energy transferred to the water= -q

as a result, the answer is positive because it is talking about the water not the products
You've lost me now
0
4 years ago
#13
(Original post by Mina_)
You've lost me now
Could you send pics with context pls? Though based on that I'd (hazardously) suggest that the teacher is wrong as it is 'heat given out' - that would normally imply a negative value.
0
4 years ago
#14
Maybe your teacher wants to show how your all Q (T)SSSS
0
Thread starter 4 years ago
#15
-----------------
0
Thread starter 4 years ago
#16
Question 8
0
4 years ago
#17
(Original post by Mina_)
My teacher wrote on my test that Q is always positive but in some questions, the mark scheme says the answer is negative

I'm wondering If heat is given out (exothermic reaction) or there's a rise in temperature, does that mean Q will be negative?
Depends on the question. In a lot of cases the question is worded in such a way that the answer will always be positive, regardless of the sign of the enthalpy change
0
4 years ago
#18
Hi Mina_

q is measured in joules. Your units were wrong also, you gave Kjmol-1 when you had in fact only worked out joules. You would need to divide by the moles to get PER mole and then divide by 1000 to get Kjmol-1.

q can be negative if your temperature change is negative, though a lot of people keep the temperature change as a positive number even for endothermic reactions. Your teacher is trying to teach your the simplest way to ensure you get the right answers.

If you express temperature CHANGE as a positive number then your q value will be a positive number. You just need to ensure that when you work out deltaH you make sure that it is a positive sign for endothermic reactions and a negative sign for an exothermic reaction. I hope that helps.

Alex
0
4 years ago
#19
(Original post by Mina_)
You've lost me now
Basically, q is the energy change in the reaction.

A negative value of q means energy is released to the surroundings,

So the energy gained by/transferred to the surroundings must be the reverse (aka positive becomes negative, vice versa) of q. (-q)

and in this case the question says assume that all of the energy lost to surroundings is gained by the water.

Therefore the energy gained by the water (causing it to heat up) is the exact amount of energy lost in the reaction, except it is positive.

Sorry if this is still hard to understand, I can't really explain it any better than this
0
4 years ago
#20
Hi, has anyone here done or is doing the Chem 3X paper?
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### What factors affect your mental health the most right now?

Anxiousness about lockdown easing (128)
4.91%
Uncertainty around my education (387)
14.84%
Uncertainty around my future career prospects (289)
11.09%
Lack of purpose or motivation (367)
14.08%
Lack of support system (eg. teachers, counsellors, delays in care) (120)
4.6%
Impact of lockdown on physical health (159)
6.1%
Loneliness (221)
8.48%
Financial worries (96)
3.68%
Concern about myself or my loves ones getting/having been ill (107)
4.1%
Exposure to negative news/social media (120)
4.6%
Lack of real life entertainment (138)
5.29%
Lack of confidence in making big life decisions (228)
8.75%
Worry about missed opportunities during the pandemic (247)
9.47%